Fishery Assessment Report to
PIRSA Fisheries
Mulloway (Argyrosomus japonicus)
Fishery
Greg Ferguson and Tim Ward
March 2003
SARDI Aquatic Sciences
Publication No. RD03/0040
Fishery Assessment Report to
PIRSA Fisheries
Mulloway (Argyrosomus japonicus)
Fishery
Greg Ferguson and Tim Ward
March 2003
SARDI Aquatic Sciences
Publication No. RD03/0040
Title:
Mulloway (Argyrosomus japonicus) Fishery
Sub-Title:
Fishery Assessment Report to PIRSA Fisheriesfor the Inland Waters
and Marine Scalefish Fishery Management Committees
Greg Ferguson and Tim Ward
Authors:
South Australian Research and Development Institute
SARDI Aquatic Sciences
2 Hamra Avenue
West Beach SA 5024
Telephone: (08) 8200 2400
Facsimile: (08) 8200 2406
http://www.sardi.sa.gov.au
This document is not to be cited without permission from the authors.
The authors warrant that they have taken all reasonable care in producing this report. This report has
been through SARDI Aquatic Sciences internal review process, and was formally approved for release
by the Chief Scientist. Although all reasonable efforts have been made to ensure quality, SARDI
Aquatic Sciences does not warrant that the information in this report is free from errors or omissions.
SARDI Aquatic Sciences does not accept any liability for the contents of this report or for any
consequences arising from its use or any reliance placed upon it.
© 2003 SARDI Aquatic Sciences
This work is copyright. Apart from any use as permitted under the Copyright Act 1968, no part may be
reproduced by any process without prior written permission from the author.
Printed in Adelaide September 2003.
Author:
Reviewers:
Approved by:
Signed:
Date:
Distribution:
Circulation:
Greg Ferguson and Tim Ward
Qifeng Ye and Simon Bryars
Anthony Cheshire
Insert date
PIRSA Fisheries, Inland and Marine Scalefish Fishery Management
Committees, SARDI Aquatic Sciences Library
Public Domain
ii
TABLE OF CONTENTS
TABLE OF CONTENTS------------------------------------------------------------------------------------------I
ACKNOWLEDGEMENTS------------------------------------------------------------------------------------- II
EXECUTIVE SUMMARY ------------------------------------------------------------------------------------ III
1
GENERAL INTRODUCTION -------------------------------------------------------------------------- 1
1.1
Overview ---------------------------------------------------------------------------------------------- 1
1.2
Description of the Fishery ------------------------------------------------------------------------- 2
1.2.1
Location and Size --------------------------------------------------------------------------------- 2
1.2.2
Environmental Characteristics------------------------------------------------------------------- 2
1.2.3
Commercial Fishery ------------------------------------------------------------------------------ 3
1.2.4
Recreational Fishery ------------------------------------------------------------------------------ 7
1.3
Management of the Fishery ----------------------------------------------------------------------- 7
1.3.1
Management Milestones-------------------------------------------------------------------------- 7
1.3.2
Current Management Arrangements ------------------------------------------------------------ 8
1.3.3
Management Objectives and Strategies -------------------------------------------------------11
1.3.4
Performance Indicators and Reference Points ------------------------------------------------11
1.4
Fisheries Biology of Mulloway -------------------------------------------------------------------11
1.4.1
Taxonomy and Distribution---------------------------------------------------------------------11
1.4.2
Stock Structure -----------------------------------------------------------------------------------12
1.4.3
Life History ---------------------------------------------------------------------------------------12
1.4.4
Growth and Size at Maturity--------------------------------------------------------------------13
1.5
Stock Assessment -----------------------------------------------------------------------------------15
1.5.1
Commercial Fishery -----------------------------------------------------------------------------15
1.5.2
Recreational Fishery -----------------------------------------------------------------------------16
1.6
Discussion --------------------------------------------------------------------------------------------16
2
FISHERY STATISTICS ---------------------------------------------------------------------------------18
2.1
Introduction -----------------------------------------------------------------------------------------18
2.2
Fresh water inflows and catches-----------------------------------------------------------------18
2.2.1
Introduction ---------------------------------------------------------------------------------------18
2.2.2
Methods -------------------------------------------------------------------------------------------18
2.2.3
Results ---------------------------------------------------------------------------------------------19
2.2.4
Discussion-----------------------------------------------------------------------------------------22
2.3
Commercial Catch, Effort and CPUE----------------------------------------------------------24
2.3.1
Overview of Statewide Catches ----------------------------------------------------------------24
2.3.2
Catch and Effort by Sector ----------------------------------------------------------------------27
2.3.3
Discussion-----------------------------------------------------------------------------------------37
3
OTHER RESEARCH-------------------------------------------------------------------------------------39
3.1
Introduction -----------------------------------------------------------------------------------------39
3.1.1
Size-structure and net selectivity ---------------------------------------------------------------39
3.1.2
Reproduction--------------------------------------------------------------------------------------42
3.1.3
Discussion-----------------------------------------------------------------------------------------44
4
GENERAL DISCUSSION -------------------------------------------------------------------------------46
4.1
Introduction -----------------------------------------------------------------------------------------46
4.2
Synopsis of information ---------------------------------------------------------------------------46
4.2.1
Status of resource and fishery ------------------------------------------------------------------46
4.3
Future research needs and potential performance indicators -----------------------------48
5
REFERENCES---------------------------------------------------------------------------------------------51
i
ACKNOWLEDGEMENTS
Funds for this study were provided by PIRSA Fisheries, SARDI Aquatic Sciences and
the University of Adelaide.
Fisheries statistics and maps were provided by SARDI Aquatic Sciences, and we
thank Malcolm Knight, Angelo Tsolos, Emily Thompson and Annette Doonan for
their assistance. Margot Sachse (AFMA) provided aggregated information on the
Commonwealth shark fishery in South Australia. We are grateful to Suzanne Bennett
and Craig Schillabeer (SARDI Aquatic Sciences) for their help with the literature
searches. Special thanks to Brenton Erdmann and Joanna Oborne of the Department
of Land, Water and Biodiversity Conservation for providing modelled flow data for
the Murray River. We also thank the staff of SA Water at the Goolwa and Mundoo
barrages for their assistance in obtaining fishery-independent size frequency data.
SAFCOL also provided bench space and help at the market.
We are grateful to LCF fishers and recreational anglers for their help collecting
biological data.
We thank Dr Qifeng Ye, Neil Wellman, Matt Pellizzare, Jason Higham, David Short
and David Fleer who’s cooperation made the fieldwork possible.
Professor Anthony Cheshire, Drs Keith Jones, Qifeng Ye and Simon Bryars (SARDI
Aquatic Sciences) and Mr Sean Sloan (PIRSA Fisheries Policy Group) reviewed a
draft of this report.
ii
EXECUTIVE SUMMARY
1
This is the first report on mulloway, Argyrosomus japonicus to be included in
SARDI’s Fishery Assessment Series.
2
This report (1) synthesises information available for the fishery, (2) assesses the
current status of the resource, and (3) identifies future research and management
needs.
3
Few data are available on the fisheries biology of A. japonicus in South Australia,
however extensive data on age and reproduction are being collected as part of a
current PhD study.
4
The South Australian commercial catch of mulloway in 2001/02 was 114 tonnes,
which was 20% above the most recent 5-year average of 95 tonnes.
5
The Lakes and Coorong Fishery (LCF) is the dominant sector in the South
Australian commercial mulloway fishery and contributed 95.7% of the total
commercial catch in 2001/02. It is a multi-species fishery, and targets mulloway
in the Coorong lagoons and adjacent ocean beaches.
6
The commercial mulloway fishery is seasonal, with most of the catch taken in the
warmer months and is characterised by marked inter-annual variability.
7
The dominant gears used to target mulloway in the LCF are: large-mesh gill nets
(≥70 mm mesh) used in the Coorong lagoons (size range of mulloway 460 – 800
mm TL, modal size 550 mm TL) and, swinger nets (150 mm mesh) used to target
large mulloway that aggregate at the Murray Mouth in spring-summer (size range
800 – 1300 mm TL, mode 1000 mm TL).
8
The Marine Scalefish Fishery (MSF) contributed 4% of the state catch, in
2001/02, mostly from the Coorong region. There are additionally 2258
recreational small mesh gill nets registered that have access to the Coorong
lagoons area.
9
Patterns of catch and effort in the recreational fishery are poorly understood,
although state-wide estimates of numbers caught have been provided recently in
the National Recreational and Indigenous Fishing Survey (2003).
iii
10 Mulloway are also taken as by-catch in the Commonwealth shark fishery. There
are anecdotal reports of juvenile mulloway being taken as by-catch in the LCF
and the recreational small mesh gill net fishery. The methods used in the LCF to
reduce by-catch have been documented recently. However, the levels of
incidental mortality in the small mesh gill net fishery are yet to be quantified.
11 Preliminary selectivity’s have been estimated for large mesh gill nets and swinger
nets used in the LCF. For large mesh gill nets the modal length was 525 mm TL
with a range of 460 – 822 mm TL (n=458). For swinger nets the range was 7571268 mm TL (n=31) and for females was 801-1245 mm TL (n=39). The modal
length for each sex was 975 mm TL.
12 The most recent five year average CPUE(TEF) (catch-per-unit-effort where effort
was targeted at mulloway) for large mesh gill nets in the LCF was
42 kg.manday-1 while for swinger nets it was 49 kg.manday-1. This was higher
than occurred in the 1980’s for both types of gear. Inter-annual variability in
CPUE was high for both types of gear also.
13 The most recent five year average CPUE(TEF) for line fishing (fishing pole) in the
MSF was 27 kg.manday-1 which was higher than occurred in the previous decade.
For MSF shark nets the most recent 5 year average CPUE(TOS) (CPUE where
effort was targeted at species other than mulloway) was 31 kg.manday-1 which
was also higher than in previous years.
14 The use of CPUE as an index of relative abundance is complicated by the
possible effects of freshwater flows on the recruitment and aggregation of fish in,
and adjacent to, the Coorong. Additionally factors are market influences and
relocation of effort in the multi-species LCF and MSF fisheries.
15 No performance indicators are currently prescribed for mulloway in South
Australia. Potential indicators include CPUE, age structure, sex ratio, and recruit
abundance. A plan is currently being developed by PIRSA fisheries in
consultation with the Inland Fisheries Management Committee.
16 Females grow more quickly than males and reach a size where they are
vulnerable to the swinger nets at a younger age. Due to this difference in
catchability between males and females, sex ratio may provide a useful
iv
performance indicator for changes in the age structure. The sex ratio from
swinger net samples in 2002/03 was 0.55 (nf/(nf + nm), n=80), indicating that
females dominated catches.
17 There is a strong need for additional data on the age structure of sexually mature
fish from recreational line and commercial swinger net catches. These data may
provide a useful potential performance indicator for the fishery. Contraction of
the age range may indicate a reduction in adult biomass and the relative
abundance of particular ages may indicate the relative strength of year classes.
v
1
1.1
GENERAL INTRODUCTION
Overview
This is the first report on mulloway, Argyrosomus japonicus, taken from South
Australian waters and is the first in SARDI’s Fishery Assessment Series. It includes
data from a PhD study that began in October 2001. The aims of the report are (i) to
provide a comprehensive synopsis of information available for this species; (ii) assess
the current state of the resource, and (iii) to identify future research and management
needs for the fishery.
The first section is the General Introduction that: (i) outlines the aims and structure of
the report; (ii) describes the history of the mulloway fishery in South Australia; (iii)
describes the existing management arrangements for the fishery (iv) provides a
synopsis of biological and ecological information for mulloway and (v) summarises
previous stock assessments and research on mulloway.
Section two provides a synopsis of data on freshwater inflows to the Coorong lagoons
and the fishery statistics for mulloway for the financial years from 1983/84 to
2001/02. This section describes inter-annual and intra-annual patterns in catch, effort
and catch-per-unit-effort (CPUE) in the Lakes and Coorong Fishery (LCF) and
Marine Scalefish Fishery (MSF).
Section three provides a summary of research conducted since October 2001 as part
of the PhD project entitled “The Biology and Ecology of Mulloway in South
Australian Waters.” This includes some preliminary data on net selectivity, the size
structure of catches and reproductive biology.
Section four uses information from sections two and three to assess the status of the
fishery and to identify future research needs. The aim of this section is to provide
sufficient information to develop relevant Biological Performance Indicators and
Reference Points and to identify areas of uncertainty.
The final (fifth) section is the bibliography which provides a list of research papers
and reports that are directly relevant to research and management of mulloway in
South Australia and that are cited in this report.
1
1.2
Description of the Fishery
1.2.1 Location and Size
Mulloway are caught throughout South Australian waters but the main commercial
catch is taken from the Coorong lagoons and adjacent coastal waters (Figure 1-1).
Similarly, most recreational catches are also taken within the Coorong lagoons and in
the nearshore zone along the South Australian coast during the warmer months.
Small catches are also taken as by-catch by shark fishers
ens
r
dne
Gair
Western
Australia
e
La k
Lake
Everard
FOWLERS BAY
##
# CEDUNA
# SMOKY BAY
PORT AUGUSTA
New S outh W ales
#
#
STREAKY BAY
WHYALLA
ELLISTON
COWELL
# PORT BROUGHTON
#
Rive
i ver Mur
ray
#
#
PORT
LINCOLN
#
# ADELAIDE
EDITHBURGH#
VICTOR
HARBOR
Legend
#
#
Vic toria
KINGSCOTE
Towns
#
Highways
Secondary Roads
# KINGSTON SE
80
0
80
160 Kilometers
Map Projection: Equidistant Conic
#
ROBE
#
BEACHPORT
PORT MACDONN
#
Figure 1-1 Map of South Australia showing locations of commercial mulloway fishery. (Red
circles show where mulloway are caught; their size reflects the contribution to the state
catch).
1.2.2
Environmental Characteristics
The larvae and juveniles of A. japonicus, in southern Australia, appear to have a
requirement for hyposaline water (Hall 1986; Gray et al 1990). Adults are found in
nearshore surf zones and occasionally in estuaries along the coast including the Port
and Glenelg Rivers and the Coorong lagoons.
The Coorong is the remnant estuary of the Murray River and is the largest area of
estuarine habitat in South Australia. It comprises two shallow (1-2 m depth) hyper
saline lagoons about 100km long and 2-3km wide that are separated from the ocean
by a 1-2 km wide barrier of sand dunes (Hall 1984; Geddes 1987; Boon 2000). The
2
lagoons are connected to the open ocean by the Murray Mouth, which can be up to
300 metres wide but can close completely during prolonged periods of drought (Hall
1984).
The main sources of water for the Murray River are winter rains and snow in the
Great Dividing Ranges of eastern Australia. Natural flows in the lower river usually
peak during spring to early summer (Newman 2000). The Murray River flows into
the Coorong lagoons via a series of barrages that were constructed in 1940. Below
the barrages, the remnant estuary, which prior to 1940, included Lakes Alexandrina
and Albert, is now confined to the Coorong Lagoons. Hence, only 11% of the natural
estuary area remains and, with the reduced freshwater input, seawater dominates the
conditions (Jensen et al 2000; Newman 2000). Additionally, because salinity in the
Coorong is determined by flows through the barrages, conditions can change abruptly
from saline to fresh water conditions and back again in unseasonal and unnatural
patterns (Geddes and Butler 1984; Geddes 1987; Geddes and Hall 1990; Newman
2000).
1.2.3
Commercial Fishery
Fishing for mulloway has been undertaken in the lower Murray Lakes and Coorong
since early European settlement. The commercial fishery for mulloway is comprised
of the LCF and the MSF. Small catches are also taken by rock lobster and shark
fishers.
The LCF is a multi-species fishery in which mulloway is one of the key species.
Fishers in the LCF also have access to freshwater species in Lakes Alexandrina and
Albert and effort may be transferred between the fresh and salt water environments
and between species. Members of the LCF fish for mulloway in the Coorong lagoons
and on the adjacent oceanic beaches (Figure 1-2). Prior to 1986, beach seines were
used in Areas 6-8 (Hall 1986). Now the primary gear is the large mesh monofilament
gill net. These nets are required, by legislation, to have >150 mm mesh and are used
to target mulloway (>460 mm TL) within the Coorong lagoons during the warmer
months. Larger mulloway (>750 mm TL) are targeted in the nearshore zone of the
beaches along the Younghusband Peninsula, using swinger nets. These gill nets are
required to have >150 mm mesh. Each is attached to several hundred meters of rope
3
and allowed to drift out through the surf with the aid of the offshore “rip” and is then
carried ashore by longshore drift and manual hauling.
Marine-scale fishers target mulloway along South Australia’s ocean beaches, in
particular the area adjacent to the Coorong and on the far west cost (Figure 1-3). The
dominant gear is reported as the fishing pole, although haul nets and shark nets are
also used.
4
WELLINGTON
N
W
MILANG
"F
E
S
# RAPID BAY
#
Ba
ck
s
KINGSCOTE
NEPEAN
BAY
LAKE
ALEXANDRINA
CLAYTON
GOOLWA
F
##
VICTOR HARBOR
"F
6
Murray 15
Mouth
4
"
7
F
"#
#
NARRUNG
LAKE
ALBERT
8
tai
rs
#
Pa
ss
PENNESHAW
ag
e
The Pages
ENCOUNTER
BAY
5
9
Mark Point
#
#
Long Point
MENINGIE
"F
10
#
Rob's Point
D'ESTREES
BAY
11
#
Hell's Gate
12
# WOOD'S WELL
F
" POLICEMAN'S POINT
13
SALT CREEK
16 "F
14
PORTS OF LANDING
Lakes and Coorong Fishery
"F
Ports of Landing
4
9
Lakes and Coorong Commercial Fishing Areas
0
9
18
27
36 Kilometers
Map Projection: Equidistant Conic
"F
KINGSTON SE
Figure 1-2 Map of central South Australia showing the Coorong Lagoons and Murray River. (Block numbers are LCF area codes)
5
3
2
1
Fowlers
Bay
#
# Ceduna
9
8
# Port
10
12
14
Bay
Whyalla #
16
13
21
17
Venus
Bay
15
23
20
18
19
27
24
25
Augusta
11
# Streaky
Ne w Sou th W ales
7
6
5
4
29
26
Port Lincoln #
28
31
30
22
32
35
33
34
36
#
Adelaide
43
40
38
39
Goolwa
#
41
42
44
Port of Landing
Marinescale Fishery
#
Ports of Landing
10
Marinescale Fishing Blocks
!+
47
48
49
52
53
50
45
46
V icto ria
37
51
# Robe
Roads
60
0
60
120 Kilometers
54
55
57
56
Map Projection: Equidistant Conic
Port
Macdonnell
#
58
Figure 1-3 Map of South Australia showing Marine Scalefish Fishery reporting blocks
6
1.2.4
Recreational Fishery
Mulloway is an icon species for land-based recreational anglers. They are valued for
their large maximum size, fighting ability, good eating qualities and availability from
beaches (Hall 1986; Jones et al 1990). Recreational fishers use surf rods to target
mulloway on the ocean beaches near the Coorong and on the far west coast during
December-January (Hall 1986). Recreational net and line fishers also operate within
the Coorong lagoons (Hall 1986).
1.3
Management of the Fishery
Total commercial catches of mulloway in South Australia have been recorded since
1951 (Hall 1986). Estimates of effort have been collected since 1983 with fishers of
both the LCF and MSF reporting effort in terms of target species and days fished
(Knight et al. 2001).
The broad statutory framework for ecologically sustainable management of this
resource is provided by the Fisheries Act 1982 (currently under review). The
regulations that govern the MSF are described in the Fisheries (General) Regulations
2000. The specific regulations that govern the LCF are established in the Scheme of
Management (Lakes and Coorong Fishery) Regulations 1991. The Inland Fisheries
Five Year Strategic Research and Monitoring Plan is currently being prepared.
1.3.1
Management Milestones
Management arrangements have evolved since the inception of the fishery. The
commercial mulloway fishery was reviewed in 1986 when a number of measures,
including restrictions on netting were established (Hall 1986).
An environmental management plan, based on qualitative information, documented
the intentions of fishers to promote fishing techniques that minimise the by-catch of
juvenile and undersize fish in the LCF (Pierce and Baker 1998). A further qualitative
study described methods for reducing the interaction of fishing gear with mulloway
and other species within the LCF (Anon. 2002). A review of the recreational fishery
was completed in 1997 and a management strategy was developed that identified
environmental health and sustainability of fish stocks as key objectives (Anon. 1995).
Major milestones are listed in Table 1-1.
7
Table 1-1 Management milestones for the mulloway fishery in South Australia. (Jones et al
1990; Rohan et al 1991)
Date
Milestone
1971
Introduction of fishing licences for all commercial fishing in South
Australia
1972
Licensed commercial fishers required to provide monthly catch data
1977
Freeze on commercial marine scalefish licences to stabilise fishing
effort
1980
“Owner/ operator” policy introduced. Class A net endorsement limited
to 1 net with a maximum length of 600m.
1982
Non-transferability of net endorsements to reduce fishing effort.
1984
Scheme of Management (Marine Scale Fishery) Regulations.
Scheme of Management (Lakes and Coorong Fishery) Regulations.
1986
Increase in legal minimum length from 460 to 750 mm TL in all waters
except the Coorong.
Restrictions on commercial net type, mesh size, net depth and net
length.
Prohibition of net use adjacent to the Murray Mouth from November 1
to March 31.
Limit of one registered recreational net per person, with 70m total
length and maximum of 1m drop.
Total prohibition on recreational netting in coastal marine waters from
Goolwa Beach Road to Kingston Jetty.
Recreational bag limit of 3 fish per person per day for fish >750 mm
TL taken in Coorong or coastal marine waters.
Recreational limit of 10 fish per person per day in Coorong waters
Non-transferrable quota of 1000 kg of mulloway for commercial MSF
and lobster fishers on the west coast, west of Cape Carnot.
Prohibition of all forms of netting in the Coorong, adjacent to the
Murray Mouth from December 25 to January 7 (Anon. 1988).
1994 (Sept)
Licence amalgamation scheme introduced to reduce number of licensed
participants in the commercial fishery
1995 (May)
Changes to Coastal Marine Net Fishing regulations result in ban on
recreational net fishing in coastal marine waters.
1.3.2
Current Management Arrangements
Commercial Fishery
The LCF is limited entry with 37 owner-operators (Knight et al 2000). Licence
holders have non-exclusive access within the Lakes and Coorong system and effort is
8
limited through gear entitlements and owner-operator provisions as designated under
the Lakes and Coorong Fishery Scheme of Management (Pierce and Doonan 1999;
Knight et al 2000). To monitor catches, fishers must complete monthly records of
catch, effort (days) and fishing location. The level of detail and nature of effort
reported in these returns is currently under review.
Size limits are in place with a Legal Minimum Length (LML) within the waters of the
Coorong of 460 mm TL, and in all other waters of 750 mm TL. In addition to size
limits the commercial fishery is managed by restrictions to gear type, and area and
time closures. Mesh nets are subject to restrictions on length, drop and mesh size.
The part of the Coorong lagoons designated as Area 1 (Figure 1-4) has a number of
additional restrictions to commercial fishing:
•
•
•
•
•
No net fishing from 25 December – 7 January;
No small mesh monofilament set nets, or small mesh haul nets from 1
November to 31 March;
Total length of small mesh nets not to exceed 400 m;
Large mesh nets must not exceed a depth of 2 m, or 33 meshes, and total
length must not exceed 500 m;
•
Total length of large mesh monofilament hauling net not to exceed 240 m;
•
No commercial fishing during weekends in Goolwa channel;
•
Year round commercial fishing ban within 500 m of the Murray Mouth;
Length restriction of mulloway same as for rest of Coorong (>460 mm
TL).
Fishers in MSF do not have access to the Coorong lagoons and are subject to the LML
of 750 mm TL. MSF monthly returns detail species, catch and effort in days
Recreational Fishery
The recreational fishery is open access and has the same size limits as the commercial
fishery in addition to size specific bag and boat limits. Netting by recreational fishers
is permitted within the Coorong and there are approximately 2258 licences with
access to the Coorong. These are subject to restrictions on net size, net numbers and
mesh size as well as area and time closures. Coorong (Area 1) is closed to
recreational netting between 1 November and 31st March inclusive, and there are also
provisions in effect for nets to be attended (Figure 1-4).
9
A
N
W
E
e
R iv
r
Mu
y
rr a
S
#
WE LLING TON
MILANG #
ML
I A NG
LA KE
ALE XA ND RINA
er
iv
R
4
nn
Fi
si
s
CLAY TON
#
CLAYTO N
GO O LW A #
GOO LWA
#
NAR RUNG
HIN D MA R S H IS LA N D
#
VICT O R HARB OR
#
Area (1 )
15
#
Yo
M ur ray M o uth
EN C OU N TE R
BA Y
G nu rlun g
P oint
un
gh
#
us
ba
nd
8
Pe
n in
su
LA K E
AL B ER T
la
#
5
9
EN C O U N TE R
BAY
Lak es and Co oron g
0
5
10
# ME NING IE
Area (2 )
10
15 K ilom e ter s
Map Pro je ction : E quid ist ant C on ic
Figure 1-4 Map of Coorong showing Lakes and Coorong Fishery management Areas 1 (red) and 2 (green)
10
1.3.3
Management Objectives and Strategies
There are few management objectives and strategies identified for mulloway in
particular. Implicit management objectives and strategies are shown in Table 1-2.
Table 1-2 Implicit general management objectives and strategies that apply to mulloway .
Objective
Protection of spawning stock
Strategy
Harvest above size at maturity to provide
adequate levels of recruitment.
Currently, 750 mm TL size limit for fish
caught outside the Coorong lagoons.
1000 kg limit on catches of mulloway for
MSF and lobster licences on West coast
Reducing conflict between sectors
Area 1 closures
Prohibition of commercial fishing within
500m of the mouth of the Murray mouth
1.3.4
Performance Indicators and Reference Points
No reference points or biological performance indicators are prescribed specifically
for mulloway at the current time. However, potential biological performance
indicators and reference points are currently being investigated for the Management
Plan for the LCF (Sean Sloan, PIRSA, pers. com.).
1.4
1.4.1
Fisheries Biology of Mulloway
Taxonomy and Distribution
Sciaenids are found around the world and are large predatory fishes that are often
associated with estuaries (Whitfield 1999). One such sciaenid is the mulloway
(Argyrosomus hololepidotus Lacepéde, 1802), which was recently reclassified as
Argyrosomus japonicus (Temminck & Schlegel, 1843)(Griffiths and Heemstra 1995).
This species occurs in both northern and southern hemispheres. It is distributed from
the east coast of South Africa (Griffiths and Heemstra 1995), along the Chinese coast
from Hong Kong northwards to southern Korea and Japan and the south coast of
11
Australia (Starling 1992; Kailola et al 1993). In southern Australia, it occurs from
North West Cape in Western Australia to the Burnett River in Queensland (Kailola et
al 1993). In South Australia juveniles are most commonly found in estuaries whereas
adults are found mainly in the high-energy surf zone (Jones et al. 1990; Kailola et al.
1993).
1.4.2
Stock Structure
In southern Australia, several genetic methods have been employed in two separate
studies to investigate the structure of the mulloway stock. (Dixon 1988) used
electrophoresis on small samples from several localities, and concluded that there
were 2 sub-populations of mulloway around Australia: one in Western Australia from
Carnarvon to Mandurah and the other from South Australia to New South Wales.
Further genetic differences were found between samples from Western Australia, the
Great Australian Bight, the Coorong and New South Wales suggesting the possibility
of further population sub-structuring (Dixon 1988).
(Black and Dixon 1992) conducted a further study using two methods: (i) direct
sequencing of mitochondrial DNA and (ii) isoelectric focussing. The first approach
supported the idea of a single interbreeding population from Sydney to the west coast
of Australia, although this was “by no means a certainty” (Black and Dixon 1992).
The second approach, isoelectric focussing, also failed to support the earlier findings
of (Dixon 1988), although there was significant inter-locality heterogeneity between
the Coorong and NSW. Interpreting the results of this study was difficult as
differences between replicate samples were as large as those between localities.
Genetic techniques are far from ideal for the task of understanding the geographic
structure of fish populations: they do not detect differences in the face of even low
levels of larval or adult mixing between populations (Hoff and Fuiman 1993).
Additionally, they cannot measure rates of individual change between sites or specify
the origin of individuals.
1.4.3
Life History
Considerable work has been done on the early life-history stages of A. japonicus in
South Africa. In this region it spawns throughout the year, with a peak in winter-
12
spring (Wallace 1975; Smale 1985). A large proportion of adults appear to migrate
along the South African coastline to spawn in warmer waters near East Cape
(Griffiths 1996). As onset of spawning is correlated with water temperature,
migration to warmer waters has been suggested as a strategy to permit earlier
spawning (Conover 1992).
In South Africa Argyrosomus japonicus is thought to spawn in the near-shore
environment and large numbers of eggs have been collected within 1 m of the surface
off the Kwa Zulu/Natal coast of Southern Cape, South Africa (Smale 1985). Early
juveniles recruited to estuaries at four weeks old (20-30 mm TL) and utilised turbid
parts of the upper estuaries where salinities were lowest (Griffiths 1996). Early
juveniles from 20–150 mm TL were found exclusively in estuarine habitats, which
may be a strategy to avoid con-specific predation. Larger juveniles (>150 mm TL)
were observed in the surf zone although never in the rest of the nearshore zone.
Tagging studies in South Africa have shown that juveniles rarely move great
distances, despite periods of liberty of up to 1713 days. This also appears to be the
case in Western Australia (Anon. 1993; Griffiths 1996). Spawning areas in South
Australia are unknown, although spent and spawning fish have been observed at the
Murray mouth and on the west coast during spring-summer (November-February)
(Hall 1986; Gray et al. 1993).
In particular, adult mulloway (800 – 1500 mm TL) aggregate in the surf zone adjacent
to the Murray mouth in October–December. This appears to be related to fresh water
outflow (Hall 1984) and may be a spawning aggregation, or a pre-spawning feeding
aggregation. Larval development may occur at sea with juveniles entering the
Coorong several months later at about 150mm TL (Hall 1986; Geddes 2000).
Juvenile mulloway utilize the Coorong for 2-5 years (Wallace 1975; Hall 1986;
Geddes 2000).
1.4.4
Growth and Size at Maturity
One South African study found that the size at maturity (L50) was 920 and 1070 mm
TL for males and females respectively (Table 1-3) (Griffiths and Hecht 1995). The
only estimate of size at maturity for A. japonicus in South Australia was is 700 mm
TL although separate estimates were not made for males and females (Hall 1986). The
13
marked difference between the size at maturity of males and females in South Africa
suggests that gender specific estimates are also needed for South Australia.
The best estimates for von Bertalanffy growth parameters, based on sample sizes and
separation of the sexes were from South Africa (Griffiths and Hecht 1995). In this
study the Linf for males and females was 1372 and 1472 mm TL respectively (Table
1-4). An earlier South African study estimated much lower values for K, and higher
values of Linf but these were probably less reliable due to smaller sample sizes
(Wallace and Schleyer 1979). Hence, size at maturity (L50) in South Africa was 67%
of the Linf for males and 73% for females (Griffiths and Hecht 1995).
The estimate of growth rate, K, for A. japonicus in South Australia is less than half
that for the South African study but is based on estimates of age obtained from scales
rather than otoliths. Scales may provide unreliable estimates of age, especially in
older fish, consequently, growth parameters for A. japonicus in South Australia
should be re-estimated for each sex using otoliths to estimate age.
Table 1-3 Size at maturity for Argyrosomus japonicus
Location
Sex
Size at Maturity
(L50, mm TL)
n
(Hall 1986; Jones et
al 1990)
South Australia
M&F
700
?
(Griffiths and Hecht
1995)
South Africa
M
920
521
(Griffiths and Hecht
1995)
South Africa
F
1070
441
Source
Table 1-4. von Bertalanffy growth parameters for Argyrosomus japonicus
Source
Sex
Method
t0
K
Linf (mm TL)
n
M&F
scales
0.862
0.116
1795
?
(Griffiths and Hecht
1995)
M
otoliths
-0.428
0.260
1372
262
(Griffiths and Hecht
1995)
F
otoliths
-2.620
0.228
1472
257
M&F
otoliths
0.33
0.0303
1720
148
(Hall 1986)
(Wallace and
Schleyer 1979)
14
The life-history strategy appears to be one of rapid growth, delayed maturity and
prolonged longevity that leads ultimately to maximised individual egg production
(Griffiths 1996). Griffiths (1996) postulated that A. japonicus evolved its life history
strategy as a result of rates of natural mortality that are lower than for other Sciaenid
species. The implication is that juveniles may experience lower natural mortality i.e.
by utilising nursery areas, or employing other predator avoidance strategies.
1.5
Stock Assessment
1.5.1
Commercial Fishery
Two reports have assessed the status of the mulloway stock in South Australia. Hall
(1986) provided biological information, analysed catch and effort data and examined
the relationship between catches and freshwater flows. Most of this work was
conducted within the Coorong lagoons. The key points were as follows:
•
•
•
•
•
•
•
trends in CPUE appeared to be similar to trends in freshwater flows;
declines in commercial catches of mulloway from 1975 to 1985 resulted from
redirection of effort to Lakes Alexandrina and Albert;
there were two size classes present in haul net samples, probably representing
two year classes;
juvenile mulloway were caught as by-catch in small mesh gill nets (2 inch, 50
mm) during winter (May-October).
the instantaneous rate of mortality (Z) for juvenile fish within the Coorong
was estimated at 0.8, although Hall (1986) noted that the assumption of no
immigration/emigration was violated;
mesh selectivity was estimated for monofilament gill net meshes: 51, 64, 76,
89, and 102 mm;
a legal minimum size limit LML of 460 mm TL was recommended for
mulloway caught within the Coorong lagoons and 750 mm TL for elsewhere.
More recently, Pierce and Doonan (1999) documented a qualitative assessment of the
sustainable harvest status of several species in the LCF. These authors estimated the
mulloway stock in South Australia to be fully-exploited, but didn’t explain the basis
upon which this conclusion was reached.
15
1.5.2
Recreational Fishery
Results from the National Recreational and Indigenous Fishing Survey suggested that,
in South Australia, between May 2000 and April 2001, the total recreational catch of
mulloway was 90.2 tonnes (Henry and Lyle 2003). This comprised 27 004 mulloway
(± 5156 SE) with an estimated mean weight was 3.34 kg (Henry and Lyle 2003).
The mortality of mulloway returned to the water by recreational fishers is unknown,
mulloway were ranked second amongst South Australian species in terms of their
susceptibility to catch and release mortality (McLeay et al 2002). However, one study
that investigated juvenile mulloway that were released, after being mouth hooked and
taken from shallow depth, found that they experienced minimal deleterious effects
(Broadhurst and Barker 2000). Mortalities of line caught and released adult
mulloway taken from high energy beaches are unknown (McLeay et al 2002).
1.6
Discussion
Argyrosomus japonicus is widely distributed, however, within Australian waters, the
structure of the stock is poorly understood. Investigations utilising a combination of
tag- recapture, regional comparisons of otolith morphology/microchemistry and
parasite tags may provide greater understanding (Hall 1986; Black and Dixon 1992;
Begg et al 1998; Lester et al 2001). An international study into the taxonomic
relationships within the genus Argyrosomus is currently being conducted by the
Department of Marine and Coastal Management in South Africa and an FRDC funded
project, through Murdoch University, is investigating the stock structure of mulloway
within Australian waters.
Little is known of the reproductive biology of mulloway (Hall 1984) and additional
information on the reproductive strategy and mode from studies at the cellular level
would provide estimates of the spawning fraction and spawning frequency to give a
clearer understanding of fecundity. The location of spawning in South Australia is
poorly understood as is the contribution to the stock of that proportion of the
population that utilise the Coorong as juvenile habitat. Larval surveys conducted
adjacent to the mouth of the Murray River, at the time of peak gonadal development
and peak flow, could confirm the location of at least one spawning ground in South
Australia.
16
Size at maturity and growth parameters have been estimated for both male and female
A. japonicus in South Africa and South Australia. However estimates from South
Australia were done by combining data from both sexes. Also the von Bertalanffy
growth function parameters, estimated for A. japonicus in South Australia, were made
using scales to estimate age. Scales tend to give poor age estimates for older fish and
consequently the South African estimates, made using ages derived from otoliths, are
likely to be more reliable. There is a clear need for accurate estimates of growth
parameters, using ages estimated from otoliths, for A. japonicus in South Australia.
Information on the age, growth and reproductive development is needed to understand
the life history of mulloway and the capacity to sustain fishing. These data are being
collected as part of a current PhD study.
17
2
FISHERY STATISTICS
2.1
Introduction
This section presents fishery statistics for mulloway in South Australia, from 1983/84
to 2001/02, as well as examining the relationship with freshwater flows into the
Coorong lagoons during this period. It describes inter-annual and intra-annual
patterns in catch, effort and CPUE in the LCF and MSF sectors. Estimates of catch
are also provided from the National Recreational and Indigenous Fishing Survey.
2.2
2.2.1
Fresh water inflows and catches
Introduction
Freshwater flows into estuarine systems have been related to catches of many fishes,
including mulloway (Bebars and Lasserre 1983; Hall 1986; Smetacek 1986; BarreraGuevara 1990). Changes to the freshwater flow following dam construction, have
been implicated as causal agents of changes to invertebrate and fish community
structures (Benson 1981; Bebars and Lasserre 1983) as well as the decline of several
fisheries for sciaenids. The construction of the locks and barrages across the Murray
River have had a profound effect on the aquatic environment of the lakes and
Coorong lagoons (Noye 1974; Hall 1986; Geddes and Hall 1990; Pierce 1995;
Griffiths 1996).
2.2.2
Methods
Monthly commercial catch and effort data for mulloway were available from June
1984. Modelled daily flow data (MSM BigMod) for the Murray River were obtained
from the Murray-Darling Basin Commission. Datasets of mean monthly flow
incorporating time lags from 0-84 months were constructed and used to generate a
correlogram to identify appropriate time lags linking these data (Chatfield 1996;
Campana 1999). Peaks in the value of the correlation coefficient were used to
identify time lags which resulted in the best fit between the two datasets.
Catch and effort data were available for mulloway in South Australia from 1983 to the
present. CPUE where the effort was targeted at mulloway (CPUE(TEF)) yielded the
most consistent results and therefore were used for correlation of flows and catches.
18
2.2.3
Results
The primary periodicity of both peak catches and peak freshwater inflow to the
Coorong lagoons was approximately 12 months. For large mesh nets, values of the
correlation coefficient increased sequentially for lags of 41, 53 and 63 months (Figure
2-1a). The highest value of the correlation coefficient was with a time lag of 63
months. The trend for targeted CPUE(TEF) i.e. CPUE calculated where effort is
targeted at mulloway, was very similar to that for catch. In general correlation
coefficients were higher for freshwater flow data and CPUE(TEF), than for freshwater
flow and catch.
Correlation coefficients for swinger net catches were high for modelled flow data with
no time lag (Figure 2-1).
0.6
a.
0.4
Correlation coefficient
0.2
0.0
-0.2
-0.4
0.6
b.
0.4
0.2
0.0
Catch
CPUE
-0.2
-0.4
0
5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85
Lag (months)
Figure 2-1 Correlation coefficients for mean monthly freshwater inflow to the Coorong
lagoons with time lags from 0 – 85 months and (a) total monthly catch (blue) and CPUE(TES)
(red) for mulloway from large mesh net catches within the Coorong Lagoon and, (b) total
monthly catch (blue) and CPUE(TES) (red) for swinger net catches on the Younghusband
Peninsula.
19
Catch (kg *1000)
35
30
25
20
15
0.0
10
0
3.0
5
100
80
60
40
20
0
Large mesh gill net catch (t)
Flow (GL)
Large mesh gill net CPUE(TES)
Jul-84
Oct-84
Jan-85
Apr-85
Jul-85
Oct-85
Jan-86
Apr-86
Jul-86
Oct-86
Jan-87
Apr-87
Jul-87
Oct-87
Jan-88
Apr-88
Jul-88
Oct-88
Jan-89
Apr-89
Jul-89
Oct-89
Jan-90
Apr-90
Jul-90
Oct-90
Jan-91
Apr-91
Jul-91
Oct-91
Jan-92
Apr-92
Jul-92
Oct-92
Jan-93
Apr-93
Jul-93
Oct-93
Jan-94
Apr-94
Jul-94
Oct-94
Jan-95
Apr-95
Jul-95
Oct-95
Jan-96
Apr-96
Jul-96
Oct-96
Jan-97
Apr-97
Jul-97
Oct-97
Jan-98
Apr-98
Jul-98
Oct-98
Jan-99
Apr-99
Jul-99
Oct-99
Jan-00
Apr-00
Jul-00
Oct-00
Jan-01
Apr-01
Jul-01
Oct-01
Jan-02
Apr-02
3.0
2.5
-1
Mean flow (gigalitres.month )
2.0
1.5
1.0
0.5
2.5
2.0
1.5
1.0
0.5
0.0
20
Figure 2-2 Coorong Lagoon large mesh gill net fishery: Mean monthly freshwater inflow (lagged by 53 months) superimposed over (a) Catches of mulloway
and, (b) CPUE(TES).Inter-annual Patterns
-1
CPUE (kg.manday )
-1
Catch (kg *1000)
CPUE (kg.manday )
10
8
6
4
0.0
2
0
3.0
3.0
2.5
-1
Mean flow (gigalitres.month )
2.0
1.5
1.0
0.5
2.5
2.0
1.5
1.0
0.5
0.0
21
200
150
100
50
0
Swinger net catch (t)
Flow (GL)
Swinger net CPUE(TES)
Figure 2-3 Younghusband Peninsula swinger net fishery: Mean monthly freshwater inflow (no lag) superimposed over (a) catches of mulloway and, (b)
CPUE(TES)
Jul-84
Oct-84
Jan-85
Apr-85
Jul-85
Oct-85
Jan-86
Apr-86
Jul-86
Oct-86
Jan-87
Apr-87
Jul-87
Oct-87
Jan-88
Apr-88
Jul-88
Oct-88
Jan-89
Apr-89
Jul-89
Oct-89
Jan-90
Apr-90
Jul-90
Oct-90
Jan-91
Apr-91
Jul-91
Oct-91
Jan-92
Apr-92
Jul-92
Oct-92
Jan-93
Apr-93
Jul-93
Oct-93
Jan-94
Apr-94
Jul-94
Oct-94
Jan-95
Apr-95
Jul-95
Oct-95
Jan-96
Apr-96
Jul-96
Oct-96
Jan-97
Apr-97
Jul-97
Oct-97
Jan-98
Apr-98
Jul-98
Oct-98
Jan-99
Apr-99
Jul-99
Oct-99
Jan-00
Apr-00
Jul-00
Oct-00
Jan-01
Apr-01
Jul-01
Oct-01
Jan-02
Apr-02
Jul-02
Oct-02
Jan-03
Apr-03
Plots of mean monthly flow data, incorporating a 53 month time lag, overlaid on plots
of mean monthly catches from large mesh gill nets, showed that annual peaks in both
datasets exhibit a periodicity of around 12 months (Figure 2-2a). The trend in the
magnitude of peaks in mean monthly catches appears to follow that for modelled flow
data although this is more evident in the plot of CPUE(TEF) (Figure 2-2b). Periods of
high CPUE were: December 1993 to June 1994, September 1994 to March 1995,
December 1996 to April 1997 and September 1997 to June 1998. These align with
periods of high freshwater flows 53 months prior. Periods of high flow also appeared
to be related to periods of CPUE(TEF) four years later. These periods of high
CPUE(TEF) may last for three years. High flows in summer of 1993/94 were followed
by high CPUE in the summer of 1997/98. High CPUE continued during the
following summer fishing season although fresh water inflow had declined.
Catches for swinger nets appeared to align with those of modelled flow data where no
lag was employed. Mean monthly flow (no lag) versus swinger net catches showed
peaks that appeared to show some correlation in both datasets (Figure 2-3a). Mean
monthly flow (no lag) versus swinger net CPUE(TEF) showed a similar trend (Figure
2-3b).
2.2.4
Discussion
Peak values for the correlation coefficients for lagged flow data versus commercial
large mesh gill-net catches imply that mulloway begin to recruit to large mesh gill
nets approximately three years after a freshwater inflow event. The appearance of
three increasingly higher peaks in the correlation coefficient implies that, following a
freshwater inflow, catches may remain high for three years until exploitation or
migration result remove individuals from the Coorong lagoons. Any effects on
mulloway catches from freshwater inflow may also be compounded by oceanic events
such as upwelling of oceanic water onto the continental shelf (Schahinger 1987).
Preliminary ageing work (PhD study) supports this with most fish caught in large
mesh gill nets aged 3, 4 or 5 years. It is likely that as mulloway grow the catchability
in relation to commercial fishing gear will change also. Thus, the vulnerability of the
2-5 year olds would be expected to increase with age. This will be validated later
with further ageing of samples, size age model (von Bertalanffy Growth function) and
net selectivity estimates.
22
Both the periodicity and magnitude of lagged freshwater inflow appears to relate to
that for CPUE. For example in the five years from July 1990 to July 1995 there are 3
years of smaller inflows followed by two years of increased inflow (Figure 2-2). The
CPUE in the first 3 years is relatively low then increases as flows increased.
Freshwater inflow may affect juvenile habitat in a number of ways i.e. reduced
salinity, increased turbidity or enhanced food supply for iliophagous prey species.
Further work will be done as part of the PhD study to characterise this relationship
and to further understand variations in the relationship over different periods of time.
Increased numbers of juveniles recruiting to the fishing gear, some years after a major
flow event may be a result of increased recruitment success. Freshwater inflow may
act to promote recruitment success by: creating low salinity environments where
juveniles are protected from predators; increasing turbidity which may provide
protection from visual predators, and food for smaller herbivorous or iliophagous prey
species; providing a high growth environment for juveniles; providing attractant flow
for larvae and early juveniles to locate favourable habitat.
In the case of the commercial swing net catch the correlation coefficients were high
for mean monthly flow datasets with no time lag. Adult mulloway aggregate at the
Murray Mouth, attracted by the flow of freshwater, and probably food. If this event is
a spawning aggregation then flow events may result in increased spawning success.
Freshwater inflow may promote spawning success by: providing an attractant flow
causing adult mulloway in spawning condition to aggregate, which may, in turn, place
larvae close to suitable nursery habitat; providing a pre-spawning food source as adult
mulloway enter spawning condition.
23
2.3
2.3.1
Commercial Catch, Effort and CPUE
Overview of Statewide Catches
Inter-annual trends
The total South Australian commercial catch of mulloway in the 2001/02 financial
year was 114 tonnes (Figure 2-4). This was 20% above the most recent 5 year
average (95 t) although less than the catch in 2000/01 (145 t) which was the highest
recorded catch. The catch of mulloway is characterised by marked variability among
years.
160
Catch
Catch (kg *1000)
140
5 Year Average
120
100
80
60
40
20
2001/02
1999/00
2000/01
1998/99
1997/98
1996/97
1994/95
1995/96
1993/94
1992/93
1991/92
1989/90
1990/91
1988/89
1987/88
1986/87
1984/85
1985/86
1983/84
0
Figure 2-4 Statewide commercial catches of mulloway for financial years 1983/84 to 2001/02
Catches remained below 60 tonnes.yr-1 between 1984/85 through 1992/93 whilst the
1993/94 catch was almost double that of most previous years although catches
declined again between 1994/95 through 1997/98.
Reasons for variability in the statewide catch may be: (i) changes in abundance of
mulloway in fished areas which appears to be related to freshwater inflow; (ii)
transfer of effort to other species or; (iii) a combination of these factors.
24
Intra-annual patterns
The fishery for mulloway, in South Australia, is seasonal and operates mainly in
Catch (kg *1000)
spring and summer (Figure 2-5).
35
2001/02
30
2000/01
25
1999/00
20
1998/99
15
1997/98
10
5
Jun
Apr
May
Mar
Jan
Feb
Oct
Nov
Dec
Aug
Sep
Jul
0
Figure 2-5 Monthly commercial catches of mulloway (Shaded area on x-axis represents
seasons).
During 2001/02, 90 tonnes, representing 79% annual statewide catch of mulloway
was taken between October and February. The financial years, 1997/98 through
1999/00 showed similar seasonality, although with lower catches. In contrast, the
period of peak catches in 2000/01 occurred in summer-autumn, from January to April.
Annual value
Average monthly prices paid for mulloway at South Australian fish processors were
Catch
Value
01-02
00-01
99-00
98-99
97-98
96-97
95-96
94-95
93-94
92-93
900
800
700
600
500
400
300
200
100
0
Catch value (x $1000)
160
140
120
100
80
60
40
20
0
91-92
Catch (kg *1000)
available from 1991/92 to 2001/02 (Figure 2-6).
Figure 2-6 Annual catches of mulloway from 1991/92 through 2001/02
25
These figures adequately represent the production value for fish sold within South
Australia although not the proportion sold on the Sydney or Melbourne markets
(Baker and Pierce 1998; Knight et al 2001). The proportion of the state catch of
mulloway that was sent to the Sydney Fish Market in 2000/01 and 2001/02 was 13.4
and 15.7% respectively.
Overall, the trend in the value of the commercial mulloway catch, for each financial
year from 1991/92 to 2001/02 followed that of the catch. Minor peaks in value,
during the mid and late 1990’s, were followed by a record catch value of $800 000 in
2000/01. Lower catches in 2001/02 resulted in a decline in value to $714 000.
The average price increased from $4.74 per kg, in 1991/92, to $6.26 per kg in 2001/02
(Figure 2-7). The decline in average price during 2000/01 coincided with
01-02
00-01
99-00
98-99
97-98
96-97
95-96
94-95
93-94
92-93
8
7
6
5
4
3
2
1
0
91-92
Average price ($ per kg)
significantly increased supply to the market in that year.
Figure 2-7 Average price ($ per kg) (±SD) for mulloway in South Australia from
1991/92 through 2001/02The average monthly price ($ per kg) for catch sold in the
Adelaide Fish Market during 2001/02 was negatively correlated to catch, i.e.
decreased linearly as catch increased (Figure 2-8).
The average monthly price ($ per kg) for catch sold in the Adelaide Fish Market
during 2001/02 was negatively correlated to catch, i.e. decreased linearly as catch
increased (Figure 2-8).
26
6
4
y = -0.0844x + 7.0675
2
R2 = 0.878
35
30
25
20
15
10
5
0
0
Value ($ per kg.year -1)
8
Catch (kg *1000)
Figure 2-8 The relationship between average price per kg.year-1 and total catch of mulloway
in South Australia.
2.3.2
Catch and Effort by Sector
The commercial fishery for mulloway in South Australia comprises four main types
of licence holders: (i) LCF, (ii) MSF, (iii) Southern and Northern Zone Rock Lobster
Fisheries, and (iv) “B” licence holders (restricted marine scale licence). For the
purposes of this stock assessment the catches from (iii) and (iv) are combined as
“other”.
Catch and effort data have been collected since 1983/84. Over this period the LCF
has been the main contributor to the mulloway catch in South Australia (Figure 2-9).
160
Catch (kg *1000)
140
120
100
80
60
Other
40
MSF
20
LCF
01-02
00-01
99-00
98-99
97-98
96-97
95-96
94-95
93-94
92-93
91-92
90-91
89-90
88-89
87-88
86-87
85-86
84-85
83-84
0
Figure 2-9 Annual commercial catches of mulloway in South Australia, subdivided by the type
of licence holder.
From 1984/85 through 1997/98 the LCF caught between 66-84% of the state catch
and from 1998/99 to 2000/01 this contribution was greater than 90%. In 2001/02 this
27
fishery caught 95.7 % of the total catch of mulloway for the state with the MSF
contributing 4%.
Lakes and Coorong Fishery
Overall, catches from the LCF have increased since 1992/93, although with strong
inter-annual variability. The dominant gear type, in terms of landed weight, was the
large mesh gill net, which accounted for 90% of the annual catch (Figure 2-10a). The
second highest contribution was from swinger nets (8% in 2001/02) on the ocean
beach of the Younghusband Peninsula. Contributions by other gears were negligible.
In general, trends in catches by large mesh gill nets were similar to those of effort
(Figure 2-10b). Peak effort occurred in 2000/01 which was also the year of highest
recorded catches. However, in the previous year, high levels of effort did not yield
similarly high catches.
Trends in catch from the swinger nets (Figure 2-10c) were generally similar to those
from the gill nets with lower catches and effort throughout the 1980’s, increased catch
in the 1990’s and a peaks in both catch and effort in 2000/01.
Because trends in effort followed those of catch in most years it appears that when
mulloway were abundant targeted effort also increased. This implies that for those
years when CPUE also increased there was a real increase in quantity of mulloway
available to the fishery.
CPUE is reported for three effort types: (i) no specific target (NST), (ii) targeted
effort (TEF), and (iii) targeting other species (TOS). The dominant effort type for
large mesh gill nets was TEF. CPUE (TEF) increased from a low in 1987/88 to a peak
in 1993/94 (Figure 2-10d). After this CPUE(TEF) continued to follow the trend in
catches with an increase in 1998/99, a decline in 1999/00 and increase in 2000/01.
Overall, CPUE(TEF) remained high from 1993/94 onwards. The most recent five year
average CPUE(TEF) was 42 kg.manday-1.
CPUE(NST) and CPUE(TOS) for large mesh gill nets followed the trends in CPUE(TEF)
but did not show an increase in 1993/94.
28
CPUE(TEF) for the swinger nets (Figure 2-10e) has tended to increase over time but
was relatively low after 1998/99. The most recent five year average was 49
kg.manday-1.
CPUE(TEF) provides the best estimate of relative abundance currently available and
the overall trend was for an increase from the beginning of the 1990’s that has been
maintained until the present. It should be noted, however, that inter-annual variability
is relatively high.
Details of catches for other gear are shown in Table 2-1.
29
160
a
140
120
100
80
Other
60
Small mesh gill net
40
Sw inger net
Large mesh gill net
20
0
2500
2000
60
1500
40
1000
20
500
0
0
100
2500
c
80
2000
60
1500
40
1000
20
500
0
84-85
85-86
86-87
87-88
88-89
89-90
90-91
91-92
92-93
93-94
94-95
95-96
96-97
97-98
98-99
99-00
00-01
01-02
0
d
60
40
20
0
80
e
60
40
20
0
84-85
85-86
86-87
87-88
88-89
89-90
90-91
91-92
92-93
93-94
94-95
95-96
96-97
97-98
98-99
99-00
00-01
01-02
80
80
CPUE (kg.man day-1)
b
Effort (man days)
Catch (kg *1000)
100
Catch
No Specific Target
Effort
Targeted Effort
Targeting Other Species
Figure 2-10 Annual catch, effort and CPUE for mulloway from the Lakes and Coorong
Fishery in South Australia: (a) Catch by gear type, (b) Large mesh gill net catch and effort, (c)
Swinger net catch and effort, (d) CPUE by 3 effort types for large mesh gill net and (e) CPUE
for swing net.
30
Table 2-1 Details of the annual catch and effort, for mulloway for 4 main gears in the Lakes
and Coorong Fishery. The other category includes drum nets, set lines and hand lines,)
Small mesh gill net Large mesh gill net
FinYr
1983/84
1984/85
1985/86
1986/87
1987/88
1988/89
1989/90
1990/91
1991/92
1992/93
1993/94
1994/95
1995/96
1996/97
1997/98
1998/99
1999/00
2000/01
2001/02
Catch
Effort
Catch
Effort
Swinger net
Catch
Effort
Haul net
(< 7cm mesh)
Catch
Effort
Other
Catch
TOTAL
Effort
Catch
Effort
(kg *1000) (M Days) (kg *1000) (M Days) (kg *1000) (M Days) (kg *1000) (M Days) (kg *1000) (M Days) (kg *1000) (M Days)
2.23
0.70
0.56
0.29
0.08
0.56
0.23
0.12
0.08
0.22
0.02
0.17
0.03
0.28
0.20
6.99
0.09
1.54
73
76
47
31
16
40
41
21
24
20
10
37
8
14
14
64
6
84
32.04
26.56
23.41
11.18
22.34
31.31
34.55
33.38
21.06
50.38
52.32
34.58
39.09
34.51
70.94
47.60
111.51
98.15
1474
1864
1714
1086
1915
1446
1435
1905
1235
1145
1303
1363
1055
1110
1594
2290
2502
2648
0.49
0.01
1.69
0.35
0.36
0.04
2.80
4.96
5.05
16.31
11.88
12.14
8.16
3.11
11.38
11.34
24.07
8.36
19
1
42
29
16
1
66
113
156
335
272
174
177
58
167
299
517
226
2.16
1.20
0.61
0.70
0.24
0.30
0.11
0.77
0.00
0.00
2.03
0.56
1.02
1.74
2.80
1.96
57
204
81
176
57
11
8
16
2
2
19
29
54
42
29
55
0.44
23
0.07198
0.05546
0.06254
0.01888
0.02242
0.05546
0.40732
0.35046
1.51704
1.5399
1.01126
0.00472
0.69722
0.17156
0.20374
1.37716
0.11092
0.75992
4
12
4
3
15
3
8
19
46
76
40
3
25
6
10
46
6
25
37.00
28.53
26.33
12.55
23.04
32.27
38.09
39.58
27.71
68.45
67.27
47.46
49.00
39.81
85.52
69.26
135.78
109.25
31
1627
2157
1888
1325
2019
1501
1558
2074
1463
1578
1644
1606
1319
1230
1814
2754
3031
3006
Marine Scale Fishery
The Marine Scale Fishery was divided into four regions: (i) Coorong (MFA’s 44, 45,
46, 51); (ii) Spencer Gulf (MFA’s 11, 20, 21, 22, 23, 30, 31, 32, 33); (iii) Gulf St.
Vincent (MFA’s 34, 35, 36, 40, 42, 43) and; (iv) the West Coast (MFA’s 1, 3, 7, 8, 9,
10, 27, 28). Catches from each region are presented in Figure 2-11. From 1992/93
through 2000/01 catches by the MSF sector were taken mainly from the Coorong
region. In 2001/02 catches from the Coorong comprised 41% of the total for this
sector, with the other main region being Gulf St. Vincent (38%).
20000
18000
16000
Catch (kg)
14000
12000
10000
West coast
8000
Spencer Gulf
6000
South-east
4000
Gulf St. Vincent
2000
Coorong
83-84
84-85
85-86
86-87
87-88
88-89
89-90
90-91
91-92
92-93
93-94
94-95
95-96
96-97
97-98
98-99
99-00
00-01
01-02
0
Figure 2-11 Annual regional catches of mulloway in the Marine Scale Fishery.
Overall, catches increased from 1992/93 to 1995/96 then declined to the present
(Figure 2-10). The main gear prior to 1993/94 was the gill net. Following restrictions
to the use of nets in most years (Figure 2-11a) fishing pole and shark net became the
dominant gears.
Gill net catches increased to 1992/93 then declined. Gill net effort declined steadily
from 1983/84 to 1988/89 then remained low until 1999/00 (Figure 2-11b). This was a
result of legislation in 1982 resulting in non-transferability of net endorsements on
licences.
Line licences remained transferable after legislative changes in 1982 and fishing pole
catches increased from 199/91 onwards (Figure 2-11c) as gill net catches declined.
Fishing pole effort increased to a peak in 1993/94 before declining to the present.
32
Shark net catches remained relatively low form 1983/84 through 190/91 then
increased to an all time peak in 1995/96 (Figure 2-11d) before declining steeply.
Shark net effort generally followed the trends in shark net catches.
Gill net CPUE(TEF) increased steeply from 1989/90 until 1992/93 then declined to
1994/95 (Figure 2-11e). CPUE(NST) increased from 1985/86 to 1990/92. Effort
levels were generally much lower in later years.
Fishing pole CPUE(TEF) was first reported in 1989/90 and generally increased to the
present with a peak in 1997/98 with several periods of sharp decline where catches
were relatively low. The most recent five year average CPUE(TEF) was
27 kg.manday-1.
CPUE(TOS) for shark net catches showed a strong increase from 1991/92 to the
present with the most recent five year average 31 kg.manday-1.
Details of catches for each of the other regions in South Australia are shown in
33
Table 2-1. These data are summarised by gear in Table 3-2.
Other Fisheries
Several Southern Zone and Northern Zone Rock Lobster licences have endorsements
for taking marine scalefish. During the peak catches in 1995/96 this amounted to
several tonnes but in 2001/02 was only 52 kg in the Southern Zone and 3.5 kg in the
Northern Zone.
Mulloway are taken from estuaries in Victoria by recreational fishers but they are not
of commercial interest, i.e. less than 7 kg in 2001/02 (Morrison pers. com.).
34
20
a
15
10
Other
Gill net
5
Shark net
Fishing pole
0
10
0
1000
c
800
10
600
8
6
400
4
200
2
0
0
d
10
0
Effort (mandays)
80
f
60
40
20
0
1000
80
800
60
g
600
8
6
40
400
4
200
2
0
0
83-84
84-85
85-86
86-87
87-88
88-89
89-90
90-91
91-92
92-93
93-94
94-95
95-96
96-97
97-98
98-99
99-00
00-01
01-02
Catch (kg * 1000)
20
200
0
12
60
40
2
14
800
e
400
4
12
80
600
8
6
14
1000
20
0
83-84
84-85
85-86
86-87
87-88
88-89
89-90
90-91
91-92
92-93
93-94
94-95
95-96
96-97
97-98
98-99
99-00
00-01
01-02
12
b
CPUE (kg.manday-1)
14
Catch
No Specific Target
Effort
Targeted Effort
Targeting Other Species
Figure 2-12 Catch, effort and CPUE for mulloway from the Marine Scale Fishery in the
Coorong region of South Australia: (a) Catch by gear type, (b) gill net catch and effort, (c)
fishing pole catch and effort, (d) shark net catch and effort, (e) CPUE(TEF and NST) for gill net, (f)
CPUE(TEF) for fishing pole and (g) CPUE(TOS) for shark net
35
Table 2-2 Details of the annual catch and effort, for mulloway from all five regions for the
Marine ScaleFishery.
West Coast
FinYr
1983/84
1984/85
1985/86
1986/87
1987/88
1988/89
1989/90
1990/91
1991/92
1992/93
1993/94
1994/95
1995/96
1996/97
1997/98
1998/99
1999/00
2000/01
2001/02
Catch
Effort
Spencer Gulf
Catch
Effort
Gulf St. Vincent
Catch
Effort
Coorong
Catch
Effort
South East
Catch
Effort
TOTAL
Catch
Effort
(kg *1000) (M Days) (kg *1000) (M Days) (kg *1000) (M Days) (kg *1000) (M Days) (kg *1000) (M Days) (kg *1000) (M Days)
2.12
5.45
1.59
1.32
0.71
0.19
1.23
0.41
0.40
1.04
0.13
1.06
0.83
1.02
0.37
0.06
0.33
1.53
0.55
403
199
147
257
49
14
62
24
40
45
14
53
42
42
17
7
82
36
119
1.00
1.07
0.96
3.15
3.37
2.14
1.72
0.81
1.13
1.15
0.06
0.31
0.11
54
270
146
233
231
120
198
49
73
69
4
19
8
0.02
0.15
0.07
0.98
0.39
5
52
42
59
21
1.99
3.98
3.09
3.21
1.63
1.73
4.11
2.46
2.26
0.37
0.17
0.95
0.68
1.20
0.64
0.54
0.67
1.53
1.77
426
525
397
379
250
213
260
155
237
45
73
98
78
172
26
56
49
149
89
11.13
2.77
0.46
1.76
2.45
2.47
3.87
3.19
2.90
11.70
12.00
11.14
16.16
6.71
7.34
7.73
1.80
4.66
1.85
871
475
343
310
255
143
198
186
230
959
742
892
954
966
323
255
120
131
86
1.12
0.20
14
9
2.53
3
0.01
18
0.03
0.24
0.16
0.04
0.04
33
54
58
10
14
17.36
13.47
6.10
9.43
10.70
6.53
10.93
6.87
6.70
14.26
12.39
13.71
17.94
8.98
8.40
8.48
2.86
8.71
4.56
1768
1478
1033
1179
788
490
718
414
598
1118
866
1116
1140
1190
385
370
293
375
315
Total
catch
Total
effort
Table 2-3 Details of the catch and effort, for mulloway in the Marine Scale Fishery, for the
four main gears. The “other’ category is mainly hand line catches.
Gill net
FinYr
1983/84
1984/85
1985/86
1986/87
1987/88
1988/89
1989/90
1990/91
1991/92
1992/93
1993/94
1994/95
1995/96
1996/97
1997/98
1998/99
1999/00
2000/01
2001/02
Catch
Haul net
Effort
Catch
Effort
Fishing pole
Catch
Effort
Shark net
Catch
Effort
Other
Catch
Effort
(kg *1000) (M days) (kg *1000) (M days) (kg *1000) (M days) (kg *1000) (M days) (kg *1000) (M days) (kg *1000) (M days)
4.50
1.49
1.25
2.36
1.94
1.70
2.77
3.22
2.01
3.39
2.44
0.65
0.60
0.65
0.10
0.30
0.54
616
479
407
349
160
97
110
104
65
62
102
85
28
34
14
15
37
0.12
10
3.77
4.24
2.88
5.00
7.36
3.47
6.09
2.55
2.58
1.34
0.05
0.57
0.22
1.30
0.36
0.82
0.10
1.88
1.08
366
702
389
497
397
305
415
150
244
89
52
68
44
112
3
16
61
171
78
0.09
0.32
0.27
0.51
4.08
6.97
5.04
3.08
1.01
5.31
4.08
0.31
3.33
1.14
1
59
37
88
217
328
300
197
107
170
164
38
102
33
7.20
4.33
0.85
1.36
0.52
0.25
1.21
0.18
0.35
3.39
2.52
6.10
13.25
4.82
1.66
2.87
0.97
0.83
0.82
642
200
176
266
178
42
90
12
119
651
341
597
814
881
146
141
124
12
144
1.89
3.41
1.11
0.71
0.87
1.01
0.54
0.65
1.25
2.05
0.40
1.34
0.78
1.19
0.97
0.42
0.95
2.67
1.41
144
97
61
67
53
45
44
111
82
99
43
66
57
56
52
34
33
90
50
17.36
13.47
6.10
9.43
10.70
6.53
10.93
6.87
6.70
14.26
12.39
13.71
17.94
8.98
8.40
8.48
2.86
8.71
4.56
36
1768
1478
1033
1179
788
490
718
414
598
1118
866
1116
1140
1190
385
370
293
375
315
Mulloway as by-catch
Mulloway are taken as by-catch by several fishery sectors. Adult mulloway are taken
in shark nets off the South Australian coast although there is a trip limit of 100 kg.
This component of the by-catch is documented in the Commonwealth Shark Fishery
commercial log. Quantities reported in logbooks for shark gill net were 5733 and
2691 kg in 2000/01 and 2001/02 respectively Figure 2-13. In these years, greater than
80% of the by-catch came from the area between western Kangaroo Island and the
Victorian border. In the years 1997/98 and 1998/99 the contribution from this region
was over 98%. In 2001/02, by-catch of mulloway from this sector represented about
3.2% of the catch from the South Australian state fishery although it was less than this
in all other years.
There is also the potential for mulloway to be taken as by-catch in small mesh gill
nets used to target mullet in the commercial and recreational fisheries operating in the
7000
6000
5000
4000
3000
2000
1000
0
Region
(longitude)
141-146 E
136-141 E
133-136 E
01-02
00-01
99-00
98-99
129-133 E
97-98
Catch (kg)
Coorong.
Figure 2-13 Catches of mulloway taken as by-catch in the Commonwealth Shark Fishery
from 1997/98 through 2001/02.
There is also anecdotal evidence that some mulloway are taken in the Gulf St. Vincent
Prawn Fishery. However, there are no data available for mulloway by-catch from this
fishery.
2.3.3
Discussion
The fishery for mulloway in South Australia is located primarily in the Coorong
lagoons and adjacent ocean beaches. Catches increased during the 1990’s but are
37
characterised by inter-annual variability. CPUE also showed marked inter-annual
variability. The LCF is the dominant sector and has accounted for greater than 90%
of the statewide catch in most years. The large mesh gill net is the dominant gear
(90% of catch) and is used to target smaller mulloway in the Coorong lagoons (see
Section 3). Larger mulloway were targeted by LCF fishers with swinger nets which
contributed approximately 8% of the annual catch.
Mulloway are also caught with fishing poles and shark nets by the MSF, and in most
years the bulk of this catch is taken in the Coorong region. However, this sector
contributes less than 5% of the statewide catch. Commonwealth shark fishers also
take mulloway as by-catch in South Australia with most of this also coming from the
Coorong region. Mulloway is also a popular target for recreational line fishers.
Hall (1984; 1986) identified a possible link between freshwater flows and subsequent
catches of mulloway which has also been supported by current research. This,
coupled with aggregation of mulloway means that a level of uncertainty must be
attached to CPUE as a measure of relative abundance for this species.
While uncertainty occurs in the magnitude of the recreational statewide catch,
preliminary information is now available from the National Recreational and
Indigenous Fishing Survey. Additional unreported by-catch from the Commonwealth
Shark Fishery may also be significant, although shark prices are generally higher than
for mulloway and it is unlikely that this fishery targets mulloway (Sachse pers. com.).
Catches of mulloway tend to follow effort in both the MSF and LCF although the
extent to which effort drives catch is unclear. Mulloway prices also decline linearly
in relation to the amount of catch placed on the relatively small Adelaide market and a
proportion of the catch is sold interstate. A better understanding of effort within this
multi-species sector may require a dedicated study of the influences driving effort
transfer between target species and between salt and fresh water fishing grounds.
Further work is also required to better understand the effect of freshwater inflow into
the Coorong lagoons and its effect on CPUE and spawning and recruitment success.
38
3
OTHER RESEARCH
3.1
Introduction
This section provides a summary of current research work carried out since October
2001 as part of a PhD project titled “The Biology and Ecology of Mulloway in South
Australian Waters.” This includes some preliminary data on net selectivity, size
structure and reproduction.
3.1.1
Size-structure and net selectivity
Introduction
Net selectivity’s for mulloway have not been estimated for gears used in the LCF,
MSF and recreational sectors since the mid 1980’s (Hall 1986). Preliminary size
distributions for several sizes of commercial and research nets, along with recreational
line catches are presented here.
Methods
Size structure data were obtained from several sources: (i) Adelaide fish market, (ii) a
fishery independent survey using multi-panel monofilament gill nets, (iii) haul nets,
and (iv) recreational line catches.
Samples from the Adelaide fish market originated from the Coorong lagoons and
were taken by the LCF, using large mesh monofilament gill nets. Additional size data
from swinger net catches were collected at the port of landing.
Mulloway were also collected in multi-panel gill nets within the Coorong lagoons.
These nets were 45 m long and 2 m deep and comprised five, 9 m long panels with
stretched mesh measurements of: (i) 40 mm; (ii) 50mm; (iii) 70 mm; (iv) 113 mm
and; (v) 153. These samples of mulloway were frozen and later thawed, measured
and dissected at the SARDI Aquatic Sciences laboratories, West Beach.
Data were also available from one commercial haul net catch from the west coast and
from recreational line fishers, who provided fish from adjacent to the Murray Mouth,
the Port River and Yorke Peninsula.
39
For each of these where there were sufficient data, length frequency histograms were
constructed. The modal length was estimated as the point midway between the upper
and lower limits of the dominant length class.
Results
Size structures from four types of fishing gear are shown in Figure 3-1. The modal
length (mm TL) of mulloway taken from large mesh gill nets was 525 mm TL (Figure
3-1a). The range was from slightly below the legal minimum size (LML) of 460 mm
TL to 822 mm TL.
For mulloway from swinger net catches, it was possible to segregate TL’s by sex.
The range was 757 to 1268 mm TL for males and 801 to 1245 mm TL for females
(Figure 3-1b). The modal length for both was 975 mm TL while the sex ratio
(nf/(nf+nm)) was 0.55.
Both haul net and line catches showed lower size selectivity when compared with gill
nets (Figure 3-1c,d). The haul net catch had a size range of 712 – 1191 mm TL while
that for recreational line catches was 400 to 1438 mm TL. The modal TL of 825 mm
was the same for both.
Fishery independent mesh selectivity’s for 50 and 70 mm (stretched) monofilament
meshes are shown in Figure 3-2. Selectivity’s for other meshes (40, 113, 153) are not
shown due to small sample sizes. The modal TL for mulloway caught in the 50 mm
mesh was 225 mm TL and the range was 126 –391 mm TL. For the 70 mm mesh the
modal size was 375 mm TL and the range was 207 to 480 mm TL.
40
150
a.
20
(n = 458)
c.
(n=46)
d.
(n=31)
15
100
10
Frequency
50
5
0
20
15
10
0
6
b.
5
Males
Females
(nm=31, nf =39)
4
3
2
5
1
400
450
500
550
600
650
700
750
800
850
900
950
1000
1050
1100
1150
1200
1250
1300
1350
1400
1450
400
450
500
550
600
650
700
750
800
850
900
950
1000
1050
1100
1150
1200
1250
1300
1350
1400
1450
0
0
Total Length (mm)
Figure 3-1 Size structures of mulloway for (a) large mesh gill net catches within the Coorong
(M, F combined), (b) swinger net catches from Younghusband Peninsula (M,F separate), (c)
haul net catch from west coast (M, F combined) and (d) recreational line fishery (M, F
combined).
30
60
20
40
10
20
0
0
b.
0
50
100
150
200
250
300
350
400
450
500
550
600
650
700
80
a.
0
50
100
150
200
250
300
350
400
450
500
550
600
650
700
Frequency
40
Total Length (mm)
Figure 3-2 Preliminary net selectivity’s for mulloway for net meshes (a) 50 mm, and (b) 70
mm
41
3.1.2
Reproduction
Introduction
Sexually mature mulloway aggregate at the mouth of the Murray River during the
warmer months where fresh water flows appear to act as an attractant. Some
information on the reproduction of mulloway was collected as part of a PhD study.
However, due to the current drought these reproductive data are limited. Therefore
the information presented here should be viewed as preliminary.
Methods
Mulloway were collected in multi-panel gill nets within the Coorong lagoons on a
monthly basis. These nets were 45 m long and 2 m deep and comprised five, 9m
mesh panels: (i) 40 mm; (ii) 50mm; (iii) 70 mm; (iv) 113 mm and; (v) 153. Mulloway
were also collected from the swinger net catches at the port of landing and from line
fishers operating at the Murray mouth from October 2001 to January 2002. The
ovaries were classified macroscopically into one of five stages of development based
on size, colour and visibility of oocytes (Table 3-1) (Hunter et al 1985; Fowler and
McGarvey 1997; Ferguson 1999).
Ovaries from stages III to V were grouped as mature. More detailed analysis of a
subset of ovaries, for which the process of maturation had started, was achieved by
histological preparation and microscopic examination. A segment was removed from
the centre of one ovary lobe and preserved in a fixative of formalin, acetic acid and
calcium chloride (FAACC). After fixing, the segment was stained with haematoxylin
and eosin, sectioned (6-7 mm) and mounted for microscopic examination. Testes
were classified into three classes only; undeveloped, developing and running ripe
(testes released sperm when gently squeezed).
42
Table 3-1 Stages of development used for macroscopic classification of the ovaries of
mulloway. The macroscopic characteristics of these stages were determined by examination
of histological preparations (Hunter and Macewitz 1985; Fowler and McGarvey 1997))
Stage
Stage I
Immature
Stage II
Developing
Stage III
Developed
Stage IV
Gravid or
Running Ripe
Stage V
Regressing or
Resting
Macroscopic Appearance
Microscopic Appearance
Small, clear to translucent, jelly-like
thread, grey to pink in colour
Close to posterior vertebral column
Oocytes invisible to naked eye
Unyolked, non atretic oocytes only
Small to medium
Translucent to yellow/brown
Ovary lobe may appear short, relative to
stage III
Oocytes invisible to naked eye
All unyolked.
Large, yellow to orange
Granular appearance due to visible
individual oocytes.
Ovary large, reaching forward into
anterior gut cavity
Dominated by advanced yolked oocytes,
unyolked with partially yolked oocytes present
in low numbers.
Lumen large and obvious.
Very large orange, reaching anteriorly
to/above stomach
Clear hydrated oocytes visible among
opaque oocytes
Oocytes of all stages present from unyolked to
hydrated.
Lumen large, obvious.
Ovaries medium in size, brown to
reddish-brown, opaque.
More flaccid than other stages
Atretic oocytes present.
Partially yolked oocytes present.
Greater number of unyolked oocytes relative to
yolked than in stage III/IV ovaries.
Results
None of the mulloway (n=251, range 70 – 660 mm TL) collected in multi-panel gill
nets within the Coorong, during the warmer months (October – March) of 2001/02
showed signs of gonad development or regression i.e. they were not sexually mature.
This sample included 31 fish that were between 400-660 mm TL.
For samples from the commercial swinger net fishery the smallest female to have
developed (stage III) ovaries i.e. with ovaries that held yolked eggs, was 882 mm TL
while the smallest female to exhibit regressing, (stage V) ovaries was 889 mm TL
(Figure 3-3).
The smallest male to show gonadal development i.e. running ripe, was 757 mm TL. It
should be noted that there were few data for either males or females in the size range
650 to 800 mm TL.
43
6
(n=75)
5
4
GONAD STAGE
Female
1 Immature
2 Developing
3 Developed
4 Hydrated
5 Regressing
Gonad stage
3
2
1
4
Male
1 Immature
2 Developing
3 Running ripe
(n=60)
3
2
0
100
200
300
400
500
600
700
800
900
1000
1100
1200
1300
1400
1
Total length (mm)
Figure 3-3 Gonad development stages of female (red) and male (blue) mulloway from
October – January 2001/02 from Younghusband Peninsula, South Australia.
3.1.3
Discussion
None of the mulloway taken within the Coorong lagoons during the summer period
showed gonad development, which suggests that mulloway <660 mm TL are not
sexually mature. The smallest female to show ovarian development and yolked
oocytes was 882 mm TL. This is consistent with reports of mulloway from the
Coorong lacking gonad development, which is also the situation in South African
estuaries (Wallace 1975; Hall 1986).
However, samples of mulloway in the size class between 660 and 800 mm TL were
not collected during this period, hence the size range where the gonads first develop is
not well defined. More data are required to construct maturity ogives (LD50) to
provide accurate estimates of size at maturity for mulloway in South Australia.
The modal size of mulloway taken in large mesh gill nets from within the Coorong
lagoons was 525 mm TL, with most of the catch smaller than 700 mm TL. Thus
44
mulloway taken by large mesh gill nets are sexually immature. The modal size of
mulloway taken in swinger net catches was 950 mm TL, which is above the size of
sexual maturity.
Data were presented for the net selectivity for mulloway in 50 mm mesh. The modal
size of mulloway taken in the 50 mm mesh panels was 225 mm TL (range 126 –391
mm TL) which is consistent with that found by (Hall 1986).
45
4
GENERAL DISCUSSION
4.1
Introduction
This section provides a synopsis of information available for the fishery, assesses the
current status of the resource, and identifies future research and management needs.
This section also aims to provide information required to establish Biological
Performance Indicators and Reference Points for the fishery.
4.2
Synopsis of information
Few data are available on the biology and ecology of mulloway in Australia.
Information from overseas, especially South Africa, suggests that the life-history
strategy of A. japonicus is one of rapid growth and delayed sexual maturity (Griffiths
1996). Data on the age, growth and reproduction of mulloway in South Australia is
needed urgently and is being collected as part of a current PhD project.
The stock structure of A. japonicus in southern Australia is also poorly understood. In
the absence of information to the contrary, the South Australian mulloway population
should be managed as if it is a self-sustaining stock, with recruitment dependent on
successful local spawning and high levels of egg production.
4.2.1
Status of resource and fishery
The prolonged juvenile stage of mulloway and the implied low natural mortality of
juveniles may make this species vulnerable to anthropogenic impacts, especially
perturbations to juvenile habitat and high juvenile fishing mortality.
Information available for assessing the status of mulloway in South Australia is
presented in Sections 2 and 3 of this report. Like all fisheries assessments, the data
and estimates presented in these sections have high associated levels of uncertainty.
This uncertainty originates from several sources.
One source of uncertainty is the lack of information on the stock structure of A.
japonicus in southern Australia. However, the apparent dependence of juveniles on
estuarine/hyposaline environments, combined with the relative paucity of such
environments in South Australia and the very high proportion of South Australian
46
catch that is taken in the LCF, suggest that the Coorong lagoons must be treated as
critical habitat for mulloway in South Australia.
One of the most significant potential sources of uncertainty in this assessment is its
reliance on fishery-dependent data. One of the major constraints with the fishery data
is the lack of information from several sectors. Comprehensive catch and effort data
are reported for the MSF and LCF only. Only retained catches are reported in
logbooks of the Commonwealth shark fishery and no data are available on the bycatch of mulloway in South Australian prawn trawl fisheries. The recreational catch
of mulloway in South Australia is poorly understood although some regional
information will become available from the National Recreational and Indigenous
Fishing Survey.
As most of the South Australian mulloway catch is taken in the LCF, this assessment
is fundamentally dependent on the consistency and accuracy of LCF logbook data,
which has not been examined empirically. Furthermore, the mixed nature of this
fishery means that non-biological factors, such as the availability of and prices paid
for other LCF species, and not just mulloway abundance, can affect mulloway
catches. The strong relationship between mulloway catch and price could also bias
the analyses of catch and effort data.
As the LCF primarily targets juveniles, relatively few data are available on the adult
population. Adults are taken by a small number of fishers using swinger nets on the
ocean beach near the Murray Mouth. However, those catches are tightly linked to the
level of freshwater flows and do not provide a reliable indication of the abundance of
adult mulloway in South Australia. Information on age structure of these catches
would provide information on the status of the adult population, and will be obtained
during a current PhD study. However an ongoing monitoring program for monitoring
the age structure of these catches should also be established.
Despite the limitations identified above, catch and CPUE from the LCF do appear to
provide a useful indicator of the abundance of juvenile mulloway in the Coorong
lagoons. Levels of effort in this fishery have been relatively stable over recent years.
Fluctuations in catch and CPUE appear to mainly reflect inter-annual variations in
recruitment associated with fluctuations in levels of freshwater input, with strong
freshwater flows associated with catch and CPUE levels 3-5 years later. This could
47
be due to high levels of spawning success resulting from the aggregation of spawning
adults at the Murray Mouth or from increased survivorship of early juveniles in the
Coorong lagoons during periods of reduced salinity.
Assessing the current status of the South Australian mulloway resource against
historical levels is impeded by the lack of detailed catch and effort information prior
to 1983. However, Hall (1986) provides a summary of total catches since 1951. The
decline in annual catches of mulloway following construction of the barrages (Noye
1974; Hall 1986; Eckert and Robinson 1990; Geddes and Hall 1990; Pierce 1995; Sim
et al 2000) has been used to suggest that the population declined as a result of the
reduction in estuarine habitat associated with construction of the barrages. Current
fishing practices and levels of harvest appear to be sustainable under current flow
conditions, however, continued increases in the levels of extractions of water from the
Murray River for agriculture could alter this situation. The prognosis for the SA
mulloway population and fishery may not be positive unless improved procedures for
ensuring adequate environmental flows are implemented.
4.3
Future research needs and potential performance indicators
Catches and CPUE of mulloway in the LCF may provide a useful indicator of
ecosystem health in the Murray River estuary in general and the Coorong lagoons in
particular. These data will also provide some insights into the status of the resource
and fishery, but other biological data are also required for this purpose – many of
which are being collected as part of a current PhD study.
The PhD project is a life history study that will be focused around validated, otolithbased estimates of age. Size, age and reproductive data will be used to calculate
reliable estimates of the growth rates and size at sexual maturity of mulloway in South
Australia. Methods developed in this study could also be used to establish an ongoing
program for monitoring the age structure of catches. The PhD study will also
investigate the timing and location of spawning by mulloway in South Australia by
conducting macroscopic and histological analyses of adult gonads and by conducting
egg and larval surveys. Information collected during the PhD will support the
assessment and development of future management arrangements for the South
Australian mulloway fishery.
48
Mulloway may be vulnerable to high levels of juvenile mortality, and it is essential to
understand the potential sources of juvenile mortality in critical areas such as the
Coorong lagoons. Catches of juvenile mulloway in small mesh gill nets used by
commercial and recreational fishers to target Yellow eye mullet in the Coorong
lagoons are potentially significant, but have not been quantified. Furthermore, the
effectiveness of (i) area closures and gear modifications in the commercial fishery and
(ii) seasonal closures in the recreational fishery in ameliorating this problem have not
been measured. A research project has been proposed, as part of the five year
strategic plan for the LCF, to quantify the levels of by-catch and to assess the
effectiveness of current and/or additional amelioration strategies. Such a study may
also provide information on the relative abundance of smaller juveniles in the
Coorong lagoons that would further enhance our understanding of the effects of
freshwater flows on recruitment success.
CPUE data are available from 1983 for the MSF and LCF, and could potentially be
used as a performance indicator(s) for the fishery. However, these data must be
interpreted carefully as factors other than mulloway abundance affect CPUE. For
example, catch rates of adults in the swinger net fishery are affected by the timing and
levels of fresh water flow through the Murray mouth. Recruitment levels and juvenile
distribution patterns are also affected by environmental factors. Furthermore, few
catch and effort data are available for sectors other than the MSF and LCF, including
the recreational fishery in which mortality rates of mulloway are also unknown.
The uncertainties in the CPUE data suggest that it may be important to monitor the
age structure of the population. Information on the age structure of catches taken in
the swinger net fishery may provide insights into the age structure of the adult
population, and could form the basis of a useful performance indicator, e.g. mean age.
Any reduction in mean age and/or the abundance of older fish in catches could
indicate a reduction in the adult biomass. Samples of the otoliths required for such a
program could potentially be collected from the Adelaide Fish Market. However, as
fish are sometimes graded by size to accommodate the needs of interstate markets the
sampling regime to support this approach would need to be designed and monitored
carefully.
49
Size can be a useful proxy of the age of fish, and the size structure or mean size, of
fish taken in swinger net fishery could be a potentially useful performance indicator.
An ongoing size monitoring program for this fishery would also provide a useful basis
for determining how well the market samples reflect the size structure of the catch.
This information could be obtained by requiring fishers to report on the numbers (as
well as the total weight) of fish caught. Such a program would also provide a costeffective (albeit less precise) tool for monitoring the age structure of catches (e.g. a
reduction in mean size would suggest a decline in the mean age of the catch).
Monitoring the age/size structure of the catches of large mesh gill nets in the Coorong
lagoons may provide a useful indicator. Although these gill nets are highly selective,
three year classes of immature mulloway tend to be caught. A performance indicator
that measured the relative contribution of these three year classes to the catch would
provide insights into the current and future status of juveniles in this critical habitat.
Female mulloway tend to grow faster than males. Consequently, the size-selectivity
of gill nets may result in different fishing mortalities for males and females and the
sex ratio could change over time if females were removed from the population at a
younger age or faster rate than males. Thus the sex ratio of swinger net catches may
also be useful as a performance indicator. These sex ratios could be validated by
comparing them with those from adults caught by line in the recreational fishery.
A range of biological performance indicators is potentially available for the South
Australian mulloway fishery. Each provides different information and a suite of
indicators will be necessary to adequately monitor the status of mulloway. CPUE
data are available and provide an indication of relative abundance but additional
biological data are also required. While critical aspects of mulloway biology are
currently being addressed in a PhD study, additional monitoring of the age/size
structure is needed for the fishery.
50
5
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