Evaluation of the temporal development of the ovaries in Gadus morhua from the Sound and Kattegat, North Sea

The gonadal development of cod Gadus morhua in the Sound and Kattegat, North Sea, was studied by investigating their histological structure on a temporal scale by intense sampling from September 2002 to May 2003. Different age classes were followed and the proportion mature in each age class within each area was analysed. Based on existing maturity criteria, a modified system, based on histological features, was developed in order to emphasize the crucial step in the developmental process, i.e . the passage from endogenous to exogenous vitellogenesis. Only fish that had attained exogenous vitellogenesis could be considered as being reproductively active in the forthcoming breeding season ( Kjesbu et al ., 2003 ). The histological based maturity scale will greatly improve the capability of separating mature from immature individuals in the studied areas, that is fundamental for an accurate and unambiguous estimate of the spawning stock biomass. Furthermore, the results showed a larger proportion of mature individuals per age class in the Kattegat stock compared to the Sound stock, which implied an earlier maturity for this stock. This difference in maturation pattern might have been related to a relaxation of competition, i.e . enhanced growth rate, as an effect of different levels of exploitation and technical fishing regulations between the two adjacent areas.