Habitat shifts and behavioural interactions between sympatric and experimentally allopatric cutthroat trout and Dolly Varden char

The role of competition in structuring a lacustrine community of two salmonid species, cutthroat trout (Salmo clarki Richardson) and Dolly Varden char (Salvelinus malma Walbaum), was investigated in three coastal B.C. lakes. Habitat utilization of both species alone (allopatric) and in coexistence with each other (sympatric) was determined by gill netting at depth contours from lake surfaces to bottoms such that littoral, epipelagic, pelagic, and epibenthic habitats were sampled. From June to October, trout utilized mainly surface habitats (littoral and epipelagic) in sympatry and allopatry. Char utilized all habitats in allopatry, and exhibited generalist feeding behaviour by opportunistically utilizing different habitats as prey abundance varied between sampling periods. However, in sympatry, char shifted to deeper habitats not occupied by trout. In sympatry, trout and char were spatially segregated with depth. However, temporal segregation was not pronounced. The habitat shift by char supports an hypothesis of competition between sympatric trout and char for habitat resources, where competition acts more strongly on char. However, food abundance partly explained patterns in fish distribution. The hypothesis that habitat segregation between sympatric trout and char is based on behavioural interactions was investigated in laboratory experiments. There were changes in the type and intensity of interaction between trout and char with irradiance level that were consistent with their distribution and depth of habitat. At high irradiance levels such as occur in surface habitats, trout were more aggressive to char than at low irradiance levels. In sympatry with trout, char may seek refuge from aggression by trout in deeper habitats with lower irradiance levels. The feeding performance of char in interspecies pairs dominated by trout increased with decreasing intensity of behavioural interactions. However, the feeding performance of these char did not improve at low irradiance levels, presumably because char continued to display subordinate behaviours while confined in an aquarium with dominant trout. Whether the shift to deeper habitats by lake-dwelling sympatric char is a result of interference mechanisms is not clear. However, an hypothesis involving an interactive mechanism of segregation and interference competition along irradiance level gradients cannot be rejected by this study.Science, Faculty ofZoology, Department ofGraduat