Ontogenetic habitat shifts of juvenile Bear Lake sculpin

Bear lake sculpin Coitus extensus exhibit ontogenetic habitat shifts during their initial year of life. Distribution and habitat switching was measure d with bimonthly bottom-trawl surveys repeated throughout the summer. Patte rns of daily growth increments on otoliths were used to measure the history of habitat residence, individual size at the time of the habitat switch, a nd habitat-specific growth rates. Laboratory experiments and known-age fish confirmed daily increment formation of otoliths. After dispersing during a n initial pelagic larval stage, postlarval juveniles settled in both the wa rm, food-rich littoral zone and the cold, unproductive profundal zone. Duri ng summer, initial profundal-zone inhabitants underwent a unidirectional ha bitat shift to the more productive littoral zone. Fish that moved to the li ttoral zone grew twice as fast as those in the profundal zone and encounter ed little apparent predation mortality risk there. Habitat shifts were ther efore consistent with both growth optimization and predatory avoidance. How ever, shifts occurred at a wide range of body sizes throughout the summer. The large spatial scale of the lake, limited swimming ability of juvenile f ish, diel migratory behavior, and lack of complex habitat structure may pre clude shifts at discrete body sizes for this species.