Report on scientific cod fishing and monitoring in 2021 in Åland, Finland

Abstract

At present, Eastern Baltic cod (Gadus morhua) in the southern Baltic Sea grows slowly, shows low condition factor and is heavily infected by the larvae of liver worms (Contracaecum spp.). It is hypothesized, that either the heavy infection by liver worms, lack of suitable food due to lack of oxygen in the deep bottoms of the Baltic Sea or both together cause severe problems for cod. The final host of the liver worm is grey seal (Halichoerus grypus), and this parasite is carried to cod via prey, smaller pelagic fish. There is a small-scale cod fishery in the Finnish waters in the Sea of Åland, where cod are large sized and in good condition. Grey seals are abundant in these waters. In this study, the occurrence of Contracaecum larvae in the livers of cod in the Sea of Åland and the food of the cod in the year 2021 was examined and presented together with the results from the year 2020. The size of measured cod in 2021 varied from 40 to 105 cm. Similarly, as in 2020, the number of Contracaecum osculatum larvae on liver surface correlated with cod length, but the number of larvae per liver weight did not. The condition factor of the cod was still very high (1.14). More importantly and similarly as in the previous year, the condition of the cod was associated neither with the number of Contracaecum larvae on the liver surface nor the number of larvae per liver weight. The most common food items of cod were Saduria and clupeid fish. The samples from both years support the conclusion that when there is enough food for the cod, the association of Contracaecum osculatum infection and the condition or growth of cod are small or even insignificant.