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

Date

Port Phillip Bay sand flathead stock status update

Paul Hamer, Simon Conron

23/5/2018

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

• Ave. max size = 38 cm

Bass Strait

• Ave. max size = 33 cm

Tasmania

• Ave. max size= 27 cm

5 10 15 20 25 Age 10 20 30 40 50 Length (cm) 5 10 15 20 25 Age 10 20 30 40 50 Length (cm) 5 10 15 20 25 10 20 30 40 50 5 10 15 20 25 10 20 30 40 50 5 10 15 20 25 Age 10 20 30 40 50 Length (cm) 5 10 15 20 25 10 20 30 40 50

Port Phillip Bay

• Growth: PPB: initial growth fast, then

slow growth and low maximum size

Biology - continued

• Females grow faster and

reach larger size than males Implication:

• Females are vulnerable

to harvesting for a greater period of their life

• Many males never reach the

27 cm LML

• Older fish - more males
• Larger fish - more females

5 10 15 20 25 Age 10 20 30 40 Length (cm) 5 10 15 20 25 Age 10 20 30 40 Length (cm) 5 10 15 20 25 10 20 30 40 5 10 15 20 25 10 20 30 40

F e m a le s M a l e s

27 cm LML

85% of PPB sand flathead >10 years old were males 90% of PPB sand flathead ≥ 30 cm were females 80% of PPB sand flathead ≥ 27 cm were females

Biology : Spawning

3 weeks, 10 mm

• 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

Small juvenile, 2 months Larval flathead otolith Larva

Diet – most recent study 1996-2007

Hirst et al. 2014 _RFL project Pelagic fish Benthic fish Crab polychaete Shrimp Opistobranch

• Unid. Amphipod

Cephalopod Bug Isopod Gastropod Algae Bivalve

• thers

The most common prey in the stomachs of sand flathead from 1996–2007:

Small crabs Small benthic fish: gobies Mollusc: Philine angasi. Bait fish: anchovies, sprats

Port Phillip Bay: fishery history

• Highly productive fishery for most of 20th century
• Prior to 1960’s: 150 - 300 tonnes / year, mostly mesh nets and long-line

Scallop dredging,

• ffshore flathead

fishery started Commercial

Cycles ~ 10 years between periods of peak catches

Recreational fishery

Elwood 1948 Elwood 1952 Most commonly caught fish by anglers in PPB over many years

In January 1962, 1208 recreational fishing boats were counted on a Sunday morning from an aerial survey (Lynch 1966).

Recent history

Recent catch history 1978 - onwards 2000/01 320 tonnes 2006/07 110 tonnes 1987: rec catch est. 437 t

MacDonald and Hall (1987)

• Catch/fishing pressure on sand

flathead has reduced over time

• Target preference for other species

LML 25 to 27 cm Bag limit 30 to 20

Sources of stock status information

Large otter trawl survey

• March 1990 - 2011

Fishery Independent (scientific surveys)

Fishery independent surveys………. Small beam trawl survey

• March 2000, night – ongoing

Fishery dependent

Recreational creel surveys

• 1989-93, 95,96, 2002 – ongoing

Commercial long-line

• 1978-2014

Diary anglers

• 2001 – ongoing

Length compositions of surveys methods:

Large trawl Small trawl < 1 year old Diary angler

% frequency Total length (cm)

2010 - post

10 15 20 25 30 35 40 45 2 4 6 8 10 12 14 16 n=4,907

2009 - pre

10 15 20 25 30 35 40 45 10 20 n=3,519

25 cm 25 cm

Length composition continued….. Creel survey – retained catches

20 25 30 35 40 45 Total length (cm) 10 20 20 25 30 35 40 45 10 20

% frequency 1996 -2009 2010-post

n=3,790 n=3,404

25 cm 27 cm

1989 -1992 50% retained < 27 cm 30% retained < 25 cm 13% retained < 27 cm 2% retained < 25 cm 5 % retained < 27 cm <1% retained < 25 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

Lots of small fish 1990 1995 2000 2005 2010 Year (March) 1000 2000 3000 4000 5000 6000 Total biomass (tonnes)

Commercial long-line

Long-line catch rate not considered indicative post-2015 due to negligible catches

Mature biomass indicator (> LML)

Annual catch rate 3 yr average catch rate

Small beam trawl Mature biomass (≥ 25 cm) indicator

0.0005 0.001 0.0015 0.002 0.0025 0.003 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018

Density (No. 1000 m-2) Year (March)

Large otter trawl ceased

Diary anglers: targeted flathead trips Mature fish ≥ 25 cm

PPB Creel survey (boat based)

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

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 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

Fish per angler hour ≥ 27 cm Fiscal Year

General recreational catch rates (adjusted) ≥ 27 cm

Length composition tends

• Stable,

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

Juvenile recruitment (replenishment) 1988-2018

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

Age composition: Diary anglers

>27 cm

• Legal fish mostly 3-7 years age,
• Born from 2000 onwards

Juvenile recruitment (replenishment) 1988-2018

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

Perspective

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 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

Depletion relative to 2004/05

Otter trawl mature biomass Creel survey => 27 cm Beam trawl mature biomass Long-line 2004/05 50% 2004/05

Comparison year: 2004/05

Diary angler mature biomass

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

• Recruitment variation shows a complex relationship with river flow

= nutrients inputs

1989 1993 ~ 40% variation explained

PPB flathead production v rainfall history

From Hirst et al. 2014

year 100 200 300 400 Commercial Catch (tonnes) 450 540 630 720 810 Annual Rainfall (mm)

5 year moving averages

Catch Rainfall Greater chances of high recruitment events in wetter periods

Thank you

Some unknowns: has productivity shifted?

• Lower egg production / reproductive potential
• Environment / habitat / prey community changes
• Species competition and predation
• Long-term genetic changes / selective harvesting

fast growers / females

• Changes in growth and mortality rates across time

Causal Effects on productivity/recovery potential

% 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

Males Females More older males Larger fish dominated by females

From Koopman et al. 2004

Length composition of Port Phillip Bay sand flathead by survey methods

Large otter trawl Small beam trawl

Total length (cm) 5 10 15 20 25 30 35 40 2 4 6 8 10 % frequency 2 4 6 8 10 12 14 16 18 5 10 15 20 25 30 35 40 % frequency

2009-pre 2010-post

n=3,661 n=3,859

2009-pre 2010-post 2009: LML 25 > 27 cm

n=2,301 n=54,507

Diary angler

% frequency Total length (cm)

2010 - post

10 15 20 25 30 35 40 45 2 4 6 8 10 12 14 16 n=4,907

2009 - pre

10 15 20 25 30 35 40 45 10 20 n=3,519

25 cm

From Hobday et al. 1999

50+ cm Bass Strait sand flathead

Maximum sizes 45 cm