Proxies for estimation of relative fishing mortality when

Proxies for estimation of relative fishing mortality when biomass is unknown Rainer Froese, GEOMAR Arlene Sampang, FIN ICES ASC, Bergen, 18 September 2012 ICES 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 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

M observed vs M = 1. 5 K M from 1. 5 K (Jensen 1996) Data from Fish. Base

If K/M = 2/3 Proxy for F/M Expexted mean length if F = M Optimum length of capture Lopt = 2/3 L∞

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 F 2005 ff 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

Thank You Rainer Froese, GEOMAR rfroese@geomar. de