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Bioeconomic modelling of seal impacts on West of Scotland fisheriesVanessa Trijoulet, 1st year PhD student – [email protected], website: http://www.strath.ac.uk/mathstat/staff/researchstudents/vanessatrijoulet/
Materials and methods
Bioeconomic modelling of seal impacts on West of Scotland fisheriesVanessa Trijoulet1, Alex Dickson2 and Robin Cook1
1Department of Mathematics and Statistics, University of Strathclyde, Glasgow, Scotland – [email protected] of Economics, University of Strathclyde, Glasgow, Scotland
IntroductionIn the north Atlantic Ocean, the 20th century has been marked by a change in fish stocks resulting in the depletion of high value commercial fish such as the Atlantic cod (Gadus morhua) (ICES 2012). Meanwhile, scientists have noticed an increase in the grey seal (Halichoerus grypus) population (SCOS 2012).
This situation has created a significant conflict between fishermen and conservationists as regards to the role that grey seals may have played in the stock depletion. This is all the more intensified by the fact that fishermen see their costs increase due to seal damage to gear or fish.
Currently, opinions remain divided, and it seems that further studies need to be done to measure the impact of seals on fisheries and to propose future seal and fishery management strategies.
ObjectivesCreate a bioeconomic model able to quantify the economic impact of grey seal predation on West of Scotland cod, haddock and whiting fisheries.
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Economic model
Multispecies: cod, haddock, whiting
Multifleet: Types: bottom trawlers (OTB), others
Mesh-sizes: TR1 (>=100mm), TR2 (70-99mm)Boat lengths: <10m, 10m-24m, >=24m
Age-structured model
Z = F + M + Mseal
Impact of seal predation on commercial fish stocks
Economic impact of seal predation on fisheries
Effort functionVariation in fleets effort according to
fishermen profits
Cost functionTotal costs = fixed costs + variable costs(effort)
Profit functionPresent value of profits discounted back to the
present
Biological model
ObjectivesCreate a bioeconomic model able to quantify the economic impact of grey seal predation on West of Scotland cod, haddock and whiting fisheries.
West of ScotlandICES VIa
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Preliminary results for codSimulations from year 2012 with test of 4 levels of seal population (S) and of fishing mortality (F), S and F constant for the whole simulation period:
- S=0 (Mseal=0), S=32,000 (SCOS 2012), S=16,000 (S/2) and S=64,000 (S*2)- F=ICES 2012, F-30% and F+30%
Conclusions• Omitting seal predation in fish assessment can significantly change the
response of the stock to changes in F.
• Necessity to decrease F or collapse in 50 years.
• Halving seal population cannot prevent cod collapse if F stays constant or increases.
• TR1 fleets (mesh-size >100 mm) are the most impacted by seal predation since they represent 70 to 80 % of cod catches in VIa.
• Following ICES assessment condition (Mseal=0), a decrease of 30% in F can induce cod recovery, which is not the case when Mseal is taken into account (decrease in SSB of 90% after 60 years).
Future directions• Develop the biological model for haddock and whiting.
• Develop the economic model to assess the impact of seal on fishery profits including fishermen behaviours.
• Test management propositions.
AcknowledgementsWe thank the University of Strathclyde and Marine Scotland for funding this project. We also thank the Sea Mammal Research Unit (University of St Andrews) for making the data on grey seal diet available.
Collapse even if S/2
S*2 has almost the same consequence as F+30% (collapse in ≈30 years)
TR1 fleets are the most impacted by the number of seals
ScenariosExpected
change in SSB after 60 years
Time before collapse (years)
S=0, F=2012 -58.37%S=0, F=-30% +870.64%S=0, F=+30% -99.83% 70S/2, F=2012 -99.08% 98S/2, F=-30% +109.79%S/2, F=+30% -100.00% 44
S=2012, F=2012 -99.98% 53S=2012, F=-30% -90.36%S=2012, F=+30% -100.00% 32
S*2, F=2012 -100.00% 28S*2, F=-30% -100.00% 44S*2, F=+30% -100.00% 21
S=0, F=ICES 2012 S=32,000, F=ICES 2012
Taking into account seal predation in fish assessment is important
Mean50% CIMedian
Bioeconomic modelling of seal impacts on West of Scotland fisheriesVanessa Trijoulet, 1st year PhD student – [email protected], website: http://www.strath.ac.uk/mathstat/staff/researchstudents/vanessatrijoulet/ 2/2
If seal population stays stable, the cod stock will collapse in≈ 50 years
