Model shows that age structure is not a viable criterion.
Fishers cannot influence age structure much.
But do the criteria serve the purpose? Wageningen fish biologists looked into the matter and came to the conclusion that age structure was not a good criterion for healthy fish stocks. What is more, fishers cannot influence it much. Thomas Brunel and GerJan Piet of IMARES reported the findings in the ICES Journal of Marine Science.
Using a model, the researchers simulated the population dynamics of North Sea cod, sole and herring over a period of 100 years. They compared five theoretical selection patterns in which young or, by contrast, older fish were spared. The model showed that the age structure of the fish populations had very little effect on the two main criteria for healthy fish stocks, namely strong resilience in the face of change and fast recovery after fishing.
The selection pattern did have one major effect, though. The researchers modeled various scenarios in which the fishing targeted small young fish or larger older fish, and various sizes and ages in between. After 100 years fish catches appeared to vary considerably across the different scenarios. If the large fish were spared, the catch was about one third the size of the catch in the scenario in which the young fish were spared. But sparing large fish offered the advantage of far fewer fluctuations in the fish stocks over the year.
The researchers concluded that a policy objective focusing on a 'healthy' age structure without any clear criteria for health does not provide a good basis for fisheries policy. Nor is it easy to implement. Fishers can make sure that they don't catch young fish by using nets with a large mesh, but it is much harder to avoid catching large fish. The findings were tabled at a meeting of scientists of the European Commission in Brussels last month.