population dynamics models state of art and future of modelling fish populations gunnar stefansson...
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Population dynamics modelsState of art and future of
modelling fish populations
Gunnar StefanssonMarine Research Institute/Univ. Iceland
Fisheries management• Advice
– (On annual quotas)– On long-term utilisation– On control systems
• Implementation– Short-term (tactics)– Systems (strategy)
• Interaction between system and advice
Single species/stock
Conclusion: Low F in long term
0
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Y/R S/R
F
050
100150200250300350400450
0 500 1000 1500 2000
S
R
Classical models - well known since 1954:
Density dependent growth
Cannibalism
...
Missing:
Age- and time-variable natural mortality
Food supply effect
Assumptions - examples of testing
Density dependent growth
Cannibalism
Time- and age-dependent natural mortality
Effects of food supply
Effects of uncertain assessments
Environmental variability
... Minor effects on policyConsiderable effects on long-term catch predictions
Capelin
Cod
Minke Fin Humpback
Simple extensions- forward projections
Shrimp
Greyseal
Harbour seal
Used for testing harvest policies
Table 1. Decomposition of total mortality (Z) of cod into components in 1994
1 2 3 4 5 Total 1-3
Grey seal 0.05 0.02 0.02 0.01 0.00 0.09Harbor seal 0.06 0.06 0.04 0.02 0.00 0.16Minke 0.04 0.04 0.04 0.04 0.04 0.13
Predation M 0.15 0.13 0.09 0.08 0.05 0.37
Cannibalism 0.19 0.00 0.00 0.00 0.00 0.19Resid. M 0.16 0.16 0.16 0.16 0.16 0.49
Total M 0.50 0.29 0.26 0.24 0.22 1.06Fishing 0.00 0.00 0.05 0.24 0.45 0.05
Total Z 0.50 0.29 0.30 0.48 0.67 1.10
M: Very easy to test various assumptions
Models - more
• Effect of reduced fishing on predator?
• Effect of increased harvest of prey?
• Effect of fishing in spawning area?
• Effect on bycatch species?
• Uncertainty in estimates?
• Predictive capability?
Need statistical multispecies spatially explicit models
Motto of the day
There are three kinds of lies:
• lies,
• damned lies and
• statisticsDisraeli
Models - statistics
• Natural variation
• Measurement errors
• Nontrivial effects of incorrect methods...
• Estimation of unknowns
• Prediction of effects with uncertainty
Conclusion: Lower F
Models - current status
• Greater uncertainty than earlier thought
• Multispecies concerns are important
• Statistical techniques essential
• Need holistic models for understanding
Control mechanisms
• Closed areas
• TAC
• Effort regulation
• Mesh sizes (fishing gear limitations)
Overcapacity
• Introduces problems in all control systems
• Reduces likelihood of efficiency in any control measure
• Increases political pressure and likelihood of deviations from earlier policy
• Needed: Models of these effects
Models and systems
• Uncertainty: – Better statistical models
• Areal closures: – Spatial models
• Effort control, analysis: – Spatial models– Multispecies, technical
interactions
• TAC control: – Multispecies, technical
interactions
• Understanding any controls: – Need to estimate effect of
major change in predator on prey abundance and vice versa
– Multispecies, biological interactions
Results from current models
• Uncertainty output: – Need lower F
• Multispecies output: – Need lower F on prey
• Areal closures: Large areas (or more controls)
• Effort control:Lower effort+annual reductions+TAC
• TAC control: Lower TAC+effort/fleet reductions
• Almost all analyses:– Need lower F
Limitation summary
• TAC: Species allocation mismatch+uncertainty
• Closed area: Migration/fishing outside+uncertainty
• Effort control: Effort reallocation+catchability
• Fleet reduction alone: Like effortCommon effects of levels of measures:
10% reductions: No effects
50% reduction: Some effect likely but can be negated
90% reductions: Almost sure effects but may lose catches
Solutions?
Extreme measures?
or
Combined systems?
or
?
No single system, set at its target will suffice in general!
Current theme
Marine resources can be harvested using the maximum fleet size economically possible up to that maximum level of fishing mortality which does not demonstrably lead to stock collapse.
A new tenet
Marine resources should be harvested using the minimum fleet size possible and at that minimum level of fishing mortality which does not demonstrably lead to a serious long-term loss of catch.