A predator´s diet : prey composition analysis of Atlantic bluefin tuna in Skagerrak and Kattegat

Sammanfattning: Atlantic bluefin tuna is an efficient predator and was a common pelagic species in the north-east Atlantic, including the North Sea and the areas of Skagerrak and Kattegat, in the first half of the 20th century. Following a population collapse in the 1960s, the tuna disappeared from Skagerrak and Kattegatt, however in recent years the tuna has returned to these waters following successful management. Little is known about this returning population, and research is conducted to get a better understanding of their origin and ecological impact. In this study, stomach content analysis was made for 19 tunas caught in Skagerrak/Kattegat to gather insights into tuna feeding habits and prey preferences in the area. The stomach content analysis revealed 17 prey species. Garfish was the most prominent prey by weight and abundance, followed by mackerel, cod and herring. Total prey consumption was calculated for four possible population sizes of Bluefin tuna in Skagerrak/Kattegatt (500, 1500, 15 000 and 25 000 individuals) over a visiting season of 90 days. The results of these calculations showed predation on cod and mackerel may be significant given high populations size scenarios of tuna, in relation to Swedish commercial harvest. The study is the first to report feeding habits of Bluefin tuna in Skagerrak/Kattegat and results are discussed from a resource conflict management perspective.