Konkurrensen mellan röding, sik och lake i Vättern : En djupdykning i tre konsumenters födopreferenser. Två tidsperioder: 1964-1978 samt 2006-2011

Sammanfattning: Lake Vättern contains Swedens most valuable population of arctic charr (Salvelinus umbla). The stock of this Salmonidae fish showed a negative trend for several decades but has now turned to a more positive one. Too minimize the risk of a new backlash it is important to obtain knowledge about the arctic charr and its interaction with other species. The purpose of this paper is to investigate the interspecific competition between arctic charr and two other valuable species i.e. whitefish (Coregonus maraena) and burbot (Lota lota). Data of stomach contents from two periods (1964-1978 & 2006-2011) were analyzed. The consumers were divided into different groups with respect to size and season. The measure of the competition was then answered quantitatively with two indexes (Pianka & Laubmeier). The results support the conclusion that small charr (<30 cm) was exposed of remarkably tough competition from whitefish regarding the older period. This derives from the fact that these two species show a very similar diet including Opossum shrimp (Mysis relicta) and Monoporeia affinis. With increasing size charr turns to a more piscivore diet and begins to compete with burbot. The piscivorous charr consumes mostly smelt (Osmerus eperlanus) while burbot shows a more diverse behavior of  its selection of fish species as a resource. The fact that these three species can coexist despite tough competition likely derives from the fact that Vätterns food web contains a lot of actors at the lower trophic levels.