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

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

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

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

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

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 statistics Disraeli

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: 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

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

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

Solutions? Extreme measures? or Combined systems? ? 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.