Fish in the Murray Valley and
Torrumbarry Irrigation Areas
J. O’Connor, A. King, Z. Tonkin, J. Morrongiello and C. Todd
2008
Arthur Rylah Institute for Environmental Research
Technical Report Series No. 176
+
Arthur Rylah Institute for Environmental Research Technical Series No. 176
Fish in the Murray Valley
and Torrumbarry Irrigation Areas
Justin O’Connor, Alison King, Zeb Tonkin,
John Morrongiello and Charles Todd
Arthur Rylah Institute for Environmental Research
123 Brown Street, Heidelberg, Victoria 3084
June 2008
Arthur Rylah Institute for Environmental Research
Department of Sustainability and Environment
Heidelberg, Victoria
Report produced by:
Arthur Rylah Institute for Environmental Research
Department of Sustainability and Environment
PO Box 137
Heidelberg, Victoria 3084
Phone (03) 9450 8600
Website: www.dse.vic.gov.au/ari
© State of Victoria, Department of Sustainability and Environment 2008
This publication is copyright. Apart from fair dealing for the purposes of private study, research, criticism or review as
permitted under the Copyright Act 1968, no part may be reproduced, copied, transmitted in any form or by any means
(electronic, mechanical or graphic) without the prior written permission of the State of Victoria, Department of
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or email customer.service@dse.vic.gov.au
Citation: O’Connor, Justin, King, Alison, Tonkin, Zeb, Morrongiello, John and Todd, Charles. (2008). Fish in the
Murray Valley and Torrumbarry Irrigation Areas. Arthur Rylah Institute for Environmental Research Technical Report
Series No. 176. Department of Sustainability and Environment, Heidelberg, Victoria
ISSN 1835-3827 (print)
ISSN 1835-3835 (online)
ISBN 978-1-74208-693-4 (print)
ISBN 978-1-74208-694-1 (online)
Disclaimer: This publication may be of assistance to you but the State of Victoria and its employees do not guarantee
that the publication is without flaw of any kind or is wholly appropriate for your particular purposes and therefore
disclaims all liability for any error, loss or other consequence which may arise from you relying on any information in
this publication.
Front cover photo: (clockwise from top left), Murray cod collected below Torrumbarry Irrigation Channel outlet;
Murray Valley Main Channel; tertiary channel in Murray Valley Irrigation Area; boat electrofishing Loddon River
syphon oin Torrumbarry Irrigation Area; Culvert on road crossing at Boothroyds Road in Murray Valley Irrigation Area.
Photos: Justin O’Connor.
Authorised by: Victorian Government, Melbourne
2
Contents
List of tables and figures ...................................................................................................................v
Acknowledgements ......................................................................................................................... vii
Summary and recommendations ......................................................................................................1
1
Introduction .............................................................................................................................3
2
2.1
Methods ....................................................................................................................................6
Study Area.................................................................................................................................6
2.2
2.1.1
Murray Valley Irrigation Area — system description ................................................ 6
2.1.2
Torrumbarry Irrigation Area — system description ...................................................9
Fish sampling ..........................................................................................................................10
2.2.1
Electrofishing...........................................................................................................10
2.2.2
Pumpouts .................................................................................................................12
2.2.3
Murray cod ageing ...................................................................................................14
2.3
Egg and larva sampling ...........................................................................................................14
2.4
Modelling the impact of larval loss on a Murray cod population ............................................. 17
2.4.1
Scenarios..................................................................................................................17
2.4.2
Model output............................................................................................................17
2.5
Water quality. ..........................................................................................................................19
3
3.1
Results ....................................................................................................................................20
Water quality — Murray Valley and Torrumbarry Irrigation Areas......................................... 20
3.2
Adult fish surveys — Murray Valley Irrigation Area 2006......................................................20
3.3
3.2.1
General ....................................................................................................................20
3.2.2
Recaptures ...............................................................................................................20
3.2.3
Length range ............................................................................................................20
3.2.4
Fish distribution .......................................................................................................20
3.2.4
Murray cod ageing ...................................................................................................21
3.2.5
2006 Murray Valley pumpouts ................................................................................21
3.2.6
Young-of-year Murray cod. .....................................................................................25
3.2.7
2004–05 vs 2005–06 Murray Valley Irrigation Area comparison ...........................25
Adult Fish Surveys — Torrumbarry Irrigation Area 2006 .......................................................27
3.3.1
General fish sampling ..............................................................................................27
3.3.2
Recaptures ...............................................................................................................28
3.3.3
Fish distribution .......................................................................................................28
3.3.4
Length range ............................................................................................................28
3
3.4
Egg and larval sampling ......................................................................................................... 31
3.4.1
Water Quality .......................................................................................................... 31
3.4.2
Drift samples ........................................................................................................... 32
3.4.3
Comparison of Murray Valley and Torrumbarry Irrigation Areas ........................... 33
3.5
Impact of larval loss on a Murray cod population ................................................................... 36
4
4.1
Discussion .............................................................................................................................. 40
Adult fish surveys .........................................................................................................................40
4.2
Early life stages in the channels .............................................................................................. 43
4.3
Impacts of larval loss on a Murray cod population ................................................................. 44
5
Conclusion ............................................................................................................................. 46
6
Recommendations ................................................................................................................ 47
References........................................................................................................................................ 48
Appendix 1......................................................................................................................................... 1
Water quality from Murray Valley Irrigation Area ............................................................................. 1
Appendix 2......................................................................................................................................... 2
Water quality from Torrumbarry Irrigation Area ................................................................................ 2
4
List of tables and figures
List of tables
Table 1. Type and number of structures present in the Torrumbarry and Murray Valley Irrigation
Areas that could act as refuges for fish after drawdown ............................................................9
Table 2. Location of drift net sampling sites ..................................................................................... 15
Table 3. Total raw abundance data of native and introduced fish species found during surveys of
the Murray Valley Irrigation Area in 2006. .............................................................................21
Table 4. Comparison of raw abundance between Murray Valley electrofishing and pumpout
sampling methodologies ..........................................................................................................25
Table 5. Total raw abundance data of native and introduced fish species found during surveys of
the Torrumbarry Irrigation Area in 2006. ................................................................................27
Table 6. Density of drifting eggs and larvae captured in both irrigation channel and the Murray
River for both Torrumbarry and Murray Valley sites. Catches are expressed as mean catch
per 1000 m3 .................................................................................................................................. 33
List of figures
Figure 1. Goulburn–Murray Water Irrigation Network .......................................................................6
Figure 2. Percentage flow diversion down (a) Murray Valley Channel and (b) combined Murray
Valley and Mulwala Channels between 2004 and December 2007...........................................7
Figure 3. Sampling sites located in the Murray Valley Irrigation Area ................................................ 8
Figure 4. Percentage flow diversion down Torrumbarry (National) Channel between 2004 and
December 2007. ......................................................................................................................10
Figure 5. Sampling sites located in the Torrumbarry Irrigation Area ................................................. 11
Figure 6. Pumpout at Grinters Road (Site 15), where 27 young-of-year Murray cod were collected
from below this structure .........................................................................................................13
Figure 7. Pumpout at Boothroyds Road (Site 22), Murray Valley Irrigation Area..............................14
Figure 8. Standard passive drift net used in the larval study .............................................................. 15
Figure 9. Drift net set from an overpass on the Murray Valley irrigation channel .............................. 16
Figure 10. An example of some trajectories produced from a stochastic population model for
Murray cod. Pale blue circles indicate the minimum population size from each trajectory. ..18
Figure 11. Examples of different risk curves (cumulative distribution of minimum population
sizes) under different scenarios identifying the concepts of added risk .................................... 18
Figure 12. Percentage species composition of CPUE of fish in the 2006 Murray Valley Irrigation
Area fish survey ......................................................................................................................22
Figure 13. Length range of Murray cod (n = 105) and golden perch (n=7) collected from the
Murray Valley Irrigation Area in 2006. ...................................................................................22
Figure 14. Length range of unspecked hardyhead collected from the Murray Valley Irrigation Area
in 2006 (n = 100) .....................................................................................................................23
Figure 15. CPUE of all, native and introduced fish with distance from source waters downstream
the Murray Valley Channel system .........................................................................................24
Figure 16. Timing and magnitude of diversions into the Murray Valley Irrigation Area during the
2004–05 and 2005–06 irrigation seasons ................................................................................26
5
Figure 17. Percentage species composition of CPUE of fish in the Murray Valley Irrigation Area
between 2005 and 2006. ......................................................................................................... 26
Figure 18. Percentage species composition of CPUE of fish in the 2006 Torrumbarry fish survey.28
Figure 19. CPUE of all native and introduced fish with distance from source waters downstream of
the Torrumbarry Irrigation Area ............................................................................................. 29
Figure 20. Lengths of Murray cod (n = 21) and golden perch (n = 6) collected from the
Torrumbarry Irrigation Area in 2006. ..................................................................................... 30
Figure 21. Comparison of percentage species composition of CPUE of fish in the Murray Valley
and Torrumbarry Irrigation areas in 2006. .............................................................................. 30
Figure 22. Water temperatures of channel and river habitats in Murray Valley and Torrumbarry
Irrigation Areas during larval sampling in 2006. .................................................................... 31
Figure 23. Species percentage composition of raw numbers of eggs and larvae found drifting in
both irrigation channel and the Murray River at Torrumbarry and Murray Valley sites. (Note
that only two instead of three drift nets were set at the Torrumbarry river site.) ...................... 32
Figure 24. Densities of total eggs captured for each sample trip at both Torrumbarry and Murray
Valley for both irrigation channel (blue bars) and Murray River (pink bars) habitats in 2006.
Densities are shown as means with 1 SE ................................................................................ 34
Figure 25. Densities of total larvae captured for each sample trip at both Torrumbarry and Murray
Valley for both irrigation channel (blue bars) and Murray River (pink bars) habitats in 2006.
Densities are shown as means with 1 SE ................................................................................ 35
Figure 26. Densities of Murray cod larvae captured for each sample trip at both Torrumbarry and
Murray Valley for both irrigation channel (blue bars) and Murray River (pink bars) habitats
in 2006. Densities are shown as means with 1 SE .................................................................. 35
Figure 27. Risk curves of the total adult female population for the ‘no fishing’ scenario. In this and
subsequent figures, the black line represents no loss of larvae; the red line represents 50%
loss, and the blue line represents 80% loss ............................................................................. 36
Figure 28. Risk curves of the adult female population aged 5–9 years old for the ‘no fishing’
scenario. ................................................................................................................................. 37
Figure 29. Risk curves of the adult female population aged 10 plus years for the ‘no fishing’
scenario. ................................................................................................................................. 37
Figure 30. Risk curves of the total adult female population for the ‘no fishing’ scenario. ................... 38
Figure 31. Risk curves of the adult female population aged 5–9 years for the ‘no fishing’ scenario.38
Figure 32. Risk curves of the adult female population aged 10 plus years for the ‘no fishing’
scenario. ................................................................................................................................. 39
Figure 33. Average population trajectory for the total adult female population under the ‘fishing’
scenario with an 80% loss of larvae. The red lines are the maximum and minimum over all
trajectories, the blue lines are ± 1 standard deviation and the black is the average overall
trajectories .............................................................................................................................. 45
6
Acknowledgements
We would like to thank Goulburn–Murray Water staff for their help in collating information about
the systems, anecdotal reports on fish in the channels and their help with field site selection, in
particular Craig Sullivan, Pat Feehan, Kevin Preist, Ross Stanton, Tony Beamish, Terry Holt,
Allan Williams and Steve Hall. Bruce McBeath, John Mahoney, Andrew Pickworth, Wayne
Koster, Damien O’Mahoney and Peter Fairbrother from ARI helped with field work. Joanne
Kearns undertook larval sorting and identification. We would also like to thank Tarmo Raadik and
Mike Smith for helpful comments on the draft report.
7
viii
Fish in the Murray Valley and Torrumbarry Irrigation Areas
Summary and recommendations
A massive amount of water is diverted each year from the natural riverine environment to artificial
irrigation channels, which can exceed flows into the natural riverine environment. At times, the
direction of many native fish movements is driven by flow volume. As a result, it appears that a
substantial number of fish are being lost to the irrigation channel environment, from which they
are unlikely to escape. Of particular concern are the Torrumbarry and Murray Valley Irrigation
Areas in the Goulburn–Murray Irrigation Network.
To build on information gathered in a study in 2005, and to undertake an assessment of adult
entrapment in both of these irrigation systems, the first comprehensive survey of adult fish in the
Torrumbarry Irrigation Area was initiated. The survey also included a repeat of the adult fish
surveys in the Murray Valley Irrigation Area in 2005 and a comprehensive assessment of early life
stage entrainment (including eggs and larvae) into both irrigation areas. Electrofishing surveys
were conducted at 30 sites in the Torrumbarry Irrigation Area and 29 sites in the Murray Valley
Irrigation Area between May and June 2006. Fortnightly sampling for drifting eggs and larvae was
conducted in both Torrumbarry and Murray Valley Irrigation Areas in November and December
2006.
This study highlighted the broad diversity and high abundance of native and introduced fish
present in the channel systems. More than 10 000 native fish (ten species) and almost 4000
introduced fish (five species) were collected from the Murray Valley and Torrumbarry Irrigations
Areas during electrofishing surveys undertaken in this study. The native fish included six
threatened species: Murray cod, Murray–Darling rainbowfish, unspecked hardyhead, golden perch
and silver perch. This study also highlighted the dynamic nature of species diversity and
abundance in the channels which has been shown to vary between years in the Murray Valley
Irrigation Area. For example, there were significantly more Murray cod and unspecked hardyhead
in the Murray Valley Irrigation Area in 2006 than in 2005. The survey results indicate that Murray
cod are probably entering the channels system early in their life history and golden perch enter
later in their life history, and also that few Murray cod are surviving beyond the juvenile phase
once they enter the channel systems.
Larval fish populations were dominated by flat-headed gudgeon, although larvae and eggs from
native species of conservation significance were also detected drifting in both channel systems.
These include Murray cod, silver perch and golden perch. The survey results indicate that drifting
eggs and larvae appear to be sourced from the river environment and not from within the channel
system. The results also indicate that there is a significant loss of eggs and larvae from the riverine
systems into the channels systems, and consequently there is a clear need to reduce this loss while
minimising the impact on irrigation water supply. Existing diversion rates, typically around 20–
30% of total passing flow, could equate to a significant loss of larvae to the channels system.
Population modelling indicates that, in conjunction with other impacts such as fishing, a loss of
over 50% of Murray cod larvae to the channel systems would have a significant impact on the
riverine population.
This study, in conjunction with the results of the 2005 study, suggests that an abundant and diverse
range of native fish are consistently being removed from the riverine environment into the channel
systems. Among these are large numbers of many smaller species such as Australian smelt,
gudgeons and unspecked hardyhead. However, numerous larger species such as Murray cod
appear to be entering the channel systems as juvenile fish, and golden perch appears to be entering
the channel systems as adults. This study indicates that native fish are being lost into channel
systems in numbers large enough to suggest the need to take action to reduce their removal from
the riverine environment. Potential solutions for reducing the loss of fish into irrigation systems
Arthur Rylah Institute for Environmental Research
1
Fish in the Murray Valley and Torrumbarry Irrigation Areas
usually involve diverting fish away from channel inlets using physical or behavioural barriers. We
suggest that management and structural options for reducing the number of fish entrained into
irrigation systems needs to be urgently assessed if we are to improve the status of native fish in the
Murray River.
Recommendations
 Investigate the feasibility of screening irrigation channel inlets to reduce or, if possible, prevent
the entrainment of native fish into channels in the Murray Valley and Torrumbarry Irrigation
Areas.
 Assess the potential impact of diversion on native fish in all existing irrigation areas in
Victoria, and establish a prioritised list with potential management options.
 Ensure that any new water diversions thoroughly consider the risks to the riverine fish
community.
 Ensure that future management strategies for the Torrumbarry Irrigation Area incorporate
environmental values. In particular, consideration should be given to providing fish passage
and habitat improvements in the first section of the National Channel and then into Gunbower
Creek, as this would aid in rehabilitating the native fish community in the Creek and in the
wetlands of Gunbower Forest.
 Determine the feasibility of reducing water extractions at night during the peak spawning
months of November and December.
 Conduct annual surveys of fish in both Torrumbarry and Murray Valley Irrigation Areas after
the drawdown at a few key sites, to determine whether there is any substantial inter-annual
variation in catches across different diversion regimes employed in different years. (For
example, during the 2006–07 season drought conditions resulted in reduced diversions into the
irrigation areas.)
 Significant fish refuge sites identified during the winter drawdown period should be targeted
for active fish removal, and the fish should then be returned to a suitable nearby river. This
should be conducted systematically by trained workers so that accurate data is obtained on the
specific locations directly after the drawdown, to determine exact numbers of fish trapped.
 Conduct further monitoring of the densities of drifting eggs and larvae, particularly given that
higher numbers of entrainment may occur during flood years.
 Determine whether the adult golden perch entering the irrigation system are in spawning
condition and how they are attracted and entrained into the diversion channels.
 Investigate the installation of fishways or other systems that could return fish to main river
systems, such as catch-and-transport operations that could be undertaken at the beginning of the
drawdown period.
 Train water management and operational staff on appropriate fish handling and release
techniques for returning fish to source waters.
2
Arthur Rylah Institute for Environmental Research
Fish in the Murray Valley and Torrumbarry Irrigation Areas
1 Introduction
The development of the Australian irrigation industry began over a century ago when
unpredictable and seasonally high flows were first harnessed to secure water for farming. In the
ensuing years, prompted principally by war and drought, the industry slowly developed into the
massive enterprise that it is today, encompassing the most productive agricultural land in Australia
(Hallows and Thompson 1995). The irrigation industry has also played a major role in the
development of rural Australia into the prosperous communities that exist today. However, along
with the undoubted economic and social benefits associated with the development of the irrigation
industry came a cost which was largely at the expense of the natural environment. The
development of the industry brought about massive alterations in the way water is moved around
the countryside, with vast networks of irrigation channels, weirs, pumps and other types of
infrastructure diverting large volumes of water from natural water courses onto dry farming land,
all of which altered the natural riverine environment.
One of the major impacts of the irrigation industry has been the alteration to streamflow.
Streamflow is strongly correlated with many physiochemical components of rivers and is critical
to the ecological functioning of all rivers. The ecological impacts of altering the natural flow
regime are now beginning to be understood (Poff et al. 1997; Lytle and Poff 2004). Evidence that
streamflow plays an important function in the life cycle of many Australian fish species is
becoming increasingly apparent (Humphreys et al. 1999). Streamflow, and the seasonal
fluctuations in streamflow, are continually being associated with, among other things, good water
quality, fish spawning and fish dispersal.
Changes in streamflow timing and magnitude is believed to have had an enormous impact on the
success of fish spawning and dispersal. However, while the impact of flow alteration on fish is
being increasingly investigated there has been comparatively little work on the impact of irrigation
infrastructure on fish. Given that a large proportion of any passing flow is often diverted down
irrigation channel systems and, given that the direction of fish movement is often dictated by flow,
the potential for the diversion of fish with this flow is enormous. King and O’Connor (2007)
indicated that this could be a problem and suggested it warranted further study. The potential for
fish entrainment in irrigation channels is further exacerbated by the natural drifting and migratory
strategies of many native fish species in the Murray–Darling system. Adult golden perch
(Macquaria ambigua) are known to undertake long-distance downstream movements that have
been associated with spawning (O’Connor et al. 2005), while the Murray cod (Maccullochella
peelii peelii), which is a nationally threatened species, is known to undertake long distance
downstream migrations after first moving upstream to spawn (Koehn 2006). Furthermore, the eggs
and larvae of golden perch, silver perch, Murray cod and trout cod are all known to drift passively
downstream (Humphreys and King 2004). Because more flow is often diverted into irrigation
channels than downstream into the natural riverine environment, it is possible that a large
proportion of fish populations migrating downstream may be diverted into the channels system.
Previous studies have identified the presence of native fish species in irrigation channels (King and
O’Connor 2007). Gilligan and Schiller (2003) combined larval density with water extraction
records and suggested that millions of drifting eggs and larvae per year may be removed from
natural systems by all types of water extraction. Koehn and Harrington (2005) reported the capture
of drifting Murray cod larvae in the Murray Valley Irrigation Channel, and suggested that there is
an urgent need to quantify the number of larvae that are lost in such systems. In a literature review
undertaken to investigate fish in irrigation supply offtakes, Baumgartner (2005) indicated that
enough scientific evidence existed to suggest that the effects on fish communities are likely to be
substantial.
Arthur Rylah Institute for Environmental Research
3
Fish in the Murray Valley and Torrumbarry Irrigation Areas
The fate of fish in the channels system is largely unknown, although it is thought that they die
fairly soon after entering the system as a result of moving through the various regulating
structures. Movement downstream through such structures can result in embolism, abrasion, eye
damage and haemorrhaging (Bell and DeLacy 1972). Then, assuming fish can survive all of these
potential hazards, they will then be subjected to the system drawdown at the end of the irrigation
season (when no water is diverted into the irrigation system) and the associated potential to be
stranded on dry ground or caught by birds (or anglers) in shallow pools. Other impacts include
creating barriers to fish movement, enhancing the dispersal of exotic species, and diverting eggs
and larvae through direct pumping and thereby reducing the number of successful recruits in the
natural environment. The mortality rates of eggs and larvae drifting into irrigation channels is also
likely to be high because of the large number of structures and the high water velocities. For
example, Marttin and De Graaf (2002) investigated the effect of a sluice gate on mortality of
drifting fish larvae in an irrigation system in Bangladesh and suggested that 25% of all hatchings
passing the main gates of the first regulator died solely because of this passage.
There have been very few studies investigating the diversity and abundance of fish in irrigation
channels anywhere in the world. However, in the United States and Canada fish entrapment in
irrigation channels has been recognised as a problem since early last century (Prince 1922), and
there have been numerous studies on the diversion of fish away from channels (Clay 1995;
Hadderingh and Bakker 1998; Odeh and Orvis 1998). Yet despite screening diversions being one
of the most common fish management practises in North America, there is a general lack of
understanding of their effectiveness (Moyle and Israel 2005).
In one of the few studies investigating the abundance and diversity of fish species in irrigation
channels, conducted in Sudan, 27 species were found in the channels, closely resembling the
community in the nearby source waters (Coates 1984). Coates (1984) also reported that species
diversity within irrigation systems declined with distance downstream into the irrigation system. In
another study, in the Gezira irrigation system in Sudan, only 19 of the 34 species present in the
source waters (a deficit of 44%) were found in the channel system (Redding and Midlen 1990).
The difference between the natural system and the channel system was attributed to a lower
diversity of instream and riparian habitat in the channels.
In Australia there has been recent interest in the diversion of fish away from the natural riverine
environment into irrigation channels. With native fish populations in many Victorian rivers under
stress, and rivers in the Murray–Darling Basin now severely degraded, there is recognition of an
urgent need to undertake research and instigate management measures to sustain native fish
populations into the future (MDBC 2003). Yet despite the massive amounts of water diverted from
our river systems into irrigation areas, there are no fish protection measures in place (Blackley
2004). However, a workshop held by the Murray–Darling Basin Commission on downstream
movements of fish recognised the need to quantify the extent and significance of fish entrapment
in irrigation systems and to allow effective management solutions to be devised where appropriate
(Lintermans and Phillips 2004).
In one of the first studies of its kind in Australia, King and O’Connor (2007) collated anecdotal
information and conducted a pilot survey on the occurrence of fish in the Goulburn–Murray
irrigation network. They highlighted that the loss of native fish from our severely degraded
riverine environment into irrigation channels is likely to be a substantial problem, particularly for
species such as golden perch and Murray cod that are known to migrate downstream during their
life cycles. However, further information is required to quantify the stages in their life history at
which fish are entering the channels systems, the significance of the losses into the irrigation
systems relative to natural populations, and the impacts on native fish.
4
Arthur Rylah Institute for Environmental Research
Fish in the Murray Valley and Torrumbarry Irrigation Areas
Once fish have entered irrigation systems in Victoria they are effectively lost from the main river
population, although their eventual fate is unknown. The consistent loss of a high number of
individuals is likely to strongly influence the long-term viability of riverine populations.
Although there are many extensive irrigation networks throughout Victoria, the project reported
here focused on the Goulburn–Murray Irrigation network in northern Victoria, and in particular the
Murray Valley and Torrumbarry Irrigation Areas, as these are two of the most important and
largest irrigation systems in Victoria and their source waters are part of the Murray–Darling river
system.
The aim of this project was to provide further information about the losses of fish into the
Torrumbarry and Murray Valley Irrigation Areas, by:
 repeating the 2005 fish surveys of the Murray Valley Irrigation Area to gain valuable temporal
data on the fish species in this system,
 undertaking the first comprehensive fish survey of the Torrumbarry Irrigation Area to establish
the abundance and species diversity of adult and juvenile fish entrained within this system, and
 investigating the entrainment of eggs and larvae into both Murray Valley and Torrumbarry
Irrigation Areas and compare the densities with those in the nearby main river channel.
Arthur Rylah Institute for Environmental Research
5
Fish in the Murray Valley and Torrumbarry Irrigation Areas
2
Methods
2.1 Study Area
The Goulburn–Murray Irrigation network covers 68 000 km2 between the Great Divide and the
Murray River, extending westward to the Loddon River and Swan Hill (Figure 1). The network
manager, Goulburn–Murray Water (GMW), maintains over 7000 km of channels and on average
delivers 2.1 million megalitres of water per year to over 24 750 serviced properties. The GMW
network controls 70% of Victoria’s stored water and is split into six management areas:
Shepparton, Goulburn, Rochester–Campaspe, Pyramid–Boort, Murray Valley and Torrumbarry.
Following preliminary surveys of the Murray Valley Irrigation Area in 2005, follow-up surveys of
this system and a preliminary survey of the Torrumbarry Irrigation Area were undertaken in 2006.
These systems were chosen for investigations because of the size of their channel networks and
associated volume of diverted water, the location of their source waters, and anecdotal evidence of
fish presence.
2.1.1
Murray Valley Irrigation Area — system description
Water impounded by the Murray Valley Weir forms Lake Mulwala (capacity 117 000 ML) on the
Murray River at Murray Valley. Water is supplied to Lake Mulwala via the Murray River from
Hume and Dartmouth reservoirs. Murray Valley Weir elevates the height of the Murray River, and
allows water to be diverted by gravity into two main irrigation channels. The Mulwala channel
(capacity 10 000 ML/day) is managed by Murray Irrigation Limited and diverts water to the
Berriquin, Denimein, Deniboota and Wakool Irrigation Districts in southern New South Wales,
which have a total serviceable area of 700 000 hectares. The Murray Valley Channel, (capacity
3100 ML/day) services the Murray Valley (Murray Valley) Irrigation Area (128 000 hectares),
extending from Murray Valley to Barmah and south to the Broken and Nine Mile Creek systems in
Victoria (Figure 1).
Yarrawonga
Weir
River
Torrumbarry
Weir
Source waters of
irrigation areas
investigated in
this study
0
5
10
Scale
Figure 1. Goulburn–Murray Water Irrigation Network.
6
Arthur Rylah Institute for Environmental Research
N
Fish in the Murray Valley and Torrumbarry Irrigation Areas
The percentage of total flow diverted down the Murray Valley Channel varies between and among
years (Figure 2a), however it is generally between 10 and 20%. In addition to the Murray Valley
Channel, water is also diverted down the Mulwala Channel and when total diversions from Lake
Mulwala are considered together then the percentage diversion increases substantially (Figure 2b)
and is generally around 30–40 % but can be often much higher than this. During the non-irrigation
period the water drains completely from most of the channels in this system, except for deeper
areas surrounding structures such as bridges, culverts and weirs, which may act as refuge areas for
fish during this period. There are many such structures in the Murray Valley Irrigation Area,
including more than 1422 culverts and 163 bridges (Table 1).
In the previous study in the Murray Valley Irrigation Area, 35 sites were sampled between May
and June during the 2005 drawdown; see King and O’Connor (2005) for further details. In 2006,
29 of these sites (Figure 3) were resampled over two weeks in May and June, enabling
comparisons to be made between the two years. The GMW irrigation network comprises a
labyrinth of channels regulated and traversed by a series of structures such as weirs, bridges,
culverts and syphons. Although depth is more or less uniform in the channels, scour pools often
form downstream of regulating structures and can be much deeper than the channels themselves.
04/05
04/05
05/06
a)
80
06/07
07/08
60
% Diversion
40
20
0
100
04/05
b)
05/06
80
06/07
07/08
60
40
20
0
Month
Figure 2. Percentage flow diversion down (a) Murray Valley Channel and (b) combined Murray Valley and
Mulwala Channels between 2004 and December 2007.
Arthur Rylah Institute for Environmental Research
7
Fish in the Murray Valley and Torrumbarry Irrigation Areas
Figure 3. Sampling sites located in the Murray Valley Irrigation Area.
After the drawdown the channels contain little or no water, but large deep pools can remain around
structures, and fish that remain in the system are likely to congregate in these deeper refuge areas.
Because of this, sampling sites were generally located around structures where scour holes had
8
Arthur Rylah Institute for Environmental Research
Fish in the Murray Valley and Torrumbarry Irrigation Areas
formed. Most sites consisted of muddy, still water with a maximum depth of approximately 2
metres.
A single site in the Murray Valley Irrigation Area was sampled twice in 2006 (Site 2). This site,
below a weir on the Murray Valley Main Channel, was sampled on 16 May immediately following
the drawdown, and again on 7 June when water levels had dropped substantially. The follow-up
sampling was undertaken to investigate the fate of a large number of young-of-year Murray cod
(0+ year old fish) that had been collected on the first sampling occasion.
Table 1. Type and number of structures present in the Torrumbarry and Murray Valley Irrigation Areas that
could act as refuges for fish after drawdown.
Irrigation Area
Structure Type
Bridge
Torrumbarry
541
Culvert
163
1417
Farm irrigation crossing
Murray Valley
1422
39
10
Offtake (regulator)
137
244
Regulator
591
542
Regulator combine (regulator
and culvert)
318
388
Subway (pipe under channel)
312
223
68
49
2
9
Syphon
Weir
2.1.2
Torrumbarry Irrigation Area — system description
Torrumbarry Weir, on the Murray River downstream of Echuca, has a capacity of 35 000 ML and
diverts water to the Torrumbarry Irrigation Area via the National Channel. The Torrumbarry
Irrigation Area delivers around 500 000 ML of water each year to about 120 000 hectares of
irrigated land around Cohuna, Kerang, Swan Hill and other areas. The National Channel system
has a total capacity of 3600 ML/day that is diverted into three main systems after exiting
Torrumbarry Weir. A further 800 ML is taken by private diversions downstream of Torrumbarry
Weir. The percentage of total flow diverted down the Torrumbarry (National) Channel varies
within and between years, but it is generally between 25 and 30% (Figure 4).
Much of the National Channel immediately downstream of Torrumbarry Weir offtake includes old
watercourses such as backwaters and lagoons (including Gunbower Creek), which hold water
during the non-irrigation season and might therefore be fish refuge areas.
The Torrumbarry Irrigation Area also has many natural waterbodies such as wetlands and lakes
with significant environmental and cultural values, unlike the Murray Valley Irrigation Area which
is fed entirely by artificial channels. The Torrumbarry system, like other systems, also has many
potential refuge areas around structures such as culverts (Table 1). The Torrumbarry system can
also be used to deliver water into the Murray River downstream of Swan Hill, so that unlike the
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Fish in the Murray Valley and Torrumbarry Irrigation Areas
Murray Valley Irrigation Area it can return water to the Murray River, along with any fish that
survive the many structures and benign habitat conditions along the way.
Until this study, no targeted fish surveys had been conducted in the Torrumbarry Irrigation Area.
To allow comparisons, a similar number of sites to that sampled at Murray Valley Irrigation Area
were also sampled in the Torrumbarry Irrigation Area using the same techniques. Thirty sites were
selected in early May 2006 and included some of the deeper natural waterbodies in the system that
were thought to be refuge areas for fish (Figure 5). Sampling was conducted over three weeks
between May and June 2006.
04/05
% Diversion
05/06
80
06/07
07/08
60
40
20
0
Figure 4. Percentage flow diversion down Torrumbarry (National) Channel between 2004 and December
2007.
2.2 Fish sampling
2.2.1
Electrofishing
All sites were sampled by electrofishing, in which fish are stunned by a controlled electric current.
Larger and deeper sites were sampled using a boat-mounted electrofishing unit (Smith-Root
model 2.5 GPP) and smaller channels (secondary and tertiary) were collected using a bankmounted electrofishing unit (Smith-Root model 7.5 GPP). Operating voltages, frequencies and
electrical currents varied between sites, depending upon water electrical conductivity and water
temperature, but were generally 1000 volts, 120 hertz and 0.5–3.0 amps.
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Fish in the Murray Valley and Torrumbarry Irrigation Areas
N
Figure 5. Sampling sites located in the Torrumbarry Irrigation Area.
Arthur Rylah Institute for Environmental Research
11
Fish in the Murray Valley and Torrumbarry Irrigation Areas
At each site the area to be sampled (usually located around a structure such as a bridge, culvert or
weir as this is where sufficient depth of water remained after the drawdown for fish to congregate)
was sampled using a single electrofishing pass. When bank-mounted electrofishing was used, two
nets (90 mm mesh) were set at the upstream and downstream end of the structure (i.e. culverts)
prior to fishing, acting as block nets. Fishing was then conducted from the downstream end in an
upstream direction. When the operator reached the upstream culvert the anode was extended into
the culvert to stun or scatter any fish in the middle of the culvert. Similarly, when sampling the
larger and deeper sites with the electrofishing boat, 90 mm mesh nets were set at each end of the
section (and also in the middle of the longer sections) to capture fish that may have been scattered
by the boat electrofisher.
After capture all fish were identified, counted, measured, examined for external tags and released
back into their capture location. If a large number of a species were collected, a subsample of 20
fish of that species was processed. Counts were also made of fish observed and positively
identified but not captured. The length to caudal fork (LCF) or the total length (TL) was measured
to the nearest millimetre. Because GMW staff thought that most of the refuge areas around
structures would hold water over the drawdown period, native fish over 300 mm in length
collected from these habitats were dart-tagged between the second and third dorsal spines and
released back into their capture location. Tagging of native fish had previously been undertaken in
similar surveys completed in the Murray Valley Irrigation Area in 2005 and was intended to
provided useful information on the movement and fate of these fish if recaptured at a later date.
Electrofishing data analysis
Sampling effort was standardised to CPUE (Catch Per Unit Effort) and was calculated as fish
collected per minute of electrofishing time, including fish collected by both electrofishing and
netting. Fish abundance data collected using boat-mounted electrofishing was separated from data
collected using bank-mounted electrofishing, as it was not possible to standardise the data
collected using these two quite different sampling methods. All standardised data was log(x + 1)
transformed.
2.2.2
Pumpouts
In addition to the targeted electrofishing surveys, fish data was also collected from three pumpouts
undertaken in the Murray Valley Irrigation Area using a 75 mm diameter pump. Three sites in the
Murray Valley Irrigation Area that had previously been sampled using our standard protocol in
both 2005 and 2006 were pumped out so that water in the scour pools was removed. A 75 mm
diameter pump powered by a diesel motor was used to remove most of the water (Figure 6), and
sand-bagging was used at some sites to stop water draining back into the area that was being
pumped out (Figure 7). The pump inlet had a filter attached to reduce the potential for removing
small fish. When no more water could be removed using the pump, larger fish were collected by
dip-netting, and a backpack electrofishing unit was used to stun smaller fish that were difficult to
net in the dirty water. Since all larger fish (> 100 mm TL) were certainly emoved using this
technique, a reasonably accurate estimate of larger fish diversity and abundance coulkd be
obtained from the refuge area. However, some smaller fish such as carp gudgeons (Hypseleotris
spp.) and Australian smelt (Retropinna semoni) would not have been completely removed. After
being examined and measured, native fish were kept in aerated plastic containers, and were not
released back into the refuge area until the pumpout was completed and water had been pumped
back into the area.
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Figure 6. Pumpout at Grinters Road (Site 15), where 27 young-of-year Murray cod were collected from
below this structure.
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Fish in the Murray Valley and Torrumbarry Irrigation Areas
2.2.3
Murray cod ageing
Eight small Murray cod were collected from the Murray Valley Irrigation Area and aged using
otoliths. Sagittal otoliths were removed with the aid of a stereo microscope and mounted in
thermoplastic cement. Transverse sections of the otoliths were then made by polishing to the level
of the primordium using 6 m lapping film and 0.5 m aluminium oxide slurry. Sections were
viewed firstly using a stereo microscope (10–100magnification) to identify any annual
increments. If there were no annual increments, otolith microstructure was examined under a
compound microscope (100–1000magnification) to identify daily increments. A photograph of
each otolith was taken under reflected light (stereo) and transmitted light (compound) for image
analysis using ImagePro Express (version 5.0.1.26, Media Cybernetics).
Figure 7. Pumpout at Boothroyds Road (Site 22), Murray Valley Irrigation Area.
2.3 Egg and larva sampling
Fortnightly sampling for drifting eggs and larvae was conducted in both Torrumbarry and Murray
Valley Irrigation Areas in 2006. Sampling was targeted at key species known to have drifting early
life stages, including Murray cod, trout cod (Maccullochelal macquariensis), silver perch
(Bidyanus bidyanus) and golden perch. The sampling was timed to coincide with the known peak
spawning months of November and December.
Sampling for drifting fish eggs and larvae was undertaken using standard passive drift nets (Figure
8) at a pair of sites in each system — one in the channel and the other in the river system upstream
of the channel offtake (Table 2). This allowed the number of eggs and larvae being diverted down
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Fish in the Murray Valley and Torrumbarry Irrigation Areas
the channel system to be measured and compared to the number of eggs and larvae in the natural
riverine environment. Because the larval drift nets require reasonable flow to function correctly,
they were usually placed some distance upstream of the channel offtakes where the water velocity
had not decreased as a result of the impoundment downstream.
Larvae were sampled at night on four separate occasions, commencing on 30 October and then at
fortnightly intervals until 13 December. Larval nets were 1.5 m long, 500 m mesh passive drift
nets with a 0.5 m diameter mouth opening tapering to a removable collection jar. A General
Oceanics flow meter was fixed in the mouth of each drift net to determine the volume of water
filtered, thus enabling raw catch data to be standardised among all nets to the number of eggs per
1000 m3 of water filtered. At all sites other than the riverine site at Torrumbarry, three nets were
deployed across the channel: left-hand bank (LHB), middle (MID) and right-hand bank (RHB).
Figure 8. Standard passive drift net used in the larval study.
Table 2. Location of drift net sampling sites.
Irrigation System
Site
Latitude & Longitude
36°01.400 S
145°58.670 E
Distance from
channel offtake
Murray Valley Channel
Channel at Reillys Rd
Murray River upstream
of Lake Mulwala
Murray River at Brimin
Rd
36°01.274 S
146°15.729 E
14.5 km upstream
Torrumbarry Irrigation
Area
National Channel
35°59.570 S
144°30.270 E
850 m downstream
Murray River upstream
of Torrumbarry Weir
Murray River at Farley
Bend
36°02.147 S
144°36.976 E
19 km upstream
Arthur Rylah Institute for Environmental Research
3.15 km downstream
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Fish in the Murray Valley and Torrumbarry Irrigation Areas
Because of desnagging and large numbers of water craft in the area, only the LHB and RHB nets
were deployed at the Torrumbarry riverine site. All nets were attached to a structure such as a
bridge or snag (Figure 9), set on dusk and retrieved as early as possible the following morning,
generally before 11 am. Channel and riverine sites associated with the same irrigation system were
sampled on the same night.
Because of the similar appearance of eggs of species such as golden perch and silver perch,
samples were sorted and any eggs were removed alive and returned to the laboratory to hatch and
identify. The rest of the sample was preserved in 95% ethanol in the field and taken to the
laboratory for further processing. In the laboratory, fish larvae were removed from the samples
under a dissecting microscope and identified using keys (Serafini and Humphries 2004) and by
comparison to the ARI larval fish reference collection. Data for eggs and larval catches were
adjusted to a standard volume of water filtered (number of eggs or larvae per 1000 m3), and the
data was then pooled across net position (LHB, MID and RHB) and averaged.
Differences between channel and riverine sites, and between regions, were analysed using Mann–
Whiney tests of log(x + 1) transformed total raw and standardised numbers of eggs and larvae of
all species to identify pair-wise differences.
Figure 9. Drift net set from an overpass on the Murray Valley irrigation channel.
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Fish in the Murray Valley and Torrumbarry Irrigation Areas
2.4 Modelling the impact of larval loss on a Murray cod population
The impact of larval Murray cod loss into the irrigation channels was determined using a matrix
population model. The model structure used to capture the life cycle of Murray cod has four stages
and eleven ages: eggs; larvae; juveniles (0, 1, 2, 3, 4 year olds); and adults (5, 6, 7, 8, 9, 10+ year
olds). Fish older than ten years are not identified by age, but are aggregated into the final age class.
Sexual maturity in the model occurs at five years of age, in accordance with Rowland (1998a), and
egg production increases with age (Rowland 1998b; Todd et al. 2004, 2005).
Variation in the survival and reproduction of individuals was modelled by demographic
stochasticity (Akçakaya 1991). Demographic stochasticity was incorporated using a binomial
distribution to model the number of individuals surviving between consecutive time steps, and a
Poisson distribution to model recruitment to one-year-olds. Environmental stochasticity was
incorporated by randomly varying the survival and fecundity rates each year. Survival rates were
drawn from normal distributions transformed to the unit interval (Todd and Ng 2001) with
specified means and standard deviations. Age-specific fecundities were drawn from log-normal
distributions with specified means and standard deviations. Todd and Ng (2001) provide a
methodology for specifying correlations among survival rates. Although no information exists to
quantify these correlations it is reasonable, given the aquatic habitat of fish, to assume that the
correlations are likely to be positive and close to unity. Survival rates were assumed to be perfectly
correlated to each other and independent of fecundity rates, fecundity rates were assumed to be
perfectly correlated with each other (Todd et al 2004, 2005), and a pre-breeding census
construction was used (Burgman et al. 1993; Caswell 2001).
The impact of disruption to the life cycle of Murray cod can be readily examined using the model
description above. It is also important to recognise that all Murray cod populations are exploited as
recreational fisheries and that the population structure determines the potential size of the larval
stage or ‘population’. Anything that affects population structure, such as fishing, will also affect
the larval ‘population’.
2.4.1
Scenarios
A Murray cod population was modelled with a maximum number of female adults of 2000. Two
fishing scenarios were considered:
 no fishing

fishing rates as measured below Murray Valley (depending on size class, up to 35% of fish are
removed annually (Todd, unpublished data held at ARI))
For each scenario, three different levels of larval loss from a Murray cod population were
considered:
 no larval loss
 50% larval loss
 80% larval loss.
2.4.2
Model output
Running the model 1000 times produces 1000 different trajectories. Collecting the minimum
population size from each trajectory is a typical method used to express risk (Todd et al., 2004,
2005) (Figure 10). A cumulative distribution of minimum population sizes, produced by
normalising the frequency of minimum population sizes, is known as the risk curve (Figure 11).
Risk curves can be easily analysed to rank different management actions as well as examining
management actions for their efficacy (Figure 11; added risk and reduced risk).
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Fish in the Murray Valley and Torrumbarry Irrigation Areas
Probability of population below TPS
(risk of quasl extinction)
Figure 10. An example of some trajectories produced from a stochastic population model for Murray cod.
Pale blue circles indicate the minimum population size from each trajectory.
1.0
0.8
0.6
0.4
0.2
0.0
0
200
400
600
800
1000
Threshold population size
Figure 11. Examples of different risk curves (cumulative distribution of minimum population sizes) under
different scenarios identifying the concepts of added risk.
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Fish in the Murray Valley and Torrumbarry Irrigation Areas
2.5 Water quality
Water temperature (°C), electrical conductivity (µS/cm), dissolved oxygen (mg/L), turbidity
(NTU) and pH were measured in situ using a TPS FL90 water quality meter at most sites during
each sampling event, in both adult and larval surveys. These parameters were measured to ensure
that water quality was within tolerable ranges for fish.
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Fish in the Murray Valley and Torrumbarry Irrigation Areas
3 Results
3.1 Water quality — Murray Valley and Torrumbarry Irrigation Areas
Water quality was generally within the normal range expected in north-eastern Victoria except at
one site, at Boothroyds Road in the Murray Valley Irrigation Area, where water electrical
conductivity was 1890 µS/cm. Turbidity at this site was low (5.9 NTU) and the water was flowing
— conditions that were unlike any of the other sites in the Murray Valley Irrigation Area on the
survey week of 7 June (i.e. usually dirty and still water). The ranges of other water quality
parameters overall were:
 dissolved oxygen 49.5–180%
 electrical conductivity 45–201 µS/cm (excluding Boothroyds Rd site)
pH 7.18–9.11
 turbidity 4.3–415 NTU.
For more detailed water quality information for the Murray Valley Irrigation Area, see
Appendix 1. Water quality in the Torrumbarry Irrigation Area during 2006 was also generally
within the range expected in north-eastern Victoria, including dissolved oxygen (range 115–
169%), electrical conductivity (59.5–2366 µS/cm), pH (7.4–8.38) and turbidity (17.2–399 NTU).
For more detailed water quality information for the Torrumbarry Irrigation Area, see Appendix 2.
3.2 Adult fish surveys — Murray Valley Irrigation Area 2006
3.2.1
General
A total of 5694 native and introduced fish were collected from the 29 sites surveyed in the Murray
Valley Irrigation Area. Of these, 4149 were native fish representing nine species and 1546
introduced fish representing five species (Table 3). CPUE indicated that the most abundant native
fish species, and the most abundant fish species overall, was the unspecked hardyhead
Craterocephalus stercusmuscarum fulvus. The least abundant fish species included six species that
each contributed less than 1% of the total abundance of fish (Figure 12). Threatened native fish
species collected included Murray cod, unspecked hardyhead, golden perch and silver perch
Bidyanus bidyanus.
3.2.2
Recaptures
One Murray cod that was captured had been collected and tagged in the previous year. The fish
was collected on both occasions at Boothroyds Road, which suggests that it was resident in that
location. This fish had grown in length from 344 mm to 484 mm, and in weight from 0.5 kg to 1.6
kg, since it was tagged.
3.2.3
Length range
The length range of two of the larger native species present in the Murray Valley Irrigation Area,
Murray cod and golden perch, indicated that the Murray cod were mainly juveniles but the golden
perch were all adults (Figure 13). In contrast, the length range of the unspecked hardyhead
appeared to be more evenly spread with the most common length class between 20–30 mm (Figure
14). There was insufficient length data of other threatened species to make further comparisons.
3.2.4
Fish distribution
CPUE data indicated that the abundance of native fish appeared to decline with increasing distance
downstream of a site from its source waters at Lake Mulwala (Figure 15). However, regression
analysis indicated this relationship was not significant (p > 0.05). And although there was a peak
in numbers around 20 kilometres downstream, it appears that this was a result of a large number of
unspecked hardyhead being collected.
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Fish in the Murray Valley and Torrumbarry Irrigation Areas
Table 3. Total raw abundance data of native and introduced fish species found during surveys of the Murray
Valley Irrigation Area in 2006.
Scientific name
Common name
Cons. status1
Natl
Gadopsis marmoratus
river blackfish
Maccullochella peelii peelii
Murray cod
Retropinna semoni
n
Mean length
(mm) (range)
Vict.
median
11
121 (74–193)
119
129
138 (76–668)
127
Australian smelt
1426
45 (25–57)
46
Hypseleotris spp.
carp gudgeons
186
35 (21–70)
32
Philypnodon grandiceps
flat-headed gudgeon
59
47 (30–74)
44
Macquaria ambigua
golden perch
V
15
444 (400–525)
430
Craterocephalus
unspecked hardyhead
DD
2322
27 (18–48)
24
Bidyanus bidyanus
silver perch
CE
1
Gambusia holbrookii*
gambusia
179
27 (18–48)
26
Cyprinus carpio*
carp
467
413 (145–600)
483
Perca fluviatilis*
redfin perch
49
254 (136–332)
284
Misgurnus anguillicaudatus*
oriental weatherloach
Carasius auratus*
goldfish
V
E
stercusmuscarum fulvus
Total
1
849
394
n/a
106 (55–165)
114
5694
Conservation status: National — under Commonwealth Environment Protection and Biodiversity Conservation
Act 1999; Victorian — under Flora and Fauna Guarantee Act 1988 and DSE (2005). CE = critically
endangered, DD = data deficient, E = endangered, V= vulnerable. * introduced species
3.2.4
Murray cod ageing
The sample of eight small Murray cod (97–172 mm total length) from the Murray Valley Irrigation
Area aged using otoliths, were found to be 0+ year old fish, indicating they were all recruited from
the most recent spawning season in spring 2005.
3.2.5
2006 Murray Valley pumpouts
A total of 2359 fish encompassing six native and four introduced species were collected from the
three pumpout sites (Table 4). The most abundant native fish species and the most abundant fish
species overall was Australian smelt, while the least abundant fish species were the introduced
gambusia and redfin. The difference in fish diversity between pumpout samples and electrofishing
surveys varied between sites and species (Table 4). At Lorenzos Road all species collected during
the electrofishing were also collected during the pumpout. At Grinters Road, all species collected
during the electrofishing surveys were also collected during the pumpout, except that a small
number of gambusias were collected only during the pumpout. Furthermore, at Grinters Road there
was also a decrease in carp and goldfish abundances between the electrofishing surveys and the
pumpout.
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Fish in the Murray Valley and Torrumbarry Irrigation Areas
Percentage species composition
60
50
40
30
20
10
0
Figure 12. Percentage species composition of CPUE of fish in the 2006 Murray Valley Irrigation Area fish
survey.
100
Percentage of Individuals
90
Murray cod
golden perch
80
70
60
50
40
30
20
10
650-699
600-649
550-599
500-549
450-499
400-449
350-399
300-349
250-299
200-249
150-199
100-149
0-49
50-99
0
Length range (mm)
Figure 13. Length range of Murray cod (n = 105) and golden perch (n=7) collected from the Murray Valley
Irrigation Area in 2006.
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Arthur Rylah Institute for Environmental Research
Fish in the Murray Valley and Torrumbarry Irrigation Areas
80
Percentage of Individuals
70
60
50
40
30
20
10
60-70
50-60
40-50
30-40
20-30
10-20
0
Length range (mm)
Figure 14. Length range of unspecked hardyhead collected from the Murray Valley Irrigation Area in 2006
(n = 100).
The greatest discrepancies between the diversity of fish collected in the electrofishing surveys and
the pumpout was at the Boothroyds Road site, where redfin, flat-headed gudgeon and silver perch
were all collected during electrofishing but not during the pumpout. However, given the small
numbers of these species collected during the electrofishing surveys, their absence in the pumpouts
was not surprising. Additionally, at the Boothroyd Road site, electrofishing failed to collect any
golden perch, but 10 were collected in the pumpout.
The length of Murray cod collected from the Grinters Road pumpout (mean 121 mm, median 124
mm, range 79–183 mm) was similar to the overall electrofishing data, but Murray cod from the
Boothroyds Road pumpout (mean 548 mm, median 503 mm, range 436–750 mm) were longer
than those captured by electrofishing. Like those at the electrofishing sites, the lengths of golden
perch collected in the pumpouts indicated they were large adult fish (mean 465 mm, median 442
mm, range 403–566 mm). Insufficient length data were available on other threatened species to
allow a comparison.
When the raw abundance electrofishing data was compared with the raw abundance pumpout
data, electrofishing collected between about 1% and 10% of what was actually present in the sites
(as assessed from pumpouts). However, there were a few discrepancies to this rule. For example,
at Lorenzos Road electrofishing collected 43 gambusia but the pumpout collected only 10, and at
Grinters Road 62 carp and 30 goldfish were collected by electrofishing but only one of each
species was collected during the pumpout.
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Fish in the Murray Valley and Torrumbarry Irrigation Areas
160
All fish
120
80
40
0
160
Abundance
Introduced fish
120
80
40
0
160
Native fish
120
80
40
100
90
80
70
60
50
40
30
20
10
0
Distance downstream from
source (km)
Figure 15. CPUE of all, native and introduced fish with distance from source waters downstream of the
Murray Valley Channel system.
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Fish in the Murray Valley and Torrumbarry Irrigation Areas
Table 4. Comparison of raw abundance between Murray Valley electrofishing and pumpout sampling
methodologies.
Species
Murray cod
Boothroyds Rd
Grinters Rd
E’fishing
E’fishing
Pumpout
Lorenzos Rd
Pumpout
E’fishing
Pumpout
3
5
8
27
0
0
31
69
29
358
65
560
Silver perch
1
0
0
0
0
0
Carp gudgeons
6
56
6
347
34
96
Flat-headed
gudgeon
1
0
3
40
0
0
Golden perch
0
10
0
0
3
10
Unspecked
hardyhead
0
0
3
68
0
0
Gambusia*
0
0
0
4
43
10
16
190
62
1
79
308
2
0
0
4
18
50
71
30
1
74
110
110
401
141
846
302
1112
Australian smelt
Carp*
Redfin perch*
Goldfish*
Total
* introduced species
3.2.6
Young-of-year Murray cod
Site 2 on the Murray Valley Irrigation Area was sampled using bank-mounted electrofishing on 16
May 2006, immediately after the drawdown had begun, and a total of 39 juvenile Murray cod were
collected (mean length = 147 mm; range 91–182 mm). This site was resampled again on 7 June
when the water level had decreased substantially, and no juvenile Murray cod were collected.
3.2.7
2004–05 vs 2005–06 Murray Valley Irrigation Area comparison
The timing and magnitude of diversions into the Murray Valley Irrigation Area varied between the
2004–05 and 2005–06 irrigation seasons (Figure 16). For many months in both seasons the timing
and magnitude of releases were similar, but compared to the 2005–06 season smaller volumes of
water were diverted in 2004–05, particularly between December and February.
A comparison of CPUE data from this survey with an identical survey of the Murray Valley
Irrigation Area undertaken in May and June 2005 (King and O’Connor 2006) indicates that there
were some changes in the fish community (Figure 17). In 2006 unspecked hardyhead comprised
41% of the total catch, but none were captured in 2005. In contrast there were large reductions in
the percentage of Australian smelt sampled in 2006 (25%) compared to 2005 (45%), and in the
percentage of carp gudgeon, which decreased from 23% of the total catch in 2005 to 3% of the
total catch in 2006. Oriental weatherloach were collected from a single site in 2005, and were also
collected only from the same site in 2006.
Arthur Rylah Institute for Environmental Research
25
Fish in the Murray Valley and Torrumbarry Irrigation Areas
2004-05
2005-06
3000
2500
2000
1500
1000
May
April
March
February
January
December
November
October
0
September
500
August
Diversion (ML/day)
3500
Month
Figure 16. Timing and magnitude of diversions into the Murray Valley Irrigation Area during the 2004–05
and 2005–06 irrigation seasons.
Percentage species composition
60
50
2005
2006
40
30
20
10
0
Figure 17. Percentage species composition of CPUE of fish in the Murray Valley Irrigation Area between
2005 and 2006.
26
Arthur Rylah Institute for Environmental Research
Fish in the Murray Valley and Torrumbarry Irrigation Areas
There were approximately 16 native fish/site and 11 introduced fish/site sampled in both years of
this study, indicating that there was no substantial change in the total CPUE between years.
However, an analysis of the CPUE abundance of individual species collected by boat-mounted
electrofishing indicated that there were significantly more Murray cod, Australian smelt and
unspecked hardyhead collected in 2006 than in 2005 (paired t test, p < 0.05). The data from bankmounted electrofishing also indicated that there were significantly more Australian smelt collected
in 2006 (paired t test, p < 0.05).
3.3 Adult Fish Surveys — Torrumbarry Irrigation Area 2006
3.3.1
General fish sampling
A total of 8418 native and introduced fish were collected in the 2006 study from 30 sampling sites.
This included 6011 native fish from nine species and 2401 introduced fish from three species
(Table 5). CPUE data indicated that the fish community was dominated by the native Australian
smelt and the introduced goldfish (Figure 18). The least abundant fish species collected include the
four native species silver perch, Murray cod, river blackfish and bony bream each of which made
up less than 1% of total fish abundance (Figure 18). Threatened native fish species collected
included Murray cod, Murray-Darling rainbowfish Melanotaenia fluviatilis, unspecked hardyhead,
golden perch and silver perch.
Table 5. Total raw abundance data of native and introduced fish species found during surveys of the
Torrumbarry Irrigation Area in 2006.
Scientific name
Common name
Cons. status1
Natl
n
Mean length
(mm) (range)
Vict.
Melanotaenia fluviatilis
Murray-Darling
rainbowfish
Maccullochella peelii peelii
Murray cod
Retropinna semoni
Australian smelt
Hypseleotris spp.
carp gudgeon
Philypnodon grandiceps
flat headed gudgeon
Macquaria ambigua
golden perch
V
Craterocephalus
stercusmuscarum fulvus
unspecked
hardyhead
DD
483
Bidyanus bidyanus
silver perch
CE
1
Nematalosa erebi
bony bream
Cyprinus carpio*
DD
V
E
median
518
121 (74–193)
45
23
255 (100–540)
148
4747
48 (37–68)
45
86
35 (31–39)
33
142
48 (31–82)
47
382 (68–590)
371
33 (25–51)
31
6
218
11
161 (68–205)
190
carp
300
394 (61–612)
432
Perca fluviatilis*
redfin perch
173
322 (217–405)
348
Carasius auratus*
goldfish
1928
177 (66–252)
187
Total
8418
Conservation status: National — under Commonwealth Environment Protection and Biodiversity Conservation
Act 1999; Victorian — under Flora and Fauna Guarantee Act 1988 and DSE (2005). CE = critically
endangered, DD = data deficient, E = endangered, V= vulnerable. * introduced species
Arthur Rylah Institute for Environmental Research
27
Fish in the Murray Valley and Torrumbarry Irrigation Areas
Percentage species composition
60
50
40
30
20
10
0
Figure 18. Percentage species composition of CPUE of fish in the 2006 Torrumbarry fish survey.
3.3.2
Recaptures
Only one previously tagged Murray cod was collected, from below a weir just downstream of the
Kangaroo Lake outlet. It had a length of 437 mm and a weight of 392 g. The fish had been
released by Fisheries Victoria 16 months earlier into Kangaroo Lake and weighed 218 g when
released.
3.3.3
Fish distribution
The CPUE abundance of native fish varied with distance from the Murray River source waters
(Figure 19). For example, a CPUE of over 120 native fish were collected at one site 120 km from
the Murray River source waters. This is likely to be due to the numerous lakes in the Torrumbarry
Irrigation Area, which would act as source waters for fish for the channels system.
3.3.4
Length range
As in the Murray Valley Irrigation Area, the Murray cod collected from the Torrumbarry Irrigation
Area were generally small juvenile fish while golden perch were generally large adult fish (Figure
20). Although the Murray cod from Torrumbarry Irrigation Area were generally small, they were
significantly larger than those collected from the Murray Valley IA (paired t test, p < 0.05).
2006 Murray Valley and Torrumbarry Irrigation Area comparisons
Diversity
Eight species of native fish were collected from 29 sites in the Murray Valley Irrigation Area in
2006, while nine species of native fish were collected from 30 sites in the Torrumbarry Irrigation
Area. However, there were considerable differences in the species composition and abundance
between the two areas (Figure 21).
28
Arthur Rylah Institute for Environmental Research
Fish in the Murray Valley and Torrumbarry Irrigation Areas
160
Total fish
120
80
40
0
160
Abundance
Introduced fish
120
80
40
0
160
Native fish
120
80
40
0
140
130
120
110
90
100
80
70
60
50
40
30
20
10
Distance downstream from
source (km)
Figure 19. CPUE of all native and introduced fish with distance from source waters downstream of the
Torrumbarry Irrigation Area.
River blackfish Gadopsis marmoratus, a native species, was collected from the Murray Valley
Irrigation Area but not from the Torrumbarry Irrigation Area, and Murray–Darling rainbowfish
and bony bream Nematalosa erebi were collected from the Torrumbarry Irrigation Area but not
from the Murray Valley Irrigation Area. Five species of introduced fish were collected from the
Murray Valley Irrigation Area in 2006, but only three of these were collected from Torrumbarry
Irrigation Area (Figure 21). The ‘missing’ species were gambusia and oriental weatherloach.
Abundance
CPUE data indicated that, in general, slightly more native and introduced fish were collected from
Torrumbarry Irrigation Area compared to the Murray Valley Irrigation Area in 2006. There were
also some significant differences in the abundance of individual species collected with the boatmounted electrofisher, including significantly more Murray cod and unspecked hardyhead from
the Murray Valley Irrigation Area (ANOVA, p < 0.05) and carp from the Torrumbarry Irrigation
Area (ANOVA, p < 0.05).
Arthur Rylah Institute for Environmental Research
29
Fish in the Murray Valley and Torrumbarry Irrigation Areas
Percentage of individuals
100
90
Murray cod
golden perch
80
70
60
50
40
30
20
10
0
600-649
550-599
500-549
450-499
400-449
350-399
300-349
250-299
200-249
150-199
100-149
50-99
0-49
Length (mm)
Figure 20. Lengths of Murray cod (n = 21) and golden perch (n = 6) collected from the Torrumbarry
Irrigation Area in 2006.
Percentage species composition
60
50
Yarrawonga Irrigation Area
Torrumbarry Irrigation Area
40
30
20
10
0
Figure 21. Comparison of percentage species composition of CPUE of fish in the Murray Valley and
Torrumbarry Irrigation areas in 2006.
30
Arthur Rylah Institute for Environmental Research
Fish in the Murray Valley and Torrumbarry Irrigation Areas
Significantly more gambusia were collected from the Murray Valley Irrigation Area using bankmounted electrofishing (ANOVA, p < 0.05) but, in contrast to the boat electrofishing data, this
data indicated that there were more carp in the Murray Valley Irrigation Area (ANOVA, p < 0.05).
The abundance data collected using the bank-mounted electrofisher also indicated that there were
significantly more redfin collected from the Torrumbarry Irrigation Area, and significantly more
Hypseleotris species collected from the Murray Valley Irrigation Area.
3.4 Egg and larval sampling
3.4.1
Water Quality
Water temperatures measured during the larval surveys were similar in the Torrumbarry and
Murray Valley Irrigation Areas, as well as in corresponding river and channel habitats in each of
these systems (Figure 22).
Figure 22. Water temperatures of channel and river habitats in Murray Valley and Torrumbarry Irrigation
Areas during larval sampling in 2006.
Arthur Rylah Institute for Environmental Research
31
Fish in the Murray Valley and Torrumbarry Irrigation Areas
3.4.2
Drift samples
A total of 1323 larvae and 116 eggs from 11 species (including seven native species) were
captured in the larval study, of which 1197 larvae and 72 eggs from nine species (seven native)
were captured drifting in the channel systems. Raw abundances of eggs and larvae collected in
irrigation channels were dominated by flat-headed gudgeons, which represented 27% and 91% of
the total larvae collected from the Torrumbarry and Murray Valley Irrigation Areas respectively
(Figure 23). The river samples from the Torrumbarry Irrigation Area were dominated by golden
perch (39%) and Murray cod (26%), while carp made up only 3% of the sample. In the Murray
Valley Irrigation Area the river sample was predominantly Murray cod (20%) and silver perch
(16%), with less than 1% of an unidentified Galaxias species collected.
Larvae and eggs of threatened native species (EPBC 1999, DSE 2005) were also detected drifting
in both channel systems. These include Murray cod, which was detected in both channel systems,
and silver perch, which was found only in the Torrumbarry Irrigation Area. In the Murray Valley
Irrigation Area, other threatened Victorian species including silver perch, golden perch, and river
blackfish, were also collected.
Torrumbary
Unidentified
12%
Goldfish
1%
Channel
Yarrawonga
Carp
<1%
Australian smelt
1%
Flat-headed gudgeon
27%
Golden perch
1%
Australian smelt
3% Carp gudgeons
2%
River Blackfish
<1%
Unidentified
3%
Murray cod
<1%
Carp
29%
Murray cod
12%
Silver perch
18%
Carp
3%
Silver perch
20%
River
Australian smelt
6%
Flat-headed gudgeon
6%
Murray cod
26%
Golden perch
39%
Flat-headed gudgeon
91%
Unidentified
14%
Redfin
6%
Australian smelt
13%
Carp gudgeons
4%
River Blackfish
1%
Flat-headed gudgeon
9%
Carp
16%
Galaxid spp.
1%
Murray cod
20%
Silver perch
16%
Figure 23. Species percentage composition of raw numbers of eggs and larvae found drifting in both
irrigation channel and the Murray River at Torrumbarry and Murray Valley sites. (Note that only two instead
of three drift nets were set at the Torrumbarry river site.)
32
Arthur Rylah Institute for Environmental Research
Fish in the Murray Valley and Torrumbarry Irrigation Areas
3.4.3
Comparison of Murray Valley and Torrumbarry Irrigation Areas
The species diversities of drifting fish fauna in the Torrumbarry and Murray Valley Irrigation
Areas were generally similar, but the abundances of particular species differed considerably
(Figure 23). For example, river blackfish and goldfish larvae were collected in the Murray Valley
Irrigation Area but not in the Torrumbarry Irrigation Area. Conversely, carp gudgeon and redfin
perch were not collected in the Torrumbarry Irrigation Area but were collected in the Murray
Valley Irrigation Area. There were also some large differences in species abundance between the
two irrigation areas, particularly in the abundance of flat-headed gudgeon, golden perch and carp.
A comparison between the Murray Valley and Torrumbarry Irrigation Areas found no significant
difference in the densities of eggs collected (Mann–Whitney, P > 0.05; Table 6), but there were
significantly higher numbers of larvae collected from the Murray Valley Irrigation Area than the
Torrumbarry Irrigation Area (Mann–Whitney, P < 0.05). This was due to the large number of
drifting flat-headed gudgeons in the Murray Valley Irrigation Area, as the exclusion of their
numbers from the analysis resulted in a non-significant difference between the systems (Mann–
Whitney, P > 0.05).
Table 6. Density of drifting eggs and larvae captured in both irrigation channel and the Murray River for both
Torrumbarry and Murray Valley sites. Catches are expressed as mean catch per 1000 m3.
Common name
Torrumbarry
Native
CHANNEL
RIVER
CHANNEL
RIVER
eggs
—
1.89
47.87
23.18
larvae
4.02
—
40.11
1.56
Carp gudgeon
larvae
—
—
23.28
0.41
River blackfish
larvae
—
—
4.89
0.06
Flat-headed gudgeon
larvae
32.47
27.46
1334.83
6.03
Murray cod
larvae
30.40
9.35
4.20
1.58
Golden perch
eggs
—
99.67
22.63
—
larvae
—
78.10
2.77
—
eggs
8.71
22.43
—
16.89
larvae
—
31.39
—
—
larvae
—
—
—
0.11
eggs
27.69
0.95
1.70
—
larvae
—
—
—
1.47
Goldfish
larvae
1.00
—
—
—
Redfin perch
larvae
—
—
—
0.58
Unidentified
larvae
8.24
—
52.07
1.33
Total
eggs
36.40
124.94
75.08
40.07
larvae
76.12
146.30
1462.15
995.56
Australian smelt
Silver perch
Unidentified Galaxias sp.
Murray Valley
Introduced
Carp
Arthur Rylah Institute for Environmental Research
33
Fish in the Murray Valley and Torrumbarry Irrigation Areas
No significant differences in the densities of drifting eggs (Mann–Whitney, P > 0.05; Figure 24)
and larvae (Mann-Whitney, both P > 0.05; Figure 25) were found between channel and
corresponding river habitats in the Torrumbarry Irrigation Area (Table 6). In the Murray Valley
Irrigation Area there was no significant difference in the density of drifting eggs (Mann–Whitney,
P > 0.05), but there was a significantly higher density of drifting larvae in the channel than the
corresponding river site (Mann–Whitney, both P < 0.05). This was caused largely by the
significantly higher densities of flat-headed gudgeon in the channel (Mann–Whitney, P < 0.001).
There was no significant difference in the density of drifting Murray cod larvae between channel
and river habitats for either the Murray Valley or Torrumbarry Irrigation Areas (Mann–Whitney, P
> 0.05; Figure 26).
Torrumbarry
700
600
500
Density (eggs 1000m-3)
400
300
200
100
0
Yarrawonga
350
300
250
200
150
100
50
0
30–Oct
13-Nov
27-Nov
12-Dec
Figure 24. Densities of total eggs captured for each sample trip at both Torrumbarry and Murray Valley for
both irrigation channel (blue bars) and Murray River (pink bars) habitats in 2006. Densities are shown as
means with 1 SE.
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Arthur Rylah Institute for Environmental Research
Fish in the Murray Valley and Torrumbarry Irrigation Areas
Torrumbarry
700
600
500
400
Density (larvae 1000m-3)
300
200
100
0
Yarrawonga
4500
4000
3500
3000
2500
2000
1500
1000
500
0
30–Oct
13-Nov
27-Nov
12-Dec
Figure 25. Densities of total larvae captured for each sample trip at both Torrumbarry and Murray Valley for
both irrigation channel (blue bars) and Murray River (pink bars) habitats in 2006. Densities are shown as
means with 1 SE.
Torrumbarry
200
150
Density (larvae 1000m-3)
100
50
0
Yarrawonga
20
15
10
5
0
30–Oct
13-Nov
27-Nov
12-Dec
Figure 26. Densities of Murray cod larvae captured for each sample trip at both Torrumbarry and Murray
Valley for both irrigation channel (blue bars) and Murray River (pink bars) habitats in 2006. Densities are
shown as means with 1 SE.
Arthur Rylah Institute for Environmental Research
35
Fish in the Murray Valley and Torrumbarry Irrigation Areas
3.5 Impact of larval loss on a Murray cod population
Under the ‘no fishing’ scenario, a 50% loss of larvae has a minor impact on the modelled Murray
cod population, and although an 80% loss of larvae has a greater impact it is also considered to be
minor (Figures 29–31). However, under the ‘fishing’ scenario a 50% loss of larvae has a moderate
impact on the modelled Murray cod population while an 80% loss has a large impact on the
modelled Murray cod population (Figures 32–34). Under the ‘fishing’ scenario the model indicates
that at a larval loss rate over 50% a significant impact occurs on the Murray cod population.
1.0
Pr(x<TPS)
0.8
0.6
0.4
0.2
0.0
0
500
1000
1500
Threshold population size
Figure 27. Risk curves of the total adult female population for the ‘no fishing’ scenario. In this and
subsequent figures, the black line represents no loss of larvae; the red line represents 50% loss, and the blue
line represents 80% loss.
36
Arthur Rylah Institute for Environmental Research
Fish in the Murray Valley and Torrumbarry Irrigation Areas
1.0
Pr(x<TPS)
0.8
0.6
0.4
0.2
0.0
0
500
1000
1500
Threshold population size
Figure 28. Risk curves of the adult female population aged 5–9 years old for the ‘no fishing’ scenario.
1.0
Pr(x<TPS)
0.8
0.6
0.4
0.2
0.0
0
500
1000
1500
Threshold population size
Figure 29. Risk curves of the adult female population aged 10 plus years for the ‘no fishing’ scenario.
Arthur Rylah Institute for Environmental Research
37
Fish in the Murray Valley and Torrumbarry Irrigation Areas
1.0
Pr(x<TPS)
0.8
0.6
0.4
0.2
0.0
0
200
400
600
800
1000
1200
Threshold population size
Figure 30. Risk curves of the total adult female population for the ‘no fishing’ scenario.
1.0
Pr(x<TPS)
0.8
0.6
0.4
0.2
0.0
0
200
400
600
800
1000
1200
Threshold population size
Figure 31. Risk curves of the adult female population aged 5–9 years for the ‘no fishing’ scenario.
38
Arthur Rylah Institute for Environmental Research
Fish in the Murray Valley and Torrumbarry Irrigation Areas
1.0
Pr(x<TPS)
0.8
0.6
0.4
0.2
0.0
0
200
400
600
800
1000
1200
Threshold population size
Figure 32. Risk curves of the adult female population aged 10 plus years for the ‘no fishing’ scenario.
Arthur Rylah Institute for Environmental Research
39
Fish in the Murray Valley and Torrumbarry Irrigation Areas
4 Discussion
This study has demonstrated that a high diversity and a large abundance of both native and
introduced fish are present in both the Murray Valley and Torrumbarry channel systems. Different
fish species were found to be entrained into the channels systems at various life stages. For
example, golden perch were mainly present as large adults, while Murray cod were present
predominantly as juveniles. A number of native species, particularly Murray cod, golden perch and
silver perch, were found to be entering the irrigation channels via the movement of drifting eggs
and larvae from the source riverine environment. This study has also highlighted the dynamic
nature of the fish community present in the channels, with the community shown to vary between
years in the Murray Valley Irrigation Area, and to differ between the two irrigation areas.
4.1 Adult fish surveys
Nine native and five introduced species were collected from the Murray Valley Irrigation Area in
the 2006 electrofishing surveys, which is slightly lower than the total species diversity that would
be expected to occur in the region (Koehn 2006). The few species not recorded in the Murray
Valley Irrigation Area were species more commonly associated with higher altitudes, such as two
native species (two-spined blackfish Gadopsis bispinosus and mountain galaxias Galaxias olidus)
and two introduced species (brown trout Salmo trutta and rainbow trout Oncorrhynchus mykiss)
Other species that were not recorded in the channel system, such as Murray hardyhead
Craterocephalus fluviatilis, flat-headed galaxias Galaxias rostratus and southern pygmy perch
Nannoperca australis are known to occur in low abundance in the Murray River, so it is not
surprising that these species were not collected in the channels system. The most significant
species that was not collected in the Murray Valley system was the endangered trout cod
Maccullochella macquariensis. However, while its abundances are high in the Murray River
below Lake Mulwala, where it has been stocked for a number of years, it has not yet established
itself in large numbers upstream of the weir where the source waters for the irrigation channels are
located. Thus the absence of these species from the channels system is probably a result of low
abundances in the source waters, and not a result of particular physical attributes of the diversion
area (such as the location of gates) or the mechanisms used to divert water into it. As work is
under way to reinstate upstream fish passage at the Yarrawonga Weir and also rehabilitate trout
cod populations in upstream reaches of the Murray and its tributaries, the incidence of entrainment
of trout cod into the Murray Valley Irrigation Area is likely to increase.
This study found nine native and three introduced species in the Torrumbarry Irrigation Area.
Intensive research on fish passage at Torrumbarry weir by Mallen-Cooper (1996) recorded seven
native and four introduced species, of which only five native and three introduced species were
recorded in the current survey. Freshwater catfish Tandanus tandanus, short-headed lamprey
Mordacia mordax and the introduced brown trout were recorded in small numbers by MallenCooper (1996), and were not recorded in the channel system in this study. Hypseleotris spp., flatheaded gudgeon, unspecked hardyhead and Murray–Darling rainbowfish were collected in the
current survey but not by Mallen-Cooper (1996), who did not target smaller fish.
The comparison of the fish communities in each irrigation area and in their source waters suggests
that the absence in the channels of any expected species is most likely to be due to their low
abundance in the source waters, and not a reflection of any particular attributes of the system’s
management or infrastructure which might be deterring the entrainment of particular species.
Indeed, the results from this study suggest that all life stages and species present within the source
waters of any irrigation area with large, open diversion channels are at a high risk of being
entrained and lost from the riverine population.
40
Arthur Rylah Institute for Environmental Research
Fish in the Murray Valley and Torrumbarry Irrigation Areas
Although water for both irrigation areas investigated in this study is sourced from the Murray
River, the fish community differed markedly between the two irrigation areas. Murray Valley
Irrigation Area was dominated by unspecked hardyhead, Australian smelt and goldfish, whereas
the Torrumbarry Irrigation Area was dominated by Australian smelt and goldfish. The introduced
oriental weatherloach and gambusia were only recorded in the Murray Valley Irrigation Area, and
the native Murray–Darling rainbowfish and bony bream were recorded only in the Torrumbarry
Irrigation Area. Murray cod and unspecked hardyhead were also captured in greater numbers in
the Murray Valley Irrigation than in the Torrumbarry Irrigation Area. Anecdotal reports also
suggest that there are major differences between the fish communities in various Victorian
irrigation areas, particularly where the fish communities in the source waters are markedly
different (King and O’Connor 2007).
Differences in the timing of irrigation releases, infrastructure or management practices, in-channel
refuge habitats, and fish communities in the source waters would all contribute to differences in
species composition and abundance between irrigation areas. This suggests that the impact of any
new diversions or diversions in other systems may be difficult to predict without a thorough
examination of the fish community present in the source waters and (if possible) in the channels.
Accordingly, an assessment of the significance of other irrigation areas and any new diversions
needs to be made on a case-by-case basis, and therefore unique management strategies may be
required for each system. For example, the Torrumbarry Irrigation Area is unique relative to other
irrigation areas, as it was constructed using a number of natural waterways and contains many
large lakes, and the first section of the system is also used to transfer environmental water into the
Gunbower State Forest, an iconic site on the Murray River. Given the mixed usage of this system,
future management strategies for the Torrumbarry Irrigation Area should incorporate
environmental values for this system. In particular, consideration should be given to providing fish
passage and habitat improvements in the first section of the National Channel and then into
Gunbower Creek, as this would aid in rehabilitating the native fish community in the creek and in
the wetlands of Gunbower Forest.
This study also highlighted that the diversity and abundance of fish present in Irrigation Areas, can
differ between years. Resampling the same sites in the Murray Valley Irrigation Area one year
later found a greater number of Murray cod, Australian smelt and unspecked hardyhead, while the
abundance of gambusia was lower. Although surveys of the riverine fish community were not
undertaken as part of this project, changes in the composition of the riverine fish community
between the two years may have contributed to the differences recorded in the fish entrained in the
channel system. However, fish communities in riverine habitats are generally fairly stable, and
other factors such as the timing, duration and amount of water diverted into the Irrigation Area is
likely to have contributed to a greater extent. Indeed, comparisons of diversions into the Murray
Valley Irrigation Area in the two years indicate that there were some differences in the volume and
timing of diversions that may account for some of the differences in the fish communities. Natural
movements and spawning times of different species may have coincided with particular irrigation
releases, resulting in greater or fewer individuals being diverted between years, which may have
caused these differences in fish entrainment between years. If this is occurring then it may be
possible to alter the diversion regime into the irrigation areas to minimise the impact on fish
migrating or drifting downstream and reduce the number of fish ultimately lost into irrigation
areas. However, at this stage we are unable to determine whether any predictable pattern does exist
between irrigation regimes and the resulting fish community entrained into the irrigation areas, and
therefore further annual surveys of fish remaining after the drawdown in the channels system are
required to allow comparisons of different diversion regimes employed in different years.
Arthur Rylah Institute for Environmental Research
41
Fish in the Murray Valley and Torrumbarry Irrigation Areas
In both irrigation areas Murray cod were commonly collected as juveniles, but a greater proportion
of larger juvenile–subadult Murray cod were collected in the Torrumbarry Irrigation Area. This
may be caused in part by the regular stocking regime of mostly on-grown fish that occurs in the
many lakes used for recreational fishing that are part of the Torrumbarry Irrigation Area (Fisheries
Victoria 2004; 2005; 2006). Only one previously tagged Murray cod, which had been released into
Kangaroo Lake 16 months earlier, was captured during our study. A large proportion of the
Murray cod collected from the Murray Valley Irrigation Area were young-of-year fish in both the
2005 and 2006 surveys. At one site close to the Murray Valley channel inlet, 39 young-of-year
Murray cod were collected almost immediately after the drawdown, but they were completely
absent just a few weeks later when the site was resurveyed. This area had drained considerably
since the first sampling event, and large numbers of birds were observed feeding in the remaining
pool following the drawdown (authors’ observations). Furthermore, in both sampling years very
few fish 1+ years old were collected, suggesting that the majority of Murray cod enter the Murray
Valley Irrigation Area as larvae or juveniles, and that some survive at least until drawdown in the
channels in May. However, the majority of individuals then do not survive the drawdown period.
Some larger Murray cod were also collected in the channels, and one tagged individual was
recaptured at exactly the same location as the previous year, indicating that some individuals can
survive the benign habitat of the channels system for at least 12 months. Whatever the ultimate
fate of each individual entering the Murray Valley Irrigation Area, the low likelihood of fish
escaping back into the natural environment means that a large number of Murray cod are being
permanently lost from the already stressed riverine populations each year.
Unlike Murray cod, most of the golden perch collected in the adult fish surveys in both systems
were large adults, and juvenile and subadults were not found. This was also the case in the 2005
survey in the Murray Valley Irrigation Area (King and O’Connor 2005). Previous studies have
observed adult golden perch undertaking long-distance downstream movements that are thought to
be associated with spawning (O’Connor et al. 2005). Hence it is possible that the fish collected
from the channels entered as downstream migrating adults that were then diverted and entrained
into the channel systems. Thr entrainment of migrating fish in spawning condition into artificial
channel habitats would not only permanently remove these individuals from the natural
environment, but also permanently remove any of their potential progeny and therefore would
reduce the amount of natural recruitment occurring in the river. Further research should be
conducted to determine whether the adult golden perch entering the irrigation system are in
spawning condition and how they are attracted and entrained into the diversion channels.
A comparison of the electrofishing and pumpout data from the three sites where both methods
were used indicated that electrofishing gave a reasonably good representation of the diversity of
species that were present. Where there were discrepancies, the fish species concerned occurred in
small abundances. In contrast, electrofishing collected between 1% and 10% of the total number of
fish removed during the pumpouts, with mostly smaller species such as carp gudgeon and
Australian smelt being underestimated. Electrofishing was also relatively inefficient at collecting
larger individuals at sites with deep, long culverts, as fish were able to avoid the electrical field.
This demonstrates that although electrofishing is a much more suitable technique for sampling a
large number and diverse range of waterbodies in the channel system, pumpouts result in a much
more accurate estimate of the total number of fish at a site. Importantly, this suggests that the
number of fish occurring at the sites sampled in the channel system using electrofishing techniques
is being massively underestimated. Considering also that only 1% of potential refuges in both
irrigation areas were sampled, and that large numbers of fish are likely to die when they enter and
move through the various regulating structures and therefore not surviving long enough to be
recorded in our surveys, the numbers of fish being permanently removed annually from the
42
Arthur Rylah Institute for Environmental Research
Fish in the Murray Valley and Torrumbarry Irrigation Areas
Murray River via these two channels systems alone is likely to be very high, and the impact of
their loss on the natural riverine populations is likely to be severe.
4.2 Early life stages in the channels
Sampling was specifically targeted at determining whether the drifting early life stages were also
being entrained into the irrigation areas, and whether the densities were similar to those in the
nearby main river channel. Flat-headed gudgeon larvae were the most commonly collected species
in the egg and larval surveys. This species is a common, highly abundant, native species in the
region and is known to have a larval and/or drifting phase (Humphreys and King 2004). Eggs and
larvae from native species of conservation significance were also detected drifting in both channel
systems, including the nationally vulnerable Murray cod (EPBC 1999) and silver perch, golden
perch and river blackfish which are recognised as threatened in Victoria (Department of
Sustainability and Environment 2005). The diversity of species drifting as eggs or larvae was
similar in both irrigation areas, but the diversity and abundance of species in the corresponding
upstream main channel environment were remarkably different. For example, flat-headed gudgeon
larvae dominated the catch in the Murray Valley channel but not at the upstream river site, and
golden perch eggs and larvae were abundant in the river at Torrumbarry but were not collected in
the channel.
Golden perch eggs and larvae were found drifting into the Murray Valley Irrigation Area, but only
adults of this species were collected using electrofishing. The absence of young-of-year and
juvenile golden perch and silver perch in the electrofishing surveys of the channels may be due in
part to the difficulties in sampling the early stages of both these species using this method (King et
al. unpublished data held at ARI). Alternatively, the lack of juvenile fish recorded from either
irrigation area may suggest that eggs and larvae entering the channels system do not survive for
long periods, perhaps because of damage as a result of drifting downstream over and under
structures such as weirs and syphons, bird or fish predation, or the unavailability of specific food
requirements. Baumgartner et al. (2006) reported mortalities of up to 95% in golden perch larvae
passing through undershot weir structures, which are found throughout the irrigation system.
In the Murray Valley Irrigation Area there was no significant difference in densities of drifting
eggs between the river and channel habitats, but there were significantly higher densities of larvae
drifting in the channel compared with the corresponding river site. This was caused largely by the
high density of flat-headed gudgeons in the channel, although the exclusion of their numbers from
the analysis still resulted in a significant difference between the two habitats. Although the reasons
for this are unclear, it may indicate that considerably more spawning activity of various species is
occurring in Lake Mulwala or in the Ovens River than upstream in the Murray River. Increased
spawning activity in Lake Mulwala would contribute more drifting larvae to the channels system
and may explain the discrepancy between the two sites.
There was no significant difference in the density of drifting Murray cod larvae between channel
and river habitats for either the Murray Valley or Torrumbarry Irrigation areas. Furthermore, the
densities of Murray cod larvae recorded entering the irrigation areas are within the normal range
recorded drifting in the Barmah–Millewa region of the Murray River (King et al. 2005a, b; King et
al. 2007). This suggests that a substantial proportion of the Murray cod drifting past these intakes
are inadvertently drifting into the channel systems. Because the adult electrofishing surveys in the
channels recorded large numbers of young-of-year Murray cod in May after the drawdown,
Murray cod appear to be entering the irrigation areas mainly at the drifting larval stage or as very
early juveniles. This consistent loss of potential recruits to the main riverine population is difficult
to quantify, but it is likely to be having a sustained and significant impact on the riverine Murray
cod population (see sections 3.5 and 4.3). For example, using the average density of drifting
Arthur Rylah Institute for Environmental Research
43
Fish in the Murray Valley and Torrumbarry Irrigation Areas
Murray cod larvae in the Murray Valley Channel and assuming they are drifting only at night, and
assuming a channel discharge of approximately 2000 ML/day, this would suggest that over 4000
individuals are entering the channel system each day. Since most Murray cod collected during the
drawdown were juveniles, it appears that few of the Murray cod entering the system are surviving
and maturing into adults.
The results of this study indicate that, in addition to adult fish, there is a significant loss of eggs
and larvae into the two irrigation areas, and that this is likely to be having a major impact on
recruitment in the riverine environment. Consequently there is a need to reduce this loss of eggs,
larvae and juveniles from the riverine system while minimising the impact on irrigation water
supply. The timing of irrigation releases into channels is probably the simplest way to help reduce
these losses. For example, in both the Torrumbarry and Murray Valley Irrigation Areas the highest
densities of Murray cod recorded in the study occurred during November. Additionally, this
species is known to exhibit a distinct diel drifting pattern, with the highest densities drifting at
night (Humphries 2005; King et al. 2007). Densities of drifting silver perch eggs have also been
reported to peak at night from 2100–0100 hours (Tonkin et al. 2007). Therefore, reducing water
extractions at night during peak spawning months of November and December might reduce the
loss of eggs and larvae fromthe riverine system. Such reductions in the volumes of water extracted
during key spawning times, as well as limiting extraction to daylight hours, has been suggested by
Boubée and Haro (2004) and Gilligan and Schiller (2003). The viability of this as a suitable
management option is unknown but should be discussed and investigated.
The application of other techniques to reduce losses of the early life stages of fish is still limited. A
variety of fish screens are used in the United States and New Zealand, but although they are
generally effective for larger fish they are less efficient for smaller ones, particularly larvae
(Boubée and Haro 2004). Other exclusion techniques such as barrier nets, lights, sound, electric
fields, louvres, spills and bypass flows can be effective for larger fish but are relatively ineffective
for eggs or smaller fish. An integrated management technique that uses these methods to minimise
adult and juvenile losses, in conjunction with controlled releases to minimise water extraction
during peak egg and larval abundances, would be ideal. However, a thorough knowledge of the
migration timing, migration pathways and diurnal cycles of the species in question is mandatory
before the implementation of any management techniques (Boubée and Haro 2004).
4.3 Impacts of larval loss on a Murray cod population
The results of the model indicate that population persistence is likely to be affected only if the loss
of larvae to irrigation channels needs is over 80%, when considered in isolation from other
impacts. However, if the impacts of fishing are included in the model then the likelihood of
population persistence is affected at levels of 50% larval loss. Fishing alters the population
structure by reducing the number of large fish in the system (either directly by angling or
indirectly by restricting the number of fish that reach this size). The fishing rates used in the
modelling were derived from an eight-year mark–recapture study undertaken below Murray Valley
weir and are the best estimates of fishing rates available (Todd, unpubl. Data held at ARI). Given
the high level of recreational fishing that occurs throughout the mid-Murray River, it is reasonable
to expect fishing rates to be similar throughout this region. Consequently, Murray cod populations
are likely to be more sensitive to larval loss where fishing occurs. For example, a loss of 80% of
larvae combined with the impacts of fishing causes the modelled Murray cod population to remain
in decline over the 50-year period modelled (Figure 33). Additional impacts such as barriers to fish
movement and altered flow regimes would make these declines even sharper, as populations will
be more sensitive to lower levels of larval loss. In other words, with each additional impact the
population persistence becomes highly sensitive to the loss of larvae. Importantly, this modelled
44
Arthur Rylah Institute for Environmental Research
Fish in the Murray Valley and Torrumbarry Irrigation Areas
scenario is likely to be an underestimate because the model did not take into account the loss of
any other life stages as the rates of loss are much harder to predict.
Diversions into both Torrumbarry and Murray Valley (including the NSW Mulwala Channel)
irrigation area vary both between and among years. Typically, however, diversion rates are around
20–30% of total passing flow, but include periods where diversions may exceed 50% and
occasionally approach 100%. However, the percentage of flow diverted does not necessarily
reflect the proportion of eggs and larvae being diverted, which will be affected by such factors as
the position of drifting eggs and larvae in the water column, mechanisms of diversion (e.g.
overshot or undershot gates) and the morphology and hydrology of the river at the point of
diversion. Consequently, the present flow diversion rates may result in a significant loss of larvae
to the channels system, given that the Murray cod population model suggested that the likelihood
of population persistence is affected at levels of 50% larval loss.
While this model has been specifically designed for Murray cod, the impacts on other fish species
as a result of diversion can also be ascertained from these results. For example, the impacts on
trout cod, which are not as fecund as Murray cod, could be expected to be greater than those on
Murray cod. On the other hand, the impacts on golden perch and silver perch, which are more
fecund than Murray cod, could be expected to be less. However, since the egg and larval stages are
known to drift, and there is some evidence to suggest that (at least for golden perch) adults may be
entering the irrigation systems on downstream spawning migration, populations of these two
species may be more vulnerable to diversions than Murray cod.
Population Size
2500
2000
1500
1000
500
0
0
10
20
30
40
50
Years
Figure 33. Average population trajectory for the total adult female population under the ‘fishing’ scenario
with an 80% loss of larvae. The red lines are the maximum and minimum over all trajectories, the blue lines
are ± 1 standard deviation and the black is the average overall trajectories.
Arthur Rylah Institute for Environmental Research
45
Fish in the Murray Valley and Torrumbarry Irrigation Areas
5 Conclusion
The massive Australian irrigation industry has, over many decades, brought enormous benefits to
the country, however, these benefits have not come without cost to the natural riverine
environment. The development of the irrigation industry has relied upon altering the natural flow
of rivers, and this has had a great impact on many of the crucial components of the life history of
fish, including spawning and migration cues (Bunn and Arthington 2002). Furthermore, dams and
weirs block crucial upstream migrations associated with spawning and dispersal. However, it has
not been until more recently that the impact of fish entrainment into irrigation channels has been
questioned by managers and scientists (Lintermans and Phillips 2004; Baumgartner 2005; King
and O’Connor 2006).
This study, in conjunction with the results of a similar study undertaken in 2005 (King and
O’Connor 2007), suggests that an abundant and diverse range of native fish at various stages in
their life history are consistently being removed from the riverine environment into both the
Murray Valley and Torrumbarry Irrigation Areas. Among these are many species of conservation
significance, including Murray cod, golden perch, silver perch, Murray–Darling rainbowfish and
unspecked hardyhead. Some species (in particular Murray cod, trout cod, golden perch and silver
perch, which have a drifting early life history stage) were shown to be vulnerable to entrainment
into the irrigation channels at all stages of their life cycle. The results of resampling sites surveyed
in the previous year in the Murray Valley Irrigation Area suggest that the majority of fish that are
trapped in the channels system do not survive past the first winter drawdown period. While it is
difficult to quantify the exact number of fish being permanently removed annually from the
Murray River via these two channels systems, it is likely to be very high and the impact of their
loss on the natural riverine populations is likely to be severe. Population modelling for Murray cod
suggests that the persistence of the population would be severely impacted where recreational
fishing occurs and larval losses to the irrigation system exceed 50%. Additionally, given the
significance of the fish fauna and the scale of the remediation strategies already underway to
restore native fish in the Murray River, any loss of native fish, particularly nationally threatened
species such as Murray cod, is a cause for considerable concern.
This study and the findings from another study conducted in NSW irrigation areas (Baumgartner et
al. 2007), indicates that the removal of native fish into channel systems is occurring in abundances
large enough to warrant investigating and implementing strategies to reduce their removal from the
riverine environment. Potential solutions for reducing loss of fish into the channel systems usually
involve diverting fish away from channel inflows using physical barriers (e.g. screens) or
behavioural barriers (e.g. aeration screens), or both (Bell and DeLacy 1972; Ruggles 1980).
We suggest that management and structural options for reducing the number of fish entrained into
irrigation systems needs to be urgently assessed if we are to improve the status of native fish in the
Murray River.
46
Arthur Rylah Institute for Environmental Research
Fish in the Murray Valley and Torrumbarry Irrigation Areas
6 Recommendations
 Investigate the feasibility of screening irrigation channel inlets to reduce or, if possible, prevent
the entrainment of native fish into channels in the Murray Valley and Torrumbarry Irrigation
Areas.
 Assess the potential impact of diversion on native fish in all existing irrigation areas in
Victoria, and establish a prioritised list of Irrigation Areas with potential management options.
 Ensure that any new water diversions thoroughly consider the risks to the riverine fish
community.
 Ensure that future management strategies for the Torrumbarry Irrigation Area incorporate
environmental values. In particular, consideration should be given to providing fish passage
and habitat improvements in the first section of the National Channel and then into Gunbower
Creek, as this would aid in rehabilitating the native fish community in the Creek and in the
wetlands of Gunbower Forest.
 Determine the feasibility of reducing water extractions at night during the peak spawning
months of November and December.
 Conduct annual surveys of fish in both Torrumbarry and Murray Valley Irrigation Areas after
the drawdown at a few key sites, to determine whether there is any substantial inter-annual
variation in catches across different diversion regimes employed in different years. (For
example, during the 2006–07 season drought conditions resulted in reduced diversions into the
irrigation areas.)
 Significant fish refuge sites identified during the winter drawdown period should be targeted
for active fish removal, and the fish should then be returned to a suitable nearby river. This
should be conducted systematically by trained workers so that accurate data is obtained on the
specific locations directly after the drawdown, to determine exact numbers of fish trapped.
 Conduct further monitoring of the densities of drifting eggs and larvae, particularly given that
higher numbers of entrainment may occur during flood years.
 Determine whether the adult golden perch entering the irrigation system are in spawning
condition and how they are attracted and entrained into the diversion channels.
 Investigate the installation of fishways or other systems that could return fish to main river
systems, such as catch-and-transport operations that could be undertaken at the beginning of the
drawdown period.
 Train water management and operational staff on appropriate fish handling and release
techniques for returning fish to source waters.
Arthur Rylah Institute for Environmental Research
47
Fish in the Murray Valley and Torrumbarry Irrigation Areas
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Appendix 1
Water quality from Murray Valley Irrigation Area
Site No.
pH
Electrical
cond.
(µS/cm)
Turbidity
(NTU)
Dissolved
oxygen
(mg/L)
Temp.
(ºC)
3
7.67
53
17
nd
14.3
4
8.15
53
7
nd
16.2
6
7.36
53.6
250
nd
12.7
7
8.04
53
63
nd
12
8
8.08
53
18
nd
15.7
11
9
52
4.3
nd
14.2
12
7.73
119
130
150
nd
13
8.18
201
19.2
100
9
14
7.76
50
115
123.7
7.1
15
7.19
118.1
415
118.1
6.5
16
8
126.3
74
55.9
8.4
17
8.22
55
42
180
6.3
18
8.42
49.8
194
123
6.8
19
7.36
45.1
334
90.6
9.3
1890
5.9
152
13.4
20
21
7.81
63.5
178.5
115.4
8.7
22
7.98
57.8
127
105.7
8.9
23
7.28
53.5
88
120.1
8.7
24
7.39
50
81
110.9
9.2
25
7.47
70.9
399
112.8
8.7
26
9.11
55
70
nd
10
27
7.31
55.8
75
149
12.3
28
7.18
54.2
56
113.6
10.8
29
7.27
120
70
49.5
6
*Note that water quality is not available for some sites
nd — no data available
Fish in the Murray Valley and Torrumbarry Irrigation Areas
Appendix 2
Water quality from Torrumbarry Irrigation Area
Site No.
pH
Electrical
cond.
(µS/cm)
Turbidity
(NTU)
Dissolved
oxygen
(mg/L)
Temp.
(ºC)
1
8.12
81.8
20.8
121
6.8
2
7.63
173
50
160
8.1
8
8.13
59.5
66
160
8.9
9
7.51
257.1
33.9
140.9
10.5
13
7.64
2366
17.2
149
11
14
7.89
477
34
169
8.3
15
7.99
87
81
149
10.8
16
8.04
82.7
107
nd
10.5
17
7.65
290
63.3
nd
8.7
18
8.34
113
65
127
12.4
19
8.1
196.4
57
156
7.8
20
8.13
192
72.5
nd
5.8
21
7.78
84
295
144
8.1
22
8.1
322
114
119
7.5
23
8.38
204
88
126
2.7
24
8.14
108
97
130
12.6
25
8.22
115.8
91
161
8.9
26
7.88
377
51
165
6
28
7.85
104
399
130
9.2
29
7.87
137
165
122
7.8
30
7.65
89
196
115
9.7
*Note that water quality is not available for some sites
nd — no data available
2
Arthur Rylah Institute for Environmental Research
Fish in the Murray Valley and Torrumbarry Irrigation Areas
Arthur Rylah Institute for Environmental Research
3