Fishing-induced early reproduction at the cost of growth in the Baltic cod Gadus morhua?

No abstracts are to be cited without prior reference to the author.

Increased adult mortality is predicted to decrease maturation age and increase reproductive effort at the cost of somatic growth. Using Swedish survey data we assessed whether the Baltic cod (Gadus morhua) stocks (ICES subdivisions 22-24 and 25-30) have undergone evolutionary changes in maturation reaction norm by age and length during years 1989 - 2003. Both male and female cod from the Baltic Proper showed decreasing trends in length at age and condition in several ages but such changes were not detected in subdivisions 22-24. Female cod from the Baltic Proper showed linear trend towards decreased length at 50% probability of maturing at age at several ages but there were no statistically significant trends in males. In the western population, detected trends were not unambiguous. Cod males in the Baltic Proper had a reaction norm for earlier maturation than females, but in the Western Baltic such difference did not exist between sexes. Western cod tended to reach the somewhat larger maturation size at the same age as their slower-growing specimens from the Baltic proper. Good condition at small size and at young age contributed to high maturation probability. Our results suggest that evolutionary effects of fishing might differ between sexes. We suggest that some fisheries-induced evolution has occurred in the heavily exploited cod population of the Baltic Proper, but further studies are needed about factors that led to the concurrent decrease in their condition.