Maturation Dynamics of Arcto-Norwegian Cod

Many commercially important fish stocks are harvested with very high exploitation rates with the consequence of substantial changes in stock parameters as well as reduced harvest rates and, in the worst case, stock depletion. On a long-term the Arcto-Norwegian cod is the most productive cod stock in the world. It has a history of exploitation many hundred of years long and has been heavily exploited since about 1950. The chosen exploitation strategy might have substantial effects on the short-term production of the stock, and a serious question is if a long-term and heavy exploitation may change genetic properties of the stock and hence its reproductive and production potential. In this paper, age and size data of spawning cod, going back to 1932, are analyzed with respect to long-term changes caused by exploitation. During the studied period, average age and size at first spawning have been reduced by about three years and 20 cm. Immature growth has also increased substantially. It is shown that a lot of the variability in age and size at first spawning can be explained by the exploitation itself and, even better, by stock biomass (density dependency). Other factors, like food availability (capelin abundance) and year class strength, also seem to play a role. Due to more or less continuous trends in the data from World War II onwards, it is difficult to disentangle temporal effects (environmental forcing) from other causal agents.