Population dynamics models State of art and future of modelling fish populations

1. Population dynamics modelsState of art and future of modelling fish populations Gunnar Stefansson Marine Research Institute/Univ. Iceland

2. Fisheries management • Advice • (On annual quotas) • On long-term utilisation • On control systems • Implementation • Short-term (tactics) • Systems (strategy) • Interaction between system and advice

3. Single species/stock Classical models - well known since 1954: Density dependent growth Cannibalism ... Missing: Age- and time-variable natural mortality Food supply effect Conclusion: Low F in long term

4. 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 policy Considerable effects on long-term catch predictions

5. Simple extensions - forward projections Minke Fin Humpback Used for testing harvest policies Grey seal Cod Capelin Harbour seal Shrimp

6. M: Very easy to test various assumptions

7. 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

8. Motto of the day There are three kinds of lies: • lies, • damned lies and • statistics Disraeli

9. Models - statistics • Natural variation • Measurement errors • Nontrivial effects of incorrect methods... • Estimation of unknowns • Prediction of effects with uncertainty Conclusion: Lower F

10. Models - current status • Greater uncertainty than earlier thought • Multispecies concerns are important • Statistical techniques essential • Need holistic models for understanding

11. Control mechanisms • Closed areas • TAC • Effort regulation • Mesh sizes (fishing gear limitations)

12. 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

13. Models and systems • 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 • Uncertainty: • Better statistical models • Areal closures: • Spatial models • Effort control, analysis: • Spatial models • Multispecies, technical interactions

14. Results from current models • Uncertainty output: • Need lower F • Multispecies output: • Need lower F on prey • Almost all analyses: • Need lower F • Areal closures: Large areas (or more controls) • Effort control:Lower effort+annual reductions+TAC • TAC control: Lower TAC+effort/fleet reductions

15. Limitation summary • TAC: Species allocation mismatch+uncertainty • Closed area: Migration/fishing outside+uncertainty • Effort control: Effort reallocation+catchability • Fleet reduction alone: Like effort Common 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

16. Solutions? Extreme measures? or Combined systems? or ? No single system, set at its target will suffice in general!

17. 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.

18. 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.