Effects of fishing on exploited fish populations: Two case studies (Pleuronectes platessa, Hippoglossus hippoglossus)

In this thesis, the effect of exploitation on harvested fish populations is studied. In the first chapter, covariation of age and length at maturity of plaice (Pleuronectes platessa) in Icelandic waters was studied from 1955 to 2000 to determine whether there have been any evolutionary changes in the reaction norms for this trait due to fishing. The results indicate that the maturation reaction norm did not change significantly for either sex during the study period. This lack of change is due to the fishing regulations used; nursery areas have been closed or partially closed to fishing for the last 50 years, so that primarily mature fish have been exploited. In the second chapter, I study the decline of abundance of Atlantic halibut (Hippoglossus hippoglossus). One explanation for this decline is that Atlantic halibut is a long-lived and late maturing species and often harvested as bycatch many years before reaching maturity. (Abstract shortened by UMI.)...