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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
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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
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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
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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
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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
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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
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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
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M as proxy for Fmsy
Relation between Fmsy and M for eight ICES stocks. On average for 34 stocks, Fmsy = 1.5 M.
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Simple vs Demanding L(F=M)
Mean length where F=M for nine ICES stocks, estimated with (X-axis) and without K and M.
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Simple vs Demanding Lopt
Length where cohort biomass is maximum, for nine ICES stocks, estimated with (X-axis) and without K and M.
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3.1 Fishing pressure F / Fmsy
Mean F2005ff relative to Fmsy for eight ICES stock
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3.1 Fishing pressure F / M
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3.2 Biomass vs 2*SSBpa
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3.2 Stock size from mean length
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3.3 Healthy Age Structure
Numbers and length at age, with reference points, similar to Eastern Baltic cod
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3.3 Healthy Age Structure
Numbers and length at age, with reference points, with optimum fishing: F=M, Lmean = Lopt.Note higher catches.
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3.3.1 Maturity
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3.3.3 Optimum Length, Old Fish
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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