Phenotypic and resource use partitioning amongst sympatric, lacustrine brown trout, Salmo trutta

Divergence into discrete foraging specialist morphs living in sympatry is relatively well described in lacustrine fishes of the Salmonidae. Although piscivorous forms of Salmo trutta have been widely reported, other trophic foraging specialists are strangely rarely recorded amongst Salmo species. Microsatellite and mitochondrial genetic data segregated S. trutta collected from Loch Laidon, Scotland, into four distinct genetic groups. Three groups analysed in this study showed significant differences in body shape, stomach contents, muscle stable isotope signature, gill raker length and spacing, and habitat use. We conclude that the three genetically defined groups comprise: a generalist foraging morph, a pelagic feeding specialist morph and a profundal macrobenthos feeding morph. The features distinguishing these morphs, however, show a degree of overlap. This appears to be only the second record of invertebrate resource use partitioning associated with expression of alternative morphologies in sympatry this species and the first report of a profundal invertebrate feeding morph. Why such polymorphisms are generally rare in Salmo species remains unclear, but potential explanations are discussed.