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Sand Flathead Stock
Assessment 2015
October 2016
Fisheries Victoria Science Report Series No. 13
Sand Flathead Stock
Assessment 2015
Paul Hamer, Simon Conron, Alastair Hirst* and Jodie Kemp
Fisheries Management and Science Branch, Queenscliff
*Deakin University, Waurn Ponds
October 2016
Fisheries Victoria
Science Re
p
ort Series No. 13
Sand Flathead Stock Assessment 2015
ii
Published by the Victorian Government, Department of Economic Development, Jobs, Transport and Resources, August
2016.
© The State of Victoria, Department of Economic Development, Jobs, Transport and Resources (2016).
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provisions of the Copyright Act 1968.
Authorised by the Victorian Government, 1 Spring Street, Melbourne
Printed by DEDJTR Queenscliff, Victoria.
Preferred way to cite this publication:
Hamer P, Conron S, Hirst A and Kemp J (2016). Sand Flathead Stock Assessment 2015. Fisheries Victoria Science
Report Series No. 13, Fisheries Victoria, Queenscliff
ISSN 2203-3122 (Print)
ISSN 2204-6933 (Online)
ISBN 978-1-925532-13-5 (Print)
ISBN 978-1-925532-14-2 (pdf/online)
Author Contact Details:
Paul Hamer
Fisheries Management and Science Branch, Fisheries Victoria
PO Box 114, Queenscliff Vic 3225
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Sand Flathead Fishery Assessment 2015
iii
Contents
Executive Summary 1
Background 2
Victoria’s sand flathead stocks and the fisheries they support 2
Overview of management arrangements 2
Sand flathead stock assessment 2015 3
Stock assessment process 3
Stock status classifications 4
Summary of stock status indicators 5
Fishery-independent information (Port Phillip Bay annual trawl and pre-recruit beam trawl surveys) 5
Recruitment 5
Stock biomass and exploitation 5
Length and age composition 5
Fishery-dependent information 6
Trends in commercial catch rates 6
Trends in recreational catch rates 6
Length composition 6
Overall summary and implications 6
Overview of the status of sand flathead indicators in Victoria 7
Stock assessment data 8
Tables 8
Figures 12
Recruitment 12
Biomass and catch rate trends 14
Length composition 18
Age composition 22
Appendices 25
Appendix 1 Life-history 25
Appendix 2 Assessment data sources 26
Acknowledgements 27
References 27
Sand Flathead Stock Assessment 2015
iv
List of Tables
Table 1. Qualitative classification ratings and descriptions applied to individual stock status indicators. ...................... 4
Table 2. Status indicator table for key performance indicators. ..................................................................................... 7
Table 3. Fishery-independent stock status indicators for sand flathead in Port Phillip Bay. .......................................... 8
Table 4. Fishery-dependent stock status catch rate indicators for sand flathead in Port Phillip Bay (a) and Western
Port (b). ........................................................................................................................................................... 9
Table 5. Fish length and age stock status indicators for sand flathead in Port Phillip Bay. ......................................... 10
Table 6. Assessment of biomass and exploitation indicators for sand flathead in Port Phillip Bay from 2000/01 to
2006/07. ........................................................................................................................................................ 11
List of Figures
Figure 1. Map of Port Phillip Bay showing previous sampling sites for the Port Phillip Bay otter trawl surveys
(ceased 2011) and the current sampling sites for the beam-trawl surveys (ongoing) used to provide
fishery independent data on sand flathead recruitment and biomass trends. .................................................. 3
Figure 2. Recruitment index from the beam trawl pre-recruit survey catch rates (number 1000 m-2) of sand
flathead ≤15 cm (age 0+, less than 1 year old). ............................................................................................. 12
Figure 3. Catch rate from the beam trawl pre-recruit survey (number 1000 m-2 of sand flathead ≤15cm (age 0+,
less than 1 year old). ..................................................................................................................................... 12
Figure 4. Standardised long-term recruitment index for 0+ age sand flathead in Port Phillip Bay generated by
combining otter trawl recruitment index (1988–2000) and beam trawl pre-recruit survey (2001–2015)
time-series. The long-term average is indicated ±20% (grey bar). ................................................................ 13
Figure 5. Sand flathead biomass in Port Phillip Bay from 1990–2011 estimated using the median trawl efficiency
scenario*. a) total biomass, b) reproductive biomass (i.e. biomass of fish ≥25 cm TL, the size at which
most males and 50% of females are sexually mature (Bani and Moltschaniwskyj 2008). ............................. 14
Figure 6. Port Phillip Bay multi-species otter-board trawl survey catch rates (kg/shot) of sand flathead at 7 m, 12
m, 17 m and 22 m from 1990–2011, all sizes. ............................................................................................... 15
Figure 7. Annual sand flathead catch (tonnes), catch rate (kg/1000-hook-lifts) and effort (1000-hook-lifts) for the
commercial long-line fishery (primarily targeting snapper) in Port Phillip Bay from 1978/79–2014/15. Grey
bar indicates ±20% of the long-term average catch rate. .............................................................................. 16
Figure 8. Recreational creel survey flathead (all species) catch rates in Port Phillip Bay 1995–2015 financial years
(i.e. 2015=2015/16). Angler diary surveys indicate approximately 80% of all flathead caught are sand
flathead. ......................................................................................................................................................... 16
Figure 9. Recreational creel survey flathead (all species) catch rates in Western Port 1998–2014 financial years. ..... 17
Figure 10. Length frequency distributions of sand flathead caught during the Port Phillip Bay multi-species otter-
board trawl surveys 1990–2000. Vertical dashed line indicates current LML (27 cm). .................................. 18
Figure 11. Length-frequency distributions of sand flathead caught during the Port Phillip Bay multi-species otter-
board trawl surveys 2002–2011. Vertical dashed line indicates current LML (27 cm). .................................. 19
Figure 12. Length-frequency distributions for sand flathead retained by recreational boat-based anglers fishing in
Port Phillip Bay recorded during creel surveys conducted in 1996, 1997 and 2006–2009. The LML during
this period was 25 cm. The vertical dashed line indicates the current LML (27 cm). ..................................... 20
Figure 13. Length-frequency distributions for sand flathead retained by recreational boat-based anglers fishing in
Port Phillip Bay recorded during creel surveys conducted 2010–2015. The vertical dashed line indicates
the current LML (27 cm). ............................................................................................................................... 21
Figure 14. Age frequency (%) histograms for sand flathead in Port Phillip Bay collected by multi-species otter-board
trawl surveys 1990–2000. Cohorts for high recruitment years are displayed using coloured bars. ............... 22
Figure 15. Age frequency (%) histograms for sand flathead in Port Phillip Bay collected by multi-species otter-board
trawl surveys 2002–2011. Cohorts for high recruitment years are displayed using coloured bars. ............... 23
Figure 16. Mean age of sand flathead superimposed onto proportion (%) of sand flathead ≥27 cm TL (LML) caught
during the Port Phillip Bay multi-species otter-board trawl surveys 1990–2011. ........................................... 24
Figure 17. Proportion (% of total catch) of male and female sand flathead ≥27 cm TL (LML) caught during the Port
Phillip Bay multi-species otter-board trawl surveys 1990–2011. .................................................................... 24
Sand Flathead Stock Assessment 2015
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Executive Summary
Fisheries Victoria conducts periodic assessments of the status of key fish species and the fisheries they support. The
assessments compile relevant data from commercial fishery catch and effort reporting, recreational fishery monitoring
programs, scientific surveys (such as juvenile pre-recruit surveys) and other data, such as age and length composition,
to support a ‘weight of evidence’ approach to assessing stock or fishery status. Formal assessment meetings are held to
present and discuss the data with stakeholders. The assessment outcomes are qualitative ratings of the current condition
of the relevant performance indicators relative to their long-term averages. The information delivered through the stock
assessment process is used by fisheries managers to identify issues and consider the need for review of current
management or monitoring arrangements.
The last formal assessment meeting for sand flathead was in July 2013, and much of the information presented was
published in a Fisheries Victoria report by Hirst et al. (2014), The decline of sand flathead stocks in Port Phillip Bay:
magnitude, causes and future prospects. Relevant key time series data reported in Hirst et al. (2014), has been updated
with commercial, recreational (2014/15) and fishery-independent juvenile recruitment data (2016) in this report. There is
limited information on the status of sand flathead stocks in most Victorian waters besides Port Phillip Bay, and to a lesser
extent Western Port. Consequently, this assessment is focussed primarily on the status of the population and fishery
within Port Phillip Bay — the principal recreational sand flathead fishery in the State.
Fishery-independent and dependent data indicate that the sand flathead biomass in Port Phillip Bay declined by 60–80%
between 2000 and 2010. Recent commercial and recreational (creel survey) catch rates remain well below the long-term
average, and while showing a slight recovery from 2010 to 2011, catch rates do not indicate significant ongoing recovery
since the last biomass estimate in 2011. Based on the trends in catch rate time series and the earlier fishery -
independent trawl survey biomass estimates, the reproductive biomass has been below the long-term average since
1990, but has stabilised at this lower level since 2011. Indications from recreational sand flathead catch rate data
suggest that the major decline observed in the Port Phillip Bay sand flathead biomass has not occurred in Western Port.
Recruitment of sand flathead in Port Phillip Bay has been well below its long-term average for all but two years since
2001. Exploitation rates of reproductive biomass for the two available estimates in Port Phillip Bay (2000/01 and
2006/07), while consistent, where relatively high at approximately 40%, given the prolonged low recruitment rates.
There is evidence that persistent low-level recruitment of sand flathead in Port Phillip Bay is linked to changing
environmental conditions, in particular prolonged drought/low river-flows (and associated nutrient inputs) into the Bay
during late spring/early summer when sand flathead spawn. The variation in nutrient inputs from river flow is thought to
influence planktonic productivity and survival of the larval stages of sand flathead, but the relationships are still not well
defined. The long-term decline and current low biomass of larger adult sand flathead presents a major risk to recovery as
the level of egg production (reproductive biomass) is likely now limiting recruitment potential and population recovery
irrespective of river-flow dynamics.
Overall the sand flathead population in Port Phillip Bay is below average. The population remains well below the most
recent productive period of the fishery during the 1990s.
The decline in abundance has moderated and efforts will now be made to replicate environmental factors that appear to
have historically triggered recruitment.
Sand Flathead Stock Assessment 2015
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Background
Victoria’s sand flathead stocks and the fisheries they support
Sand flathead occur from south-western Tasmania to the mid-coast of New South Wales including the coasts of South
Australia, Victoria and Tasmania (Gomon et al. 2008). Sand flathead are bottom dwellers living on sandy, shelly or
muddy bottoms to 100 m depth (Gomon et al. 2008). In Port Phillip Bay sand flathead are most abundant in deeper
habitats >15 m, on silty and muddy bottoms (Hirst et al. 2014). See Appendix 1 for more information on the life-history of
sand flathead.
The broad stock structure of sand flathead across Victorian waters is uncertain. However, earlier tagging studies and
more recent research on biological characteristics, such as growth rates, suggest that populations in bays such as Port
Phillip Bay and Western Port are largely separated from each other, and from those in coastal waters (Brown 1977;
Koopman et al. 2004; Hamer et al. 2010; Hirst et al. 2014).
Recreational fishery
Sand flathead is targeted by recreational fishers in Victoria’s bays and inlets. It is the most commonly caught and
retained recreational fish species in Port Phillip Bay (Hirst et al. 2014). Because the species is more abundant in deeper
water, most of the catch is by boat-based anglers using bottom fishing rigs. An estimated 107 tonnes of sand flathead
was caught and retained by recreational anglers in Port Phillip Bay in 2006/07, down by two-thirds from an estimated 322
tonnes in 2000/01 (Hirst et al. 2014). Due to slow growth and small maximum sizes of sand flathead in Port Phillip Bay,
discard rates for recreationally caught sand flathead are high under the current legal minimum legal length (LML) of 27
cm. Hirst et al. (2014) estimated that >60% of hooked fish are released. Most lip-hooked sand flathead will survive after
release if handled appropriately (Lyle et al. 2007). The sand flathead fishery in Port Phillip Bay is dominated by the
recreational sector who catch in excess of 95% of the sand flathead in the Bay.
Commercial fishery
In recent times sand flathead has been a by-catch species of the Victorian commercial finfish fishery, with negligible
targeted fishing pressure and very low catches. Sand flathead is caught by a range of commercial fishing methods (e.g.
long-line, mesh net and haul seine net). Victoria’s total commercial harvest of sand flathead in 2014/15 was 5 tonnes, 3.6
tonnes of which was caught in Victoria’s bays and inlets, mostly in Port Phillip Bay. This constitutes 0.8% of the total
Victorian commercial finfish catch.
Overview of management arrangements
Recreational fishery
Recreational fishing licence
Since 15 July 1999, recreational fishers have been required to hold a recreational fishing licence (RFL) to fish in
marine and inland waters. Recreational fishers under 18 years of age or over 70 years of age, or those holding
one of a range of concession cards are not required to hold a licence.
Gear restrictions
Recreational fishing remained unrestricted, other than size limits, until 1992, when regulations specified a
maximum of four lines per person and two hooks per line when fishing in marine waters, and a maximum of two
lines with two hooks per line in inland waters. This regulation was modified in 2009 to include two hooks per line
or one bait jig when fishing in both marine and inland waters.
Bag and size limits
Since 2009, the daily bag limit (DBL) for sand flathead is 20 and the legal minimum length (LML) is 27 cm TL.
Prior to 2009, the DBL was 30 and the LML was 25 cm TL.
Sand Flathead Stock Assessment 2015
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Commercial fishery
Victorian commercial fisheries are currently managed primarily by Input (effort) controls (including limited entry licensing,
restrictions on fishing equipment and methods, and closed areas, i.e. marine protected areas).
The number of licensed commercial fishers in Victorian waters was reduced by two-thirds between 1986/87 and 2010/11.
In 2015 there were 18 commercial licences for the Corner Inlet fishery and 42 licences for the Port Phillip Bay/Western
Port fishery. In April 2016, the number of licences for the Port Phillip Bay/Western Port fishery was reduced further under
a Government buy-out scheme aimed at removing net fishing from Port Phillip Bay —10 licences remain for this fishery
in 2016, with a further reduction to 8 hook method only licences to occur by 2022.
Figure 1. Map of Port Phillip Bay showing previous sampling sites for the Port Phillip Bay otter trawl surveys
(ceased 2011) and the current sampling sites for the beam-trawl surveys (ongoing) used to provide fishery
independent data on sand flathead recruitment and biomass trends.
Sand flathead stock assessment 2015
The last formal assessment meeting for sand flathead was in July 2013. This stock assessment report provides an
update of sand flathead stock status since the Hirst et al. (2014) report.
Stock assessment process
Stock assessments are designed primarily to provide information on the status of fish stocks and their associated
fisheries. They contribute towards assessing biological sustainability objectives and the performance of management
arrangements in achieving these.
Fisheries Victoria has developed a process to conduct periodic formal assessments of the status of key marine and
estuarine finfish stocks and the fisheries they support.
The assessment activities include:
The synthesis of all relevant fisheries dependent data,
Sand Flathead Stock Assessment 2015
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Evaluation of fisheries-independent monitoring and research data,
Convening a workshop for scientists, resource users and resource managers to assess and discuss the status of the
stock/fishery in question,
Production of an assessment report which provides scientific information to support fishery management decision
making.
The assessment process provides:
Scientific evidence on the status of the fish stocks and harvest pressures on those stocks,
Opportunity to draw on the knowledge of stakeholder groups,
Information to underpin evidence-based decisions in an ecologically sustainable development management context,
Information that complements Victorian fisheries management planning processes,
An accountable and transparent fishery assessment process.
The assessment process for sand flathead uses a ‘weight-of-evidence’ approach that assesses fishery-dependent
recreational and commercial data, fishery-independent pre-recruit survey data, and other data including size and age
composition (Table 3) and local fisher knowledge.
This report is structured to provide an overview of the main outcomes of the stock assessment in the “Summary of stock
status indicators” section including overall stock status classification tables. The assessment data is then summarised in
tables of key performance parameters. Time series of the data are then included as figures. Other supporting information
is included in appendices. Data up to and including financial year 2014/15 is included, and to 2016 for the Port Phillip
Bay pre-recruit beam trawl survey. See Appendix 2 for more information on data sources for this assessment.
Stock status classifications
A qualitative classification framework has been developed as an indicative summary of status for individual indicators
(Table 1). This classification approach provides an assessment of how the most recent individual indicators (primarily
catch rate time series, or other abundance proxies) compare with their long-term averages. The classifications aim to
readily identify the condition of fishery and stock performance indicators, and in particular to highlight areas of concern
from a management and fishery performance perspective. Classification of individual indicators is part of a weight of
evidence approach that considers a variety of indicators in order to assess the overall status of the fish stock (see
‘Overall Stock Status’).
Table 1. Qualitative classification ratings and descriptions applied to individual stock status indicators.
Indicator status
classification
Description
Above average
The indicator is ≥ 20% higher than the long-term average.
Average
The indicator is within 20% of the long-term average.
Below average
The indicator is ≤ 20% lower than the long-term average.
Limited data
A limited amount of information has been collected, or, the available data is
inappropriate/insufficient to confidently assess stock status, or, there are
inconsistent or contradictory signals in the data that preclude determination of
stock status.
Red flags identify specific concerns/issues or uncertainties regarding an
indicator’s status.
* Long-term refers to the duration of the time-series.
Sand Flathead Stock Assessment 2015
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Summary of stock status indicators
Fishery-independent information (Port Phillip Bay annual trawl and pre-
recruit beam trawl surveys)
Recruitment
Sand flathead recruitment continues to remain poor when compared to levels recorded during the late-1980s to mid-
1990s (Table 4 and Figure 4).The long-term average for the standardised otter trawl/beam trawl recruitment index
(Figure 4) is now biased by the prolonged period of low recruitment since 1998, however, despite this there were only
three years since 1998 when recruitment reached the long-term average, and none where recruitment clearly exceeded
the long-term average (Figure 4). Over the last five years (2012–2016), recruitment reached the long-term average in
only one year (2013).
Stock biomass and exploitation
Sand flathead total biomass, determined by the Port Phillip Bay otter trawl surveys, declined by an estimated 87%
between 2000 and 2010 (Table 4 and Figure 5a), and the reproductive biomass (defined here as the biomass of fish ≥ 25
cm TL; the size at which most males and 50% of females are sexually mature (Bani and Moltschaniwskyj 2008) declined
by 60–70% (Figure 5b). In 2011 the reproductive biomass was approximately 25% lower than the levels estimated during
the early 1990s, the previous low period in the available time-series of biomass estimates (Figure 5b).
The declining abundance of sand flathead in Port Phillip Bay since the mid 1990s was consistent across the different
depth zones sampled by the otter trawl surveys (Figure 6).
The exploitation rate of the reproductive biomass in 2000/01 and 2006/07 was estimated to be 44.4 and 37.5%,
respectively, under the median trawl survey capture efficiency scenario for estimating biomass (Table 7). These
exploitation rates also represent exploitation of legally available biomass at those time as the LML was 25 cm (currently it
is 27 cm). While these exploitation rate estimates appear relatively high, further review and analysis of exploitation rates
for Port Phillip Bay sand flathead would be required to inform fisheries managers of sustainable and optimal exploitation
rates for the fishery .
Length and age composition
The size structure of the sand flathead population remained relatively constant in Port Phillip Bay from 1990–2011,
despite the large decline in the overall population (Figures 10 and 11). Median fish length was slightly lower in the 1990s
(median=22.9 cm TL; Figure 10) compared with the 2000s (median=24.5 cm TL; Figure 11). The majority of sand
flathead in the trawl survey catches were below LML (27 cm) and this pattern has remained consistent since 1990 when
the trawl survey program began. The largest and oldest sand flathead recorded during the Port Phillip Bay trawl surveys
was 41 cm in length and 23 years old (Table 6). In 2011 the largest and oldest sand flathead captured was 36 cm TL and
19 years old (Table 6).
The proportion of sand flathead caught by the trawl survey that exceed the LML (i.e. ≥27 cm) varied from 1990 to 2011
(Figures 10 and 11). During the early part of the 1990s (1990–1995) <10% of sand flathead caught by the trawl surveys
were legal size, however, from 2002 onwards (to 2011) this had increased to >20% of the sand flathead caught. This
trend in the proportion of fish that were above legal size is most likely related to trends in recruitment and its effect on the
size-structure of the population. During the early 1990s the population was dominated by smaller/younger sand flathead
(Figure 14). As recruitment declined, the mean age increased into the early 2000s (Figure15), and the proportion of
older/ larger, and hence above LML, increased. The mean age decreased in the later years of the time series, and in
2011 was similar to the mean age in the early 1990s (Figure16). The recent decline in mean age reflects the reduction in
older fish from natural mortality and fishing removals.
Changes in the age composition of the Port Phillip Bay sand flathead population (Figures14 and 15) are consistent with
the absence of any large recruitment cohorts contributing to the population since the late 1990s. The age composition
from 2006 onwards displays increasing truncation towards younger ages (Figure 15). This is consistent with the ongoing
reduction in the biomass of older fish from fishing and natural mortality, in combination with low rates of new recruitment
into the adult population. Importantly, it signifies a clear decline in the larger and older spawning component of the
population, which is a concern for ongoing egg production and future recruitment potential.
Sand Flathead Stock Assessment 2015
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Fishery-dependent information
Trends in commercial catch rates
Commercial long-line catch rates for sand flathead (≥27 cm TL) in Port Phillip Bay (taken as by-catch when targeting
snapper) are currently well below the long-term average (Table 5a, Figure 7). In 2009/10 catch rates had declined by
over 80% from the previous peak period between 1993/94–2002/03 (Figure 7). The catch rates since 2009/10 have been
variable and at levels well below the long-term average. Consistent with the trends of trawl survey reproductive biomass
estimates, the long-line catch rates (of adult sand flathead) since 2009/10 are approximately 40% of those recorded in
the previous low catch-rate period during the early 1990s (Figure 7).
Trends in recreational catch rates
Recreational catch rates of flathead (80% of which are sand flathead), derived from on-site creel surveys of boat-based
anglers, show a decline of 60% between 1996 and 2011. There has been a slight improvement in recreational catch
rates since 2011, although they remain well below the long-term average for this fishery (Table 5a and Figure 8).
In contrast to the recreational catch rates of Port Phillip Bay flathead, recreational catch rates for flathead in Western Port
(also dominated by sand flathead) have been variable, but since 2000 the trend has not shown the same rate of decline
as for Port Phillip Bay (Table 5b and Figure 9).
Length composition
Length-frequency distributions of recreationally caught sand flathead from Port Phillip Bay (retained sand flathead
measured during creel surveys 1996–1997 and 2006–2015; Figures 12 and 13) remained consistently within historical
ranges, noting that the LML was increased after 2009 (Figure 13). The majority of the catches were consistently less
than 35 cm TL. In 2015 the modal (most frequently caught) length of recreationally caught sand flathead in Port Phillip
Bay was 28 cm TL (Figure 13). Age-structure data was not available for the recreational samples.
Overall summary and implications
Fishery-independent and dependent data indicate that the sand flathead biomass in Port Phillip Bay is below the long-
term average (since 1978), and declined by 60–80% between 2000 and 2010 however has improved slightly since then.
Recent commercial and recreational (creel survey) catch rates also remain well below their long-term averages,
suggesting little recovery since 2010. Reproductive biomass is now considerably lower than the previous low period of
the biomass time series in the early 1990s. Indications from recreational catch rate data suggest that the major decline of
catch rates of Port Phillip Bay sand flathead has not similarly occurred in Western Port.
There is evidence that the persistent low recruitment is linked to changing environmental conditions, in particular
prolonged drought and low river-flows (and associated nutrient inputs) into the Bay during late spring/early summer
(Parry and Hirst 2016; Hirst et al. 2014).
Sand Flathead Stock Assessment 2015
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Overview of the status of sand flathead indicators in Victoria
Table 2. Status indicator table for key performance indicators.
a) Port Phillip Bay
Indicator Status Red flags — concerns
Recruitment
(standardised otter trawl/beam trawl recruitment
index)
Below average
Prolonged period of low
recruitment.
Fishery-independent biomass (1990–2011)
Total biomass and reproductive biomass
Below average
Long-term decline.
Indicator not available since 2011
Fishery-independent age/length
Stable
Decreasing proportions of older fish
may indicate truncation is
occurring. However, age
composition data is not available
since 2011. Current status is
uncertain.
Commercial CPUE
Below average
Long-term decline
Recreational CPUE
Below average
Long-term decline
Recreational length composition Stable
Length composition of retained
sand flathead is stable.
b) Western Port
Indicator Status
Red flags
—
concerns
Recreational CPUE Average Catch rates are highly variable.
Sand Flathead Stock Assessment 2015
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Stock assessment data
Tables
Table 3. Fishery-independent stock status indicators for sand flathead in Port Phillip Bay.
Location Port Phillip Bay
Source Fishery-independent data
Indicator Recruitment Biomass
Otter trawl/beam trawl survey: combined
standardised recruitment index (number of 0+ year
.1000m-2) (1988−2016)
(Figure 4)
Otter trawl survey: median scenario total
biomass (tonnes) of sand flathead (1990−2011)
(Figure 5a)
Otter trawl survey: median scenario
reproductive biomass (tonnes) of sand
flathead (1990−2011)
(Figure 5b)
Minimum 2006: 0.02 2010: 400 2009: 213
Maximum 1989: 28.8 1991: 3950 2002: 762
Long-term
average (± 20%) 1988−2015: 4.2 (±0.8) 1990−2011: 1990 (±397) 1990−2011: 422 (±84)
Recent five-year
average 2011−2015: 1.5 2007−2011: 738 2007−2011: 257
Most recent year 2016: 0.4 2011: 465 2011: 252
Trend in five-year
moving average
over last five
years
Stable Decreasing Decreasing
Status
(five-year
average)
Below average
Below average
Below average
Notes
Long-term low recruitment since 1998.
Total biomass of stock declined by 87% between
2000 and 2010.
Reproductive biomass declined by 65%
between 2000 and 2010.
Sand Flathead Stock Assessment 2015
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Table 4. Fishery-dependent stock status catch rate indicators for sand flathead in Port Phillip Bay (a) and
Western Port (b).
a) Port Phillip Bay
Location Port Phillip Bay
Indicator Catch rates
Source Commercial Fishery Recreational Fishery
Indicator
Long-line catch rate (kg/1000
hook-lifts) (1978/79−2014/15
(Figure 7)
On-site survey retained catch
rate (no. fish/angler hour)
(1995−2015)
(Figure 8)
Minimum 2009/10: 1.43 2011/12: 0.21
Maximum 2002/03: 13.21 1995/96: 0.74
Long-term average (±20%) 1978/79−2014/15: 6.31 (±1.26) 1995/96−2014/15: 0.36 (±0.07)
Recent five-year average 2010/11−2014/15: 2.72 2010/11−2014/15: 0.24
Most recent year 2014/15: 3.22 2014/15: 0.27
Trend in five-year moving
average over last five years Stable Stable
Status (five year average) Below average Below average
Status Notes Commercial catch rates are
currently well below the long-term
average and historic maximum.
Catch rates have increased
slightly since 2009/10.
Recreational catch rates declined by
60% between 1996 and
2011.Recreational retained catch
rates are currently below the long‐term
averages. There has been a slight
increase since 2011.
b) Western Port
Location Western Port
Indicator Catch rates
Source Recreational Fishery
Indicator
On-site survey retained catch
rate (no. fish/angler hour)
(1998/99−2014/15)
(Figure 9)
Minimum 2009/10: 0.06
Maximum 2006/07: 0.19
Long-term average (±20%) 1998/99−2014/15: 0.11 (±0.02)
Recent five-year average 2010/11−2014/15: 0.10
Most recent year 2014/15: 0.08
Trend in five-year moving
average over last five years Stable
Status (five-year average) Average
Status Notes Recreational catch rates show high variation in
Western Port, with evidence of cycles of higher
catch rates at about 4-year intervals. The current
catch rate is low relative to peaks in 1998/99,
2001–2003 and 2006/07.
Sand Flathead Stock Assessment 2015
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Table 5. Fish length and age stock status indicators for sand flathead in Port Phillip Bay.
Location Port Phillip Bay
Source Fishery-Independent Recreational (retained)
Indicator
Otter trawl survey size (TL)/age -
frequency time series (1990−2011)
(Figures 10, 11 and 16)
On-site survey retained catch (TL) –
frequency time series (1996,1997,
2006–2015)
(Figures 12 and13)
Maximum length/age All years: 41 cm TL / 23 years
2011: 36 cm TL / 19 years
All years: 50 cm TL
2015: 45 cm TL
Minimum length/age All years: 5 cm TL /<1 year
2011: 9 cm TL /<1 year
All years: 20 cm TL
2015: 26 cm TL
Mean length/age most recent 2011: 24 cm TL / 4 years 2015: 30.2 cm TL
Mean length/age ten-year average 24 cm TL / 5 years 30 cm TL
Mean length/age five-year average 24 cm TL / 7 years 30.2 cm TL
Modal length/age (most recent year) 2011: 24 cm TL / 1 year 2015: 28 cm TL
Proportion of catch ≥27 cm TL (LML)
Trend over last five years
2011: 20%
Decreasing NA – LML is 27 cm
Trend in mean age time-series over last
five years (2007–2011)
Decreasing
(Figure 16) NA
Status Notes
Size-frequency distributions are
within historical ranges, but the age
structure displays absence of
major recruitment events since
1990s, and reducing proportions of
larger and older fish from mid-
2000s (Figure 16). Signs of
truncation in age composition. No
ageing conducted since 2011.
Size-frequency distributions are
within historical ranges.
Sand Flathead Stock Assessment 2015
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Table 6. Assessment of biomass and exploitation indicators for sand flathead in Port Phillip Bay from 2000/01 to 2006/07.
Location Port Phillip Bay
Indicator
Estimated
stock
biomass
(tonnes)
Estimated
reproductive
biomass
(tonnes)
Commercial
fishery
mortality
(tonnes)
Retained
recreational
fishery
mortality
(tonnes)
Discarded
recreational
fishery
mortality
(tonnes)
Total fishery
mortality
(tonnes)
Total biomass
exploitation rate
(% of total
biomass
removed by
fishing each
year)
Exploitation rate of
reproductive biomass*
(% of biomass ≥ 25 cm
TL removed by fishing
each year)
Source
Bay-wide
trawl survey
(max-min
efficiency)
median
efficiency
Bay-wide
trawl survey
(max-min
efficiency)
median
efficiency
Log-books Phone
survey
Phone
survey
Commercial
and
recreational
(>95% is
recreational)
Commercial and
recreational
(>95% is
recreational)
(lower-upper)
median
Commercial and
recreational (>95% is
recreational
(lower-upper)
median
2000/01
2002:
(1210−2289)
1600
2002:
(578–1083)
762
5.0
322.8
10.6
338.4
(15−28%)
21%
(31–58%)
44.4
2006/07
2007:
(478−906)
631
2007:
(232–436)
306
4.5
107.3
3.6
115.4
(13−27%)
18%
(26–50%)
37.5
Trend ↓ 61% ↓ 60% ↓ 10% ↓ 67% ↓ 67% ↓ 66% Stable Stable
High exploitation
rate
Exploitation rates have remained stable over time due to reductions in total catch.
While long-term biomass decline has been driven by prolonged low recruitment, the estimated exploitation rate of reproductive biomass was
moderately high during the 2000s.
Given the poor recruitment over the past 15 years, the low current mature biomass is of concern in relation to sustainability and ongoing egg
production and stock replenishment.
*Reproductive biomass = biomass of fish ≥25 cm TL, the size at which most males and 50% of females are sexually mature (Bani and
Moltschaniwskyj 2008). Note: this also equates to exploitable biomass as the legal minimum length in 2000/01 and 2006/07 was 25 cm.
See Hirst et al. (2014) for detailed methodology on biomass and catch estimation.
Sand Flathead Stock Assessment 2015
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Figures
Recruitment
Figure 2. Recruitment index from the beam trawl pre-recruit survey catch rates (number 1000 m
-2
) of sand
flathead ≤15 cm (age 0+, less than 1 year old).
Figure 3. Catch rate from the beam trawl pre-recruit survey (number 1000 m
-2
of sand flathead ≤15cm (age 0+,
less than 1 year old).
Sand Flathead Stock Assessment 2015
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Figure 4. Standardised long-term recruitment index for 0+ age sand flathead in Port Phillip Bay generated by
combining otter trawl recruitment index (1988–2000) and beam trawl pre-recruit survey (2001–2015) time-series.
The long-term average is indicated ±20% (grey bar).
Note: Data prior to 2001 from the otter trawl survey was standardised to the ongoing beam trawl survey metric using the
linear relationship between the otter trawl survey age 2+ index (lagged by 2 years) and the 0+ age pre-recruit beam-trawl
survey index (r
2
= 0.72) (see Hirst et al. 2014).
Sand Flathead Stock Assessment 2015
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Biomass and catch rate trends
a) Total biomass
b) Reproductive biomass (≥ 25 cm TL)
Figure 5. Sand flathead biomass in Port Phillip Bay from 1990–2011 estimated using the median trawl efficiency
scenario*. a) total biomass, b) reproductive biomass (i.e. biomass of fish ≥25 cm TL, the size at which most
males and 50% of females are sexually mature (Bani and Moltschaniwskyj 2008).
*The median trawl efficiency scenario estimates biomass with the assumption that fishing efficiency in the path swept by
the trawl net’s foot-line is 85% and 40% for the adjacent region swept by the bridles (see Hirst et al. 2014).
Sand Flathead Stock Assessment 2015
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Figure 6. Port Phillip Bay multi-species otter-board trawl survey catch rates (kg/shot) of sand flathead at 7 m, 12
m, 17 m and 22 m from 1990–2011, all sizes.
0
2
4
6
8
10
12
1990
1991
1992
1993
1994
1995
1996
1997
1999
2000
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
Depth=7 m kg/shot
5-year moving average
0
5
10
15
20
25
30
1990
1991
1992
1993
1994
1995
1996
1997
1999
2000
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
Depth=12 m kg/shot
5-year moving average
0
5
10
15
20
25
30
35
40
45
1990
1991
1992
1993
1994
1995
1996
1997
1999
2000
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
Depth=17 m
kg/shot
5-year moving average
0
5
10
15
20
25
30
35
40
45
1990
1991
1992
1993
1994
1995
1996
1997
1999
2000
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
Depth=22 m
kg/shot
5-year moving average
Catch rate (kg/shot)
Sand Flathead Stock Assessment 2015
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Figure 7. Annual sand flathead catch (tonnes), catch rate (kg/1000-hook-lifts) and effort (1000-hook-lifts) for the
commercial long-line fishery (primarily targeting snapper) in Port Phillip Bay from 1978/79–2014/15. Grey bar
indicates ±20% of the long-term average catch rate.
Figure 8. Recreational creel survey flathead (all species) catch rates in Port Phillip Bay 1995–2015 financial
years
(i.e. 2015=2015/16)
. Angler diary surveys indicate approximately 80% of all flathead caught are sand
flathead.
Sand Flathead Stock Assessment 2015
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Figure 9.Recreational creel survey flathead (all species) catch rates in Western Port 1998–2014 financial years.
Sand Flathead Stock Assessment 2015
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Length composition
Figure 10. Length frequency distributions of sand flathead caught during the Port Phillip Bay multi-species
otter-board trawl surveys 1990–2000. Vertical dashed line indicates current LML (27 cm).
Sand Flathead Stock Assessment 2015
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Figure 11. Length-frequency distributions of sand flathead caught during the Port Phillip Bay multi-species
otter-board trawl surveys 2002–2011. Vertical dashed line indicates current LML (27 cm).
Sand Flathead Stock Assessment 2015
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Figure 12. Length-frequency distributions for sand flathead retained by recreational boat-based anglers fishing
in Port Phillip Bay recorded during creel surveys conducted in 1996, 1997 and 2006–2009. The LML during this
period was 25 cm. The vertical dashed line indicates the current LML (27 cm).
Sand Flathead Stock Assessment 2015
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Figure 13. Length-frequency distributions for sand flathead retained by recreational boat-based anglers fishing
in Port Phillip Bay recorded during creel surveys conducted 2010–2015. The vertical dashed line indicates the
current LML (27 cm).
Sand Flathead Stock Assessment 2015
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Age composition
Figure 14. Age frequency (%) histograms for sand flathead in Port Phillip Bay collected by multi-species otter-
board trawl surveys 1990–2000. Cohorts for high recruitment years are displayed using coloured bars.
Sand Flathead Stock Assessment 2015
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Figure 15. Age frequency (%) histograms for sand flathead in Port Phillip Bay collected by multi-species otter-
board trawl surveys 2002–2011. Cohorts for high recruitment years are displayed using coloured bars.
Sand Flathead Stock Assessment 2015
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Figure 16. Mean age of sand flathead superimposed onto proportion (%) of sand flathead ≥27 cm TL (LML)
caught during the Port Phillip Bay multi-species otter-board trawl surveys 1990–2011.
Figure 17. Proportion (% of total catch) of male and female sand flathead ≥27 cm TL (LML) caught during the
Port Phillip Bay multi-species otter-board trawl surveys 1990–2011.
1990 1995 2000 2005 2010 2015
year
0
10
20
30
40
%legalsize
0.0
2.5
5.0
7.5
10.0
meanage
mean age
1990 1995 2000 2005 2010 2015
year
0
20
40
60
80
%legalsize
Females
Males
Sand Flathead Stock Assessment 2015
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Appendices
Appendix 1 Life-history
Distribution Sand flathead occur from south-western Tasmania to the mid-coast of New South Wales
including the coasts of South Australia, Victoria and Tasmania (Gomon et al. 2008).
Sand flathead are bottom dwellers living on sandy, shelly or muddy bottoms to 100 m
depth (Gomon et al. 2008). In Port Phillip Bay sand flathead are most abundant in deeper
habitats >15 m, on silty and muddy bottoms (Hirst et al. 2014).
Reproduction Sand flathead spawn from September to March, with the peak of the spawning season
observed in September–December (Jordan 2001, Koopman et al. 2004, Hamer et al.
2010). Larval stages can be found throughout Port Phillip Bay in November–January
(Hamer et al. 2010).
Growth Maximum length of 50 cm (Hirst et al. 2014).
Growth across geographic range is highly variable. Sand flathead in Port Phillip Bay grow
more slowly and reach a smaller maximum length than fish from Bass Strait and south-
eastern Tasmania.
Females grow faster and attain a higher maximum size than males (Hirst et al. 2014).
Twice as many females attain LML than males in Port Phillip Bay.
Diet Sand flathead are ambush predators that conceal themselves in fine sediments. Sand
flathead diets are dominated by pelagic fish (anchovies), benthic fish (gobies) and crabs
(Hirst et al. 2014).
Sand Flathead Stock Assessment 2015
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Appendix 2 Assessment data sources
Source Description
Fishery-
independent Snapper beam trawl survey
A small beam trawl (Hamer and Jenkins 2004, 2007) has been used to survey the relative
abundance of 0+ age sand flathead (i.e. < 1 year old, ≤ 15 cm TL) in Port Phillip Bay since
2000. This survey is ongoing, and involves sampling at nine areas around the bay in March
to coincide with the end of the larval settlement period. Each sampling event consists of
five non‐overlapping trawls of 10 minutes bottom time within each of the nine sampling
areas. The survey is conducted at night.
Multi
‐
species otter
‐
board trawl survey
This survey of fish communities in Port Phillip Bay involved a large wing‐ or otter‐board
trawl net (Hirst et al. 2014).The survey occurred from 1990 to 2011. Trawl samples were
taken at 22 fixed sites stratified by depth (7, 12, 17, and 22 m) along six transects spread
around the Bay. Sites were sampled typically in March. At each site two trawls were
undertaken, so that a total of 44 trawl samples were completed annually. The duration of
each trawl shot was ~5 minutes. Most of the snapper taken by the otter‐board trawl survey
were 1 year old.
This program provided information on sand flathead:
Abundance and biomass, Bay-wide and at four depth profiles i.e. 7,12, 17 and 22 m (see
Hirst et al. 2014)
Length composition
Age composition — sand flathead from trawl survey have been aged routinely following
standard procedures outlined in Morison et al. (1998).
Combined trawl recruitment index
A standardised long-term recruitment index for 0+ age sand flathead in Port Phillip Bay is
generated by combining otter trawl recruitment index (1988–2000) and beam trawl pre-
recruit survey (2001–2015) recruitment index time-series. Data prior to 2001 from the otter
trawl survey was standardised to the ongoing beam trawl survey metric using the linear
relationship between the otter trawl survey age 2+ index (lagged by 2 years) and the 0+
age pre-recruit beam-trawl survey index (see Hirst et al. 2014).
Commercial
fishery-
dependent
Victorian commercial fishers are required as a condition of their licence to record their
fishing activities in a logbook and to submit this information to Fisheries Victoria.
Commercial fishers provide the following information:
• Gear and net length/number of hooks/jigs
• Area
• Number of shots
• Fishing time
• Weight of species caught and retained.
The commercial catch rate indicator used in this assessment is the long-line method catch
rate, as this method accounts for most of the commercial catch and represents the catch of
adult flathead.
Recreational
fishery-
dependent
Off-site telephone diary surveys
Off‐site telephone diary surveys of recreational fishing were carried out in 2000/01 (Henry
and Lyle 2003) and 2006/07 (Ryan et al. 2009) to provide regional and State‐wide
estimates of total recreational catches of key target species, including sand flathead.
On-site access point creel surveys
On-site access point surveys of recreational fishing provide time‐series information on
catch rates and the size structure of a range of fish species, including sand flathead. For
more detailed description of creel survey methodology see PoMC (2010).
Length
composition
Since the mid-1990s on-site access point creel surveys have routinely sampled length
compositions of sand flathead retained by boat-based recreational fishers in Port Phillip
Bay and Western Port.
Sand Flathead Stock Assessment 2015
27
Acknowledgements
Thanks to the Catch and Effort Unit (Fisheries Victoria) staff: Paula Baker, Ursula Koliba, Jeanne Cinel, Monique Nelis
and Allister Coots. Thanks also to Ross Winstanley for chairing the 2013 assessment meeting, and James Andrews, Bill
Lussier and Harry Gorfine for comments on this report. Thanks to Kylie Hall for editing the final draft.
References
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