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Proxies for estimation of relative fishing mortality when biomass is unknown

Principles for sustainable fisheries management using proxies in the absence of biomass data. Reference points and methodologies outlined for effective ecosystem-based approach.

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Proxies for estimation of relative fishing mortality when biomass is unknown

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  1. 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

  2. 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

  3. 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

  4. 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

  5. Equations Mean length in catch Proxy for F/M Expexted mean length if F = M Optimum length of capture

  6. M observed vsM = 1.5 K M from 1.5 K (Jensen 1996) Data from FishBase

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

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

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

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

  11. 3.1 Fishing pressure F / Fmsy Mean F2005ff relative to Fmsy for eight ICES stock

  12. 3.1 Fishing pressure F / M

  13. 3.2 Biomass vs 2*SSBpa

  14. 3.2 Stock size from mean length

  15. 3.3 Healthy Age Structure Numbers and length at age, with reference points, similar to Eastern Baltic cod

  16. 3.3 Healthy Age Structure Numbers and length at age, with reference points, with optimum fishing: F=M, Lmean = Lopt. Note higher catches.

  17. 3.3.1 Maturity

  18. 3.3.3 Optimum Length, Old Fish

  19. Summary of MSFD Status

  20. Thank You Rainer Froese, GEOMAR rfroese@geomar.de

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