Proxies for estimation of relative fishing mortality

when biomass is unknown

Rainer Froese, GEOMARArlene Sampang, FIN

ICES ASC, Bergen, 18 September 2012ICES CM 2012/G:2815

Principles of Ecosystem-Based Fisheries Management

• Overall goal: Extract pretty good catches with least impact on the stocks and on the ecosystem

• Consider species-interactions: – Do not cause more mortality than all other

predators combined (F <= M)– Leave enough biomass of forage fish (F <= 2/3 M)

(Pikitch et al. 2012 )• Do not use destructive gears

2

Binding MSFD to the Rescue

(Descriptor 3): “Populations of all commercially exploited fish and shellfish are within safe biological limits, exhibiting a population age and size distribution that is indicative of a healthy stock.”•3.1: F <= Fmsy or U <= Umsy

•3.2: SSB >= SSBmsy

•3.3.1: Proportion of fish larger than the mean size of first sexual maturation•3.3.3: 95 % percentile of the fish length distribution

MSFD Problem: Reference points and data are lacking

• 3.1: F <= Fmsy or U <= Umsy

– If no Fmsy is available, use natural mortality M instead: F <= M

– If no F is available, get F/M proxy from mean length in catch

• 3.2: SSB >= SSBmsy

– If no biomass is known, use mean length in the catch: Lmean >= LF=M

• 3.3.1: Proportion of fish larger than the mean size of first sexual maturation– Use mean length in catch relative to maturity: Lmean >= 1.2 Lm

• 3.3.3: 95 % percentile of the fish length distribution– Use mean length in catch relative to the length where the biomass of

cohorts is maximum: Lmean >= 0.9 Lopt

Equations

Mean length in catch

Proxy for F/M

Expexted mean length if F = M

Optimum length of capture

t

ttmean N

NLL

MK

MLKLL c

MF 2

2

K

MLLopt

3

3

1)(

)(

cmean

mean

LLM

LLK

M

F

M observed vs M = 1.5 K

0.01

0.10

1.00

10.00

100.00

0.01 0.1 1 10 100

M from 1.44 K

M o

bse

rved

M from 1.5 K (Jensen 1996)Data from FishBase

If K/M = 2/3

Proxy for F/M

Expexted mean length if F = M

Optimum length of capture Lopt = 2/3 L∞

4

3

LLL c

MF

1)(3

)(2

cmean

mean

LL

LL

M

F

M as proxy for Fmsy

Relation between Fmsy and M for eight ICES stocks. On average for 34 stocks, Fmsy = 1.5 M.

Simple vs Demanding L(F=M)

Mean length where F=M for nine ICES stocks, estimated with (X-axis) and without K and M.

Simple vs Demanding Lopt

Length where cohort biomass is maximum, for nine ICES stocks, estimated with (X-axis) and without K and M.

3.1 Fishing pressure F / Fmsy

Mean F2005ff relative to Fmsy for eight ICES stock

3.1 Fishing pressure F / M

3.2 Biomass vs 2*SSBpa

3.2 Stock size from mean length

3.3 Healthy Age Structure

Numbers and length at age, with reference points, similar to Eastern Baltic cod

3.3 Healthy Age Structure

Numbers and length at age, with reference points, with optimum fishing: F=M, Lmean = Lopt.Note higher catches.

3.3.1 Maturity

3.3.3 Optimum Length, Old Fish

Summary of MSFD Status

Stock F/Fmsy F/M SSB /(2*SSB pa) L mean/L m L mean/L opt L mean/L (F=M ) MSCspr-2232 1.1 1.4 1.7 1.1 1.1her-30 0.7 0.7 1.0 1.0 1.2her-31 2.0 0.9 1.0 1.1cod-2532 1.6 2.4 1.1 0.6 0.8 Yesple-nsea 1.1 2.9 0.8 1.3 0.7 1.0 Yessai-nsea 1.0 1.5 0.7 0.8 0.7 0.9 Yesher-nsea 0.6 0.4 0.7 1.5 1.2 1.6 Yescod-arct 1.0 1.9 1.1 0.8 0.7 1.0 Yesher-noss 1.0 1.0 0.8 1.3 1.3 1.5 Yes

Thank You

Rainer Froese, GEOMAR rfroese@geomar.de