proxies for estimation of relative fishing mortality when biomass is unknown rainer froese, geomar...
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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
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
