MERTOR

The environmental conditions in the Baltic Sea have deteriorated during the latest decades, with negative consequences for cod stocks in the Eastern and Western Baltic Sea, and in Kattegat. This has challenged the assessments of these stocks, due to limited process knowledge and -understanding, resulting in uncertainties in the advice from ICES, and targeted fishing for cod in all three areas is now closed. This makes cod a ‘choke species’ in all areas, as it is difficult to avoid cod bycatch when targeting other species.
Three central things have changed markedly for the cod; the extent of areas affected by oxygen depletion, the composition of available food, and predation by seals. Cod, or fish in general, will if everything else is equal choose well-oxygenated waters. But the deteriorating oxygen conditions, and the need to search for food and avoid predators like seals, may result in cod occupying waters which have suboptimal oxygen conditions. We do however lack knowledge about how limited oxygen availability affects the growth of the fish. Deteriorating oxygen conditions likewise have changed the composition of benthic prey available for cod, which is especially important for the small cod to be able to grow large enough to catch pelagic prey. Especially the crustacean Saduria which used to be an important part of the diet of both small and larger cod in the Baltic region has disappeared from the sea bottom. We do however not know how the lack of Saduria in the diet affects the growth potential of cod. Besides poor oxygen and food conditions, also the grey seal population has increased markedly, especially in the areas also occupied by cod. At present, the natural mortality due to seal predation is assumed to be constant, which with the increasing seal abundance may not hold true. These changes we, however, need to quantify, to improve stock assessments.
The aims of the present project are to generate new knowledge on the degree to which moderate oxygen depletion and food composition affects the growth of cod, and to quantify how the abundance and distribution of grey seal through predation affects the natural mortality of cod. This knowledge is central in evaluating the current status of the cod stocks, where targeted fishery is closed in Danish waters and all stocks reveal negative trends in a number of biological and demographic traits.