Port Phillip Bay sand flathead stock status update 23/5/2018 Date - PowerPoint PPT Presentation

Port Phillip Bay sand flathead stock status update 23/5/2018 Date Paul Hamer, Simon Conron

Background – stock structure • Broad stock structure – uncertain • Port Phillip Bay – isolated population (tagging studies, growth data and larval surveys)

Species ID Managed as one despite different growth/biology Sand flathead Yank (blue-spot) flathead

Also rock flathead

Background - biology • Max age/length: PPB, 23 years / ~ 40 cm TL • Age/length maturity: males 2 - 4 y / 22 cm, females 3 - 5 y / 25 cm • Fecundity (egg production): Moderate, annual egg production per female weight - unknown 50 50 50 50 50 50 Ave. max size = 38 cm 40 40 40 40 40 40 Bass Strait Tasmania Ave. max size = 33 cm Length (cm) Length (cm) Length (cm) 30 30 30 30 30 30 Port Phillip Bay Ave. max size= 27 cm 20 20 20 20 20 20 10 10 10 10 10 10 • Growth: PPB: initial growth fast, then slow growth and low maximum size 0 0 0 0 0 0 0 0 0 0 0 0 5 5 5 5 5 5 10 10 10 10 10 10 15 15 15 15 15 15 20 20 20 20 20 20 25 25 25 25 25 25 Age Age Age

Biology - continued 40 40 40 40 • Females grow faster and reach larger size than males 30 30 30 30 27 cm LML Length (cm) Length (cm) Implication: 20 20 20 20 • Females are vulnerable F e m a le s M a l e s to harvesting for a greater period of their life 10 10 10 10 • Many males never reach the 27 cm LML 0 0 0 0 0 0 0 0 5 5 5 5 10 10 10 10 15 15 15 15 20 20 20 20 25 25 25 25 Age Age • Older fish - more males 85% of PPB sand flathead >10 years old were males • Larger fish - more females 90% of PPB sand flathead ≥ 30 cm were females 80% of PPB sand flathead ≥ 27 cm were females

Biology : Spawning • Spawning period September– March, peak from November – January • Broadcast, serial batch spawners • Larvae / small juvenile can be found throughout the bay from 5 - 24 m • No studies of individual egg production rates and how these vary with size/weight Larva 3 weeks, 10 mm Small juvenile, 2 months Larval flathead otolith

Diet – most recent study 1996-2007 Hirst et al. 2014 _RFL project The most common prey in the stomachs of sand flathead from Algae 1996–2007: Gastropod Isopod Bivalve others Cephalopod Small crabs Mollusc: Philine angasi . Bug Unid. Amphipod Opistobranch Shrimp Pelagic fish polychaete Benthic fish Crab Bait fish: Small benthic fish: anchovies, sprats gobies

Port Phillip Bay: fishery history Commercial Highly productive fishery for most of 20 th century • • Prior to 1960’s: 150 - 300 tonnes / year, mostly mesh nets and long-line Scallop dredging, offshore flathead fishery started Cycles ~ 10 years between periods of peak catches

Recreational fishery Elwood 1948 In January 1962, 1208 recreational fishing boats were counted on a Sunday morning from an aerial survey (Lynch 1966). Most commonly caught fish by anglers in PPB over many years Elwood 1952

Recent catch history 1978 - onwards Recent history • Catch/fishing pressure on sand flathead has reduced over time 1987: 2000/01 • Target preference for other species rec catch est. 437 t 320 tonnes MacDonald and Hall (1987) LML 25 to 27 cm Bag limit 30 to 20 2006/07 110 tonnes

Sources of stock status information Large otter trawl survey Fishery Independent (scientific surveys) • March 1990 - 2011

Fishery independent surveys………. Small beam trawl survey • March 2000, night – ongoing

Fishery dependent Commercial long-line • 1978-2014 Recreational creel surveys • 1989-93, 95,96, 2002 – ongoing Diary anglers • 2001 – ongoing

Length compositions of surveys methods: 25 cm Diary angler Large trawl 25 cm 20 2009 - pre 10 n=3,519 % frequency 0 10 15 20 25 30 35 40 45 16 Small trawl 14 12 2010 - post < 1 year old 10 8 6 n=4,907 4 2 0 10 15 20 25 30 35 40 45 Total length (cm)

Length composition continued….. 13% retained < 27 cm Creel survey – retained catches 2% retained < 25 cm 20 25 cm 1996 -2009 1989 -1992 10 % frequency n=3,790 50% retained < 27 cm 30% retained < 25 cm 0 20 25 30 35 40 45 27 cm 20 2010-post 10 n=3,404 5 % retained < 27 cm 0 <1% retained < 25 cm 20 25 30 35 40 45 Total length (cm)

Performance indicators • Biomass (large otter trawl): absolute biomass estimates • Catch rates (catch per unit of effort, CPUE): proxy for abundance/biomass • Length and age compositions: fishery selectivity, growth, mortality/fishing impacts • Biomass and catch rates indices can be determined by life-stages, • Mature fish ≥ 25 cm • O-age (young of year) ≤ 15 cm TL

Large otter trawl – ceased 2011 Total PPB biomass (tonnes) > 15 cm Mature PPB biomass (tonnes) ≥ 25 cm 6000 5000 Total biomass (tonnes) Lots of small fish 4000 3000 2000 1000 0 1990 1995 2000 2005 2010 Year (March)

Commercial long-line Mature biomass indicator (> LML) Annual catch rate 3 yr average catch rate Long-line catch rate not considered indicative post-2015 due to negligible catches

Small beam trawl Mature biomass (≥ 25 cm) indicator 0.003 0.0025 Density (No. 1000 m- 2 ) 0.002 0.0015 0.001 0.0005 0 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 Year (March) Large otter trawl ceased

Diary anglers: targeted flathead trips Mature fish ≥ 25 cm

PPB Creel survey (boat based) Fish per angler hour ≥ 27 cm General recreational catch rates (adjusted) ≥ 27 cm 0.2 0.4 0.6 0.8 1.2 1.4 1.6 0 1 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 Fiscal Year 2002/03 2003/04 2004/05 2005/06 2006/07 2007/08 2008/09 2009/10 2010/11 2011/12 2012/13 2013/14 2014/15 2015/16 2016/17

Length composition tends Recreational Creel survey – legal fish all length data Scientific survey Small beam trawl Sub-set ≥ 25 cm • Stable, slight increase last 3 years

Juvenile recruitment (replenishment) 1988-2018 Sand flathead (3-6 months old, ≤ 15 cm) Combined otter trawl - beam trawl recruitment index

Age composition: Diary anglers >27 cm • Legal fish mostly 3-7 years age, • Born from 2000 onwards

Juvenile recruitment (replenishment) 1988-2018 Sand flathead (3-6 months old, ≤ 15 cm) Current stock Combined otter trawl - beam trawl recruitment index

Perspective Diary angler mature biomass Comparison year: 2004/05 1.8 Otter trawl mature biomass Creel survey => 27 cm Depletion relative to 2004/05 1.6 Beam trawl mature biomass Long-line 1.4 2004/05 50% 2004/05 1.2 1 0.8 0.6 0.4 0.2 0 1978/79 1980/81 1982/83 1984/85 1986/87 1988/89 1990/91 1992/93 1994/95 1996/97 1998/99 2000/01 2002/03 2004/05 2006/07 2008/09 2010/11 2012/13 2014/15 2016/17

Stock status summary • Mature (spawning) biomass has shown three regimes since late1970’s :  Intermediate: 1978 -1993  High: 1994-2003  Low: 2006-current • Current low regime stabilised from 2010, recent signs of increase • Recent low biomass regime driven by pro-longed low recruitment levels • Given similar fishing pressure and recent recruitment levels, current low biomass regime expected to continue, with slight further increase expected in the short-term.

Environmental influences on recruitment: river flow – nutrients inputs / larval food 1987/88 - 2017/18 1989 ~ 40% variation explained 1993 • Recruitment variation shows a complex relationship with river flow = nutrients inputs

PPB flathead production v rainfall history From Hirst et al. 2014 5 year moving averages Catch Rainfall 810 400 Commercial Catch (tonnes) Annual Rainfall (mm) 720 300 630 200 100 540 0 450 year Greater chances of high recruitment events in wetter periods

Thank you

Some unknowns: has productivity shifted? • Changes in growth and mortality rates across time Effects on productivity/recovery potential • Lower egg production / reproductive potential • Environment / habitat / prey community changes • Long-term genetic changes / selective harvesting Causal fast growers / females • Species competition and predation

% males v females at age PPB Aged samples, n=1247: 2008, 2009, 2010, 2011, 2017 85% of sand flathead >10 years old are males

Males v females across age and length More older males Males Females Larger fish dominated by females From Koopman et al. 2004

Length composition of Port Phillip Bay sand flathead by survey methods 2009: LML 25 > 27 cm Small beam trawl Large otter trawl 18 16 2009-pre 2009-pre 14 % frequency 12 10 8 6 n=3,859 4 n=54,507 2 0 0 5 10 15 20 25 30 35 40 10 % frequency 8 2010-post 2010-post 6 4 n=3,661 2 n=2,301 0 0 5 10 15 20 25 30 35 40 Total length (cm)

25 cm 20 2009 - pre 10 n=3,519 % frequency 0 10 15 20 25 30 35 40 45 Diary angler 16 14 12 2010 - post 10 8 6 n=4,907 4 2 0 10 15 20 25 30 35 40 45 Total length (cm)

From Hobday et al. 1999

45 cm Maximum sizes 50+ cm Bass Strait sand flathead