The spatial distribution of cod (Gadus morhua L.) spawning grounds in the Kattegat, eastern North Sea

Similar to many other commercial marine fish species, Atlantic cod (Gadus morhua L.) migrate towards specific sites at spawning. These temporal aggregations are generally the most targeted by the commercial fishery. The Kattegat cod has undergone a substantial reduction over the past 25 years and both stock size and spawning stock biomass have remained at very low levels since the end of the 1990s. There is, therefore, an urgent need to map and document spawning grounds still in use. In the present study, spawning sites of Atlantic cod were identified in the Kattegat using a combination of commercial and fishery independent data from 1996 to 2004. Moreover, putative spawning and non-spawning areas were also sampled before and during the reproductive season between 2002 and 2006, and the proportion of mature females together with the individual physiological status were used to validate and strengthen the spatial analyses.

The spatial analyses identified several spawning areas in the region and two areas in the southeastern part of the Kattegat which appeared to be the most important. The results showed the presence of cod spawning aggregations, although reduced in size, in areas that have been utilized for more than 25 years according to historical information. Some local spawning grounds may have also disappeared. The proportion of mature females was higher in putative spawning than in non-spawning areas (p < 0.001) and females from spawning areas had higher gonadosomatic (p < 0.05) and hepatosomatic (p < 0.001) indices than those from non-spawning areas.

Knowledge of stock spatial and temporal distribution is essential in designing recovery strategies for depleted fish populations. The unambiguous stability of the locations of spawning aggregations over time, as shown in this study, represents a useful aid in order to efficiently implement a recovery plan for the collapsing cod stock in this area.