Oecologia (2008) 157:553–560 DOI 10.1007/s00442-008-1103-1 123 POPULATION ECOLOGY - ORIGINAL PAPER Asymmetric competition drives lake use of coexisting salmonids B. Jonsson · N. Jonsson · Kjetil Hindar · T. G. Northcote · S. Engen Received: 22 February 2008 / Accepted: 4 June 2008 / Published online: 16 July 2008  Springer-Verlag 2008 Abstract To what degree are population diVerences in resource use caused by competition and the occupation of adjacent positions along environmental gradients evidence of competition? Habitat use may be the result of a competi- tive lottery, or restricted by competition. We tested to what extent population diVerences in habitat use of two salmo- nids, cutthroat trout (Oncorhynchus clarki) and Dolly Var- den charr (Salvelinus malma) were inXuenced by interspeciWc competition. We hypothesized that the depth distribution of Dolly Varden charr would be aVected by competition from the more littoral and surface-oriented cut- throat trout, and that the depth distribution of cutthroat trout would be little aVected by competition from Dolly Varden charr. Sympatric populations of cutthroat trout and Dolly Varden charr were created by reciprocal transfers of previ- ously allopatric populations in two experimental lakes. We found evidence of asymmetric competition, as Dolly Var- den charr were displaced from littoral habitats when sym- patric with cutthroat trout, whereas cutthroat trout remained unaVected by the presence of Dolly Varden charr. Evolved diVerences between the species, and diVerences between experimental lakes, also contributed to population diVerences in habitat use, but asymmetric competition remained as the main driver of diVerent depth distributions in sympatry. Keywords Habitat shift · Interactive segregation · Oncorhynchus clarki · Reciprocal transfer experiment · Salvelinus malma Introduction There has been considerable debate to what degree popula- tion diVerences in resource use are caused by competition, and whether occupation of adjacent positions along envi- ronmental gradients is evidence of competition (Taniguchi and Nakano 2000; Connolly and Muko 2003). Is it a com- petitive lottery (Munday 2004) or coexistence where one or both species are restricted in the presence of the other (Bøhn and Amundsen 2001)? Studying two species of coral-dwelling goby [Gobiodon histrio (Valenciennes) and Gobiodon erythrospilus Bleeker] with similar patterns of habitat use, Munday (2004) demonstrated that there was a priority eVect where the Wrst species to occupy a vacant coral patch excluded an interspeciWc intruder of similar body size. On the other hand, a whiteWsh [Coregonus lavar- etus (L.)] population invaded by vendace [Coregonus albula (L.)] changed its habitat from pelagic to littoral areas (Bøhn and Amundsen 2001). The habitat segregation Communicated by Roland Brandl. Electronic supplementary material The online version of this article (doi:10.1007/s00442-008-1103-1) contains supplementary material, which is available to authorized users. B. Jonsson (&) · N. Jonsson Norwegian Institute for Nature Research, CIENS, Gaustadalléen 21, 0349 Oslo, Norway e-mail: bror.jonsson@nina.no K. Hindar Norwegian Institute for Nature Research, Tungasletta 2, 7485 Trondheim, Norway T. G. Northcote Department of Zoology, University of British Columbia and Elderstrand, 10193 Morrison Close, Summerland, BC, Canada, V0H 1Z7 S. Engen Department of Mathematical Sciences, Norwegian University of Science and Technology, 7491 Trondheim, Norway