Fisheries-induced trends in reaction norms for maturation in North Sea plaice. ICES CM 2002/Y:04

We analyse the potential of intensive exploitation to have caused evolutionary changes in the age and length at maturation in North Sea plaice Pleuronectes platessa. Such evolutionary change in the onset of maturation is expected given that fishing mortality is more than four times higher than natural mortality. In order to disentangle phenotypic plasticity from evolutionary change, we employ the probabilistic reactionnorm approach. This technique allows us to estimate the probabilities of maturing at each relevant age and size, and to disentangle the plasticity in age and size at maturation that results from changes in growth rates from evolutionary changes in maturation propensities themselves. This recently developed method is here applied to females of 41 cohorts (1955-1995) of North Sea plaice. We focus on trends in fishing mortality, in growth rates, and in the probabilities of maturing, and test the hypothesis that the decrease in age and length at maturation is partly caused by fisheries-induced adaptive change. We find that the reaction norm for age and length at maturation has indeed significantly shifted towards younger age and smaller length. The reactionnorm analysis suggests a picture in which short-term fluctuations originating from plastic responses are superimposed on a persistent long-term trend resulting from genetic responses.