Development of northern pike (Esox lucius) populations in the Baltic Sea, and potential effects of grey seal (Halichoerus grypus) predation
Abstract: Worldwide, marine mammal populations are increasing after considerable efforts to turn the downward trends caused by hunting, accidental mortality and pollution. The ecosystem effects from the increases of these top predators may be pronounced, but are in most cases poorly known. In the Baltic Sea, the grey seal (Halichoerus grypus) has been increasing by 5-9% annually since the middle of 1980, with potential effects on fish populations, food webs and fisheries. One possible prey for the grey seal is the northern pike (Esox lucius L.), a large, predatory fish abundant in the Baltic Sea archipelagos. Rcreational fishers are experiencing declines in abundances of pike, and partly blame the decline on seal predation. To assess a potential influence of grey seal on pike population, I have analysed trends in pike abundance and size and correlate it with trends in seal abundance. In addition, I have analysed diet composition of grey seal to study what seals in the archipelago eat. To follow trends in pike abundance I have used data from the Swedish coastal fish monitoring programme and data from fishing competitions. For trends in seal abundance, I used data from the Swedish national grey seal survey. Grey seal diet was estimated from scats and stomach content collected during 2016-2020 from Uppsala County and Stockholm County, in the inner and central parts of the archipelago, i.e. the habitats where pike is most abundant. Analyses were performed for five counties; Stockholm, Södermanland, Östergötland, Kalmar and Blekinge from 2000 to 2020. The fish monitoring data showed that the abundance of pike smaller than 40 cm has decreased significantly in all counties whereas the abundance of pike larger than 40 cm has decreased significantly in three counties, Stockholm, Östergötland, and Kalmar. The fishing competition data showed that both the abundance and the maximum size of pike have decreased significantly only in Stockholm County. Grey seal abundance has increased in all counties except Kalmar. In the grey seal diet, pike constituted 20% of the diet by weight, after perch (Perca fluviatilis) and herring (Clupea harengus). Overall, there was a negative relationship between the abundance of grey seal and the catch of both smaller and larger pike in the fish monitoring data. There was also a negative relationship between the abundance of grey seal and the maximum size of pike in Stockholm County, while no relationships between the abundance of grey seal and the maximum size of pike were observed in the other counties. Taken together, the results from this study indicate that the increasing grey seal population is associated with the negatively development of pike population in the archipelagos of the western Baltic Sea. Management of coastal fish has traditionally only taken the effects of fishing into account. The results of this thesis emphasize the importance of a transition towards an ecosystem-based fisheries management, where the effects of increasing populations of top predators are also taken into account in the assessments, to maintain viable populations of fish.
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