Fisheries-induced adaptive change in reproductive investment in North Sea plaice (Pleuronectes platessa)?

Abstract

Life history theory predicts that fishing may select for increased reproductive investment. A model of the reaction norm for reproductive investment in a capital breeder was developed to disentangle changes in reproductive investment from changes in growth rate in North Sea plaice (Pleuronectes platessa). Trends in reproductive investment since 1960 were estimated as (i) the decrease in body weight of mature males and females between the start and end of the spawning period, (ii) the difference in weight of ripe and spent females, and (iii) the ovary weight of prespawning females. These estimates were related to somatic growth estimated by back-calculation of otoliths and temperature. The ovary weight and weight loss of females that had just started and just finished spawning did not reveal any trends. There was a significant increase in weight loss over the spawning season in both sexes, but much of this increase was likely due to changes in environmental conditions. Evidence for a fisheries-induced change in reproductive investment from our analyses thus remained inconclusive. However, fecundity and ovary-weight data from previous studies tentatively suggest that an increase in reproductive investment occurred between the late 1940s and the 1960s. Such an increase is consistent with a fisheries-induced evolutionary change.