The influence of northern pike on the diet of brown trout

The predation of the northern pike (Esox lucius) has recently been found to initiate divergence in the size structuration of whitefish (Coregonus lavaretus) in Scandinavian lakes. The whitefish community responds to the predation with two different strategies: avoiding the pike by feeding in the pelagic zone with the cost of limited growth, or feeding in the littoral zone, delaying maturation to reach a size which is less subject to predation (Öhlund 2012). Brown trout (Salmo trutta) is an opportunistic predator that coexists with pike in these lakes under certain conditions (Klemetsen et al. 2003). The size structuration of the whitefish could provide the brown trout with a novel resource. The aim of this study was to investigate if the diet of trout differs between lakes with pike and lakes with no pike. A stomach content analysis was carried out on trout from thirteen Scandinavian lakes and the diet of trout from systems with pike present and absent was compared. The results indicate that the presence of pike has a substantial influence on the diet of brown trout. In systems without pike the trout are generally insectivorous; occasionally feeding on Arctic char. Trout in systems with pike are more piscivorous, probably taking advantage of the small pelagic whitefish morph. The predation of pike leads to a size-structuration of the whitefish community that the brown trout can benefit from. The novel resource could widen the window in which these two predators coexist making it a case of emergent facilitation