INDIVIDUAL VARIATIONS IN HABITAT USE AND MORPHOLOGY IN BROOK CHARR

The specific objectives of this study were to determine if there is in dividual specialization in habitat use by lacustrine brook charr Salve linus fontinalis and if so, if specialization is related to fish morph ology. Localizations of 28 brook charr equipped with thermosensitive r adio-transmitters were recorded during three summers (1991, 1992, and 1993) in two lakes of the Mastigouche Reserve (Quebec, Canada). Fifty per cent of the fish were found mainly in the benthic zone (hereafter benthic individuals), 18% in the pelagic zone (pelagic individuals), a nd 32% travelled regularly between the two zones (generalist individua ls). The observed interindividual differences in habitat preference we re related to differences in body morphology and coloration: (i) the p ectoral fins of benthic and generalist individuals were significantly longer than those of pelagic ones; and (ii) the coloration of the lowe r flank of benthic and generalist individuals was silver-grey while th at of pelagic individuals was red. The results of this study suggest t hat brook charr inhabiting oligotrophic lakes of the Canadian Shield e xhibit trophic polymorphisms, where some individuals are specialists b etter adapted to feeding in the littoral zone whereas others are speci alists better adapted to feeding in the pelagic zone. The potential fo r reproductive isolation between the two morphs is discussed. (C) 1997 The Fisheries Society of the British Isles.