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

2. Fisheries management Advice Implementation
(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 Conclusion: Low F in long term
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. Used for testing harvest policies Simple extensions
- forward projections
Minke
Fin
Humpback
Used for testing
harvest policies
Grey
seal
Cod
The earlier work included only cod, capelin and shrimp. The cod dynamics include a stock-recruitment function, which also includes a term for cannibalism by immature fish. The cod consume capelin and shrimp, so that an increase in the cod stock is likely to reduce the future shrimp and capelin catches. Density dependent growth of cod is also taken into account as is the effect of capelin biomass on cod growth.
The model introduced in this paper also includes population dynamics on the three whale stocks: Minke, fin and humpback. The minke has been observed to eat cod and all the whale stocks eat capelin.
The population model for the whales is the usual Pella-Tomlinson model.
The effect of minke whales eating cod is modelled by using the current estimate that 6% of the minke diet is cod and translating that into a part of the current natural mortality, M=0.2. When the minke stock size increases, this will have the effect of increasing M from the current value.
Similarly, all predation on capelin is translated into M values, and each predator is assumed to increase the natural mortality inflicted on capelin in proportion to the number of predators.
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 Uncertainty: Areal closures:
Better statistical models
Areal closures:
Spatial models
Effort control, analysis:
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

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? ?
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.