Joint assessment of demersal species

WG demersal spp. 16-18 Sept

Graham Pilling

Cefas

Aims

• Progress in the enhancement of joint practical stock assessment

• “Joint” – “participation of scientists from different GSAs providing their data and sharing them with their colleagues, using a standard methodology and analyzing together the results and options for fisheries management”

What did we do?

• Discussion of biological parameters

• Development of new stock assessments

• Discussion of reference points

Discussion of biological params

• No conclusions– Use of a ‘standard’ set of parameters for a

spp.– Area-specific parameters

Perform joint stock assessments• Many assessments pre-performed & discussed• New data available for GSA03 for hake

– Survey data time series– Use of SURBA

• Data for hake for GSA16– Survey data time series

• Data for hake for GSA01– Use of SURBA

• Data for sole for GSA17– Use of SURBA – insufficient time series

• Discussion of GSA24 hake data (presented)

Hake in GSA-03

0

5

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45

1992 1994 1996 1998 2000 2002 2004 2006 2008

Bio

mas

s d

ensi

ty (

kg,k

m^

-2)

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10

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35

1992 1994 1996 1998 2000 2002 2004 2006 2008

Mea

n s

ize

(cm

)

Hake in GSA-03

• Use of SURBA for assessment of survey time series

• SURBA – based on tracking cohorts– Requires age data– We have length frequency data

Hake in Morocco Survey 2001

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Length (cm)

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bers

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e

Hake in Morocco Survey 2000

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Length (cm)

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bers

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Hake in Morocco Survey 1999

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Length (cm)N

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rs p

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ille

Implications

• Age-slicing to estimate age distribution open to uncertainty

• Assuming constant growth

• Ability of SURBA to track resulting ‘cohorts’ is compromised

SURBA results - F

Y ear

0

.5

1

1.5

2

2.5

3

3.5

4

4.5

Me

an

F (

1-3

)

G S A 03H ake: M ean F

1995 2000 2005

SURBA results - SSB

Y ear

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10000

20000

30000

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80000

S S B at survey tim eG S A 03H ake: S S B

1995 2000 2005

Discussion

• Preliminary assessment only

• Model diagnostics suggest considerable uncertainty

• VIT analysis (on survey data only NOTE) suggests F2007~1.1

• Is it better to use swept area estimates?

Hake in GSA 01

• Preliminary assessment

• Very early stages – data completed today

SURBA results - F

Y ear

-.6

-.4

-.2

0

.2

.4

.6

.8

1

1.2

Mea

n F

(1-

3)

M E R LME R G S A 01 ME D ITS Q 3: M ean F

1995 2000 2005

SURBA results - SSB

Y ear

0

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S S B at survey tim eM E R LME R G S A 01 ME D ITS Q 3: S S B

1995 2000 2005

Discussion

• Preliminary assessment only

• Need to look at underlying age/length structure to understand

Hake in GSA 16

• Run on MEDITS data

• Used SURBA for GSA16

SURBA results - F

Y ear

-.2

0

.2

.4

.6

.8

1

1.2

1.4

1.6

Mea

n F

(1-

3)

hakegsa16m editsQ 3: M ean F

1995 2000 2005

SURBA results - SSB

Y ear

0

200

400

600

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1000

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1400

S S B at survey tim ehakegsa16m editsQ 3: S S B

1995 2000 2005

Discussion

• F from SURBA close to that from LCA

Reference points

• General discussion on reference points as a basis for definition of ‘fully exploited’, ‘overexploited’ etc.

ICES framework

• Advice given vs limit and precautionary ref points• Moving towards targets (e.g. WSSD MSY requirements)• Approach currently incompatible with GFCM data (SRR and

data time series requirements)• Values have not changed for years, while biology may

Summary of discussions

• Overall aim: avoid stock collapse and recruitment impairment

• YPR-based methods cannot identify recruitment overfishing

• Fmax does not inform about recruitment overfishing

• Growth overfishing – a political choice? (new mesh size rules may help)

Summary of discussions

• Role is to supply advice to managers– Alternative future management options and their

consequences, on which managers can make a decision

Summary of discussions• Discussion of reference directions, rather

than reference points– Hilborn et al. suggestions

• Aim for improvement in the situation for the stock, and the fishery

• Works in a multispecies context?

Summary of discussions

• Provocative suggestion of:– Aim for reductions in effort required to double

SSB

• Needs to be on a case-by-case basis (different biology, fishing practices, etc.)

• Scientific justification for recommendations

Summary of discussions

• Need to be discussed in collaboration with managers– If they want an improvement in the situation,

then need to agree the basis– Stock assessment scientists provide advice

based upon the best scientific information available

• Don’t forget to simulation test!

Did ‘joint’ WG meet ToR?

• Yes– New assessments (GSA 01, 03, 16)– Attempt for Sole in GSA 17– Assessments reviewed– Familiarity with SURBA gained along with

understanding of limitations– Basis for future analyses of MEDITS data– Reference point issues discussed

Did ‘joint’ WG meet ToR?

• No– Assessments performed before the meeting– Limited updates of presented assessments– Data not necessarily available at the meeting– Quality of most ‘new’ assessments too low for

management advice