Interim report for: 1115 Sports Fishery Research
Investigation of the Mawheraiti River Brown Trout Fishery
May 2015
Rhys Adams, Fish & Game Officer, May 2015
Introduction
The Mawheraiti or Little Grey River is a tributary of the Grey River. Its catchment incorporates
tributaries from the Inland mountainous flanks of the Paparoa Ranges and from the rolling hills of the
Reefton and Ikamatua areas. The Mawheraiti River joins the Grey River near the township of
Ikamatua.
The Mawheraiti River fishery is identified as ‘regionally significant’ in its rural reaches (FGWC,
2011) and receives between 150 & 480 angling day each season (Unwin, 2008). The Mawheraiti
River has long been regarded by anglers as a nursery for the greater Grey River fishery and this is
confirmed by the large number of small and medium size brown trout that have been observed in drift
dive surveys.
Fish & Game has been undertaking drift dive surveys in the river intermittently since 1993. Although
the West Coast trout populations are known to fluctuate over years it is evident that the population of
brown trout in the drift dived sections of the Mawheraiti are currently at depressed levels (Adams,
2014).
The reasons for the decline in the fishery are unknown. The catchment is both prone to high rainfall
events and summer time dry spells. Anthropogenic influences from the surrounding intensive dairy
farming land use may have some effect on the fishery. Surface water takes are prevalent in the
catchment and these could increase in the future.
To research the factors that may be affecting the brown trout abundance in the fishery Fish & Game
have installed a water temperature and water level logger at the SH7 Mawheraiti Bridge, and
undertaken field work including; water sampling, drift dive surveys, and electric fishing within the
catchment. As part of the WCRC State of the Environment monitoring programme quarterly
monitoring of water quality and habitat is undertaken in the Mawheraiti River at Maimai, this data is
reviewed alongside the field work and monitoring undertaken.
This report is intended to provide an overview on information gathered and reviewed for work plan
project 1115 – Sports fishery Research in the past year, and build on former reports where relevant.
The information gathered is also intended to inform resource consent processing.
Drift dives
The Mawheraiti River at the Mawheraiti SH7 bridge site (see appendix) has been drift dived
intermittently since 1993. The data from these dives is presented in Chart 1. It is important to note the
abundance of trout fluctuates between years, however several years were not dived including a three
year period between 1998 and 2000. The dives are generally undertaken in the summer months of
January or February. To obtain information on the seasonal changes in trout abundance three dives
were undertaken in both the 2011/12 and 2012/13 spring to autumn periods and for the 2013/2014 and
2014/2015 year two dives were undertaken with the months noted for reference.
Chart 1: Drift dive counts of brown trout at the Mawheraiti River – Mawheraiti bridge dive
site.
There are several years that provide a snap shot of the potential of the Mawheraiti River to support
high numbers of trout. 1996 has the highest combined total of 560 trout/km including; 42 Large, 210
medium and 308 small.
From December 2011 to February 2015 the counts of trout, both for each size class and combined
total have been at low levels. The February 2015 dive revealed an improvement in the total counts
from 2013 and 2014 but only to levels similar to the 2001 dives. The 2001 counts were considered
low prior to 2012.
In 2011 the Mirfins bridge dive site was established in the lower reaches of the Mawheraiti River (see
appendix). Unfortunately due to river conditions no valid dives were completed during the 2011/2012
season. Table 1 below details the seven seasonal dives completed to date during the 2012 and 2015.
The February 2015 dives reveal a slight improvement in total count. Most notably small fish number
have increased which is consistent with the counts at the SH7 site.
Table 1: Drift dive observations (trout/km) at the
Mirfins Bridge dive site
2012 (Nov)
2013 (Jan)
2013 (May)
2013 (Nov)
2014 (Feb)
2014 (Dec)
2015 (Feb)
Average
Large/km
7
11
9
7
2
15
10
9
Med/km
10
38
17
15
2
25
21
22
Small/km combined
1
18
27
76
1
27
4
26
22
26
12
52
102
133
24
54
The only consistent and distinct observation that has occurred over three years is that small fish
increase in abundance from spring to summer at the Mirfins Bridge site (Adams, 2015). The value of
performing seasonal dives may only be evident when the fishery has recovered.
Temperature & water level
The Fish & Game temperature and water level logger was installed at the SH7 bridge site in
December 2011 (see appendix for location). Data is captured at intervals of 30 minutes and is accurate
to ± 0.5°C. The data collected spans from 12/12/2011 to 17/04/2015 (see chart 2.0). Due to a technical
error data was not collected between 01/06/14 and 21/08/2014.
Chart 2: Water level and temperature from the Fish & Game Mawheraiti River SH7
monitoring site.
Temperature
Chart 2 displays temperature data including almost four summers and three winters. It is important to
note that the summer of 2012/2013 included region wide flooding (2-4 January 2013) and a very dry
spell which led to the Minister of Primary industries declaring a drought for the Buller and Grey
Districts. From both a Hydrological and Climatological point of view this drought event has been
described as significant, with NIWA scientists stating that it equated to approximately a 1 in 80 year
drought (WCRC, 2013).
Daily maximum temperatures regularly exceeds 20ºC in the months of December, January, February,
and March except during freshet or flood events. The maximum recorded temperature was 27.2°C,
recorded on January 29, 2015. The lowest temperature recorded was 3.2°C on the July 3, 2012. June
and July are recorded as the coldest months for water temperature.
Hey et al (2006) recommends to maintain values of significant sports fishery, water temperature
should not exceed 19°C as a daily maximum. During the 2014-15 summer (Dec-March) the daily
maximum temperature exceeded 19.0°C on 80 days.
A water temperature logger was installed at the Fish & Game Maimai monitoring site as part of the
Cumulative Effects project run by the Cawthron Institute. The temperature data is useful for looking
at catchment level differences in water temperature. 2013 summer temperature data was compared
between the Maimai site which is in the upper part of the catchment and the SH7 Mawheraiti
monitoring site in the middle part of the catchment. The data was selected as being in a period from
January to March when river flows were low and little rainfall fell during the declared drought period.
A single factor ANOVA was performed on the data and found that the mean temperatures were
significantly different (P-value=0.00) with the downstream Mawheraiti site averaging 19.2°C for the
period compared to 18.5°C at the Maimai site. For the period the downstream site was 0.7°C warmer
on average.
Water Level
Water level is measured as height in cm above the submerged data logger unit. The highest water
level recorded during a flood was 394cm on June 6, 2012. The lowest water level recorded occurred
during Early Autumn low flows on April 2, 2014 at 36cm.
High flows, at levels measured during the research period are known to cause changes in riverbed
morphology. A high flow event recorded on the 11th of September, 2013 measured as 344cm on the
water level gauge caused some changes to the morphology of the river bed. Adjacent and upstream of
Mirfins Bridge on Atarau Road a channel was cut but when the flow abated the original channel
remained. During the next high flow event the channel cut further and the full flow of the river
switched to the newly formed channel. The high flow event was the second largest flood event since
the monitoring equipment was installed.
Habitat & Water monitoring
Fish & Game have undertaken only a limited amount of water quality monitoring other than
temperature monitoring. The West Coast Regional Council (WCRC) have assisted Fish & Game with
short term low flow monitor specific to this project and also undertake annual monitoring as part of
their ‘State of the Environment’ (SOE) programme. The SOE monitoring site is located above the
SH7 Bridge on the Mawheraiti River at Maimai.
Table 2 lists a selection of variables monitored at the Mawheraiti River SH7 Bridge, Maimai. The
data presented was collected from January 1999 - October 2014. Data is compared to WCRC
recommendations for aquatic ecology and where applicable Hey et al (2006) recommendation for the
protection of significant trout fisheries. The WCRC recommendations are derived from various
sources and are designed to protect general aquatic ecosystem and are not specific for brown trout.
Table 2: Water Quality variables measured for the West Coast Regional Councils Mawheraiti
River ‘State of the Environment’ (SOE) monitoring site at SH7 Maimai 1999-2014.
Variable
Range
MCI - Macroinvertebrate
96.4 - 148
Community index
SQ MCI
5.42 - 7.75
Temperature
3.9 - 22.4
Total Ammonia
0.002 - 0.11
Turbididty
0.1 - 7.5
Clarity
0.2 - 5.56
E. Coli
5 - 24000
Periphyton
5.17 - 10
Disolved Reactive Phosphorus0.003 - 0.018
Disolved Oxygen
85.2 - 128
Nitrate
0.13 - 1.03
Median
121.5
WCRC Recommendations
120+ MCI
6.8
12.5
0.028
0.6
2.83
155
8.2
0.006
102
0.28
>6
<20ºC
<0.021mg/l
<5.6NTU
>1.6m
<260
>5
<0.03mg/l
>80%
<0.1mg/l
fishery recomendations
>120
<19ºC
<0.021mg/l & <0.01mg/l **
0.5
>5m
>80%
concern
yes
yes
yes
yes
yes
yes
yes
no
no
no
yes
**lower value for upland streams >150m above sea level
A one off ‘spot’ water sample was taken by Fish & Game during February 2012 during low flows and
when notable algal growth was observed. A second spot sample was taken in November 2014 as the
river was receding after a high flow event. Nitrogen and Nitrate levels were measured to assess their
potential impact on aquatic ecology. Dissolved Inorganic Nitrogen (DIN) and Dissolved Reactive
Phosphorus (DRP) where measured to assess their influence on algal growth. All variables except
Nitrate and DIN levels on the November sample were measured at levels below guideline limits based
on WCRC recommended guidelines for protection of aquatic ecosystems (WCRC, 2011). The Nitrate
levels measured were within the range observed by the WCRC during the annual SOE monitoring of
the Mawheraiti River although in exceedance of the levels recommended to protect ecosystem values.
It is evident by referring to WCRC SOE, and Fish & Game monitoring that there are many water
quality and habitat variables that are at times beyond recommended levels for both the protection of
aquatic ecology in general but also the protection of regionally significant trout fisheries. In general
water quality variables measured are more often than not within the guideline limits as indicated by
the median values. There are however water quality variable that exceed recommendations often
including Nitrate and Ammonia (see Table 2). The SOE reporting includes analysis of 10 year trends
in the variables monitored. None of the variables measured in Table 2 showed evidence of a trend
other than turbidity which showed a significant trend of improvement (Horrox et al, 2015).
Electric Fishing
In 2012 the Mawheraiti River was electric fished as part of the ‘Cumulative Effects’ project run by the
Cawthron Institute at the Fish & Game’s Maimai site (see appendix). Surveys were undertaken on two
occasions; December 20, 2012 and March 14, 2013. In December the method was to catch as many
brown trout as possible for baseline data. The method in March was a three pass quantitative survey.
A follow up two pass quantitative electric fishing survey was undertaken on the site in March 2014.
& 2015. The river bed morphology at the site changes annually, therefore habitat changes at least on
an annual basis at the site. Habitat modification may affect the suitability of the site for various size
classes of trout. The data is useful as a comparison between surveys as a general indicator of fish
abundance and size in the site.
Comparison can be made between 2014 and 2015 surveys as the methodology was the same. A larger
reach was fished in 2015 and more fish caught, however when the number of fish caught in total is
compared to the area fished, the 2014 and 2015 surveys had very similar densities of trout.
Table 3: Electric fishing survey results for brown trout at the Fish & Game Maimai site
DecemberAge/ Length
2012
March-2013
March-2014
March-2015
0+ / <130mm
25
14
7
26
1+ / 130-220 mm
4
0
3
1
2+ / 220-300 mm
1
2
1
0
3+ / 300-450mm
0
0
0
0
Older / 450+ mm
0
1
1
0
total trout/hectare
n/a
260
220
220
Length of reach
fished
620m
80m
100
114
Discussion
Drift dive surveys have shown that the Mawheraiti River fishery has had relatively low abundance of
brown trout in the period of December 2011 to present. This period of low abundance is a concern, as
until recently the fishery has provided excellent fishing and drift dives surveys have revealed healthy
populations of brown trout. The cause of the decline in the fishery is unknown but this investigation
has so far highlighted some of the variables that may have a part to play. The drift dive data is
considered the critical component of investigations into the decline in the fishery. When trout
abundance is relatively high in each size category the fishery will be considered in a healthy state. It
will not be evident as to what are the most likely factors to be effecting the abundance of trout and
therefore the quality of the fishery until the fishery returns to former levels and a retrospective look at
the factors can be made.
Water temperatures have been shown to exceed recommended levels on a substantial number of days
through the summer months. Trout have been shown to cease feeding at around 19°C and fish kills
have been recorded in New Zealand at 26°C (Hey et al 2006). The Mawheraiti River trout population
is likely to be effected negatively by high water temperatures according to this information. Although
monitoring has shown temperatures have reached 27.2°C no fish kills have been observed by staff.
There is no historic temperature data to show summer water temps in the Mawheraiti River in the past
therefore we cannot rule out that temperatures have always been high in the river during summer and
may not have a significant impact on the abundance of trout.
Water temperatures are slightly higher in the middle reaches of the fishery than the upper reaches. The
temperature in the lower river has not been measured. Monitoring temperatures in the lower reaches
of the river during the summer period of 2016 to see how temperature compare between the middle
and lower reaches could provide valuable insight to the temperature dynamics of the river.
The installation of the Fish &Game water level and temperature gauge has provided a near continuous
data set of water levels and temperature in the middle reaches of the river. The water level data is
important for understanding how frequently extreme flow events, both low and high are occurring.
High flows are known to detrimentally affect trout and their aquatic food sources through direct kills,
destruction of spawning habitat, and modification of habitat. High flows causing changes in the river
bed morphology have been evident during the term of flow monitoring.
The WCRC has installed a river flow measuring station on the Mawheraiti River near Mirfins Bridge
in late 2014. Communications with WCRC staff reveal that the equipment will be fully operational
when more flow measurement have been undertaken and that the data will be available to Fish &
Game. When this occurs it will be logical for this WCRC monitoring equipment and data to take the
place of the water level logger maintained by Fish & Game at the SH7 bridge site.
Extreme low flows are associated with reduction of available habitat for trout and their aquatic food.
Current climatic conditions have led to an increase in the amount of surface water taken for irrigation
of pasture in the Mawheraiti catchment therefore it is important to continue monitoring water levels
and temperatures through this period to see how water takes effect the Mawheraiti River flows.
Although a high percentage of the surface flow is currently allocated for consumptive takes for
consent holders, further applications to take surface water are currently being processed by the
WCRC. It is important that Fish & Game stay active in the consenting process and have information
like water temperatures and flow level measurements to use as a tool to rationalise concern for the
fishery. Work is currently being undertaken by the WCRC to identify flow levels that protect values
of species including brown trout. This work is undertaken in response to concerns that water
allocation in the catchment may affect the aquatic ecology of the river.
The WCRC’s SOE surface water quality monitoring was compared to recommended levels of both
the protection of aquatic ecology and trout fishery values. Several water quality variables are shown
to be at times above recommended levels therefore may have a negative effects on aquatic ecology
and the trout fishery. To understand how each of these variables could have a negative effects on the
fishery many considerations must be taken into account. For example; natural water quality,
frequency of data collected, and rational and science behind the recommendations. Water quality is
shown to be impacted at times but generally is at levels that could maintain a sports fishery.
Ultimately water quality, temperature, and flow monitoring should continue until the fishery returns to
a former state, where drift dive counts reflect ‘above average’ years. This could provide insight into
the changes water quality and habitat and how that relates to trout abundance. In the mean-time the
information provides a tool to assist in managing the fishery effectively through the resource
consenting process.
Recommendations


Maintain the Fish & Game water level monitoring equipment until the WCRC flow
monitoring site is fully operational.
Continue to monitor water temperature at the Mawheraiti Bridge site.




Monitor 2015-2016 summer water temperatures in the lower reaches near Mirfins Bridge.
Drift dive the Mawheraiti River site twice annually in spring and summer.
Conduct an Electric Fishing survey at the Maimai monitoring site annually in March.
Encourage the WCRC to proactively protect the Mawheraiti River trout habitat considering
their duty to do so under Section 7(h) of the Resource Management Act 1991.
References
Adams, R (2014). Drift Dive Surveys. Fish & Game internal report.
Adams, R (2015). Drift Dive Surveys. Fish & Game internal report.
Adams, R (2015). Investigations of the Mawheraiti brown trout fishery. Fish & Game internal report.
Fish & Game West Coast Region (FGWC) (2011). Sports Fish and Game Management Plan for the
West Coast Fish & Game Region. Fish & Game West Coast, internal report
Hay J, Hayes J, Young R 2006. Water quality guidelines to maintain trout fisheries values. Prepared
for horizons Regional council. Cawthron report No. 1205. 17p
Horrox J, Chaney E, Eaves A, 2015. West Coast Surface Water Quality – state of the environment
technical report 14001 for the West Coast Regional Council (WCRC).
Unwin, M.J.; Image, K. (2003). Angler usage of lake and river fisheries managed by
Fish & Game New Zealand: results from the 2001/02 National Angling
Survey. NIWA Client Report CHC2003-114. 48 p.
The West Coast Regional Council (WCRC), May 2013. Agenda and supporting papers for Councils
May meetings.
Appendix