Planktivorous feeding and habitat utilization of sympatric and experimentally segregated populations of coastal cutthroat trout (Salmo clarki clarki) and Dolly Varden char (Salvelinus malma)

Sympatric populations of Dolly Varden (Salvelinus malma) and cutthroat (Salmo clarki clarki) are spatially and food segregated in Loon Lake, British Columbia. Dolly Varden are found mainly in the lower water column, while cutthroat are found in the middle and upper water column. Zooplankton, the only major prey type found in both diets, appear more important to the cutthroat. The purpose of this study was to determine whether this segregation was due to behavioural interactive processes or genetically based selective processes. This was investigated by the segregation and transferral of the members of the two populations to separate fishless lakes. In addition, field studies examined selective feeding on zooplankton and laboratory studies compared the feeding behaviour and abilities of Dolly Varden and cutthroat on three different types of zooplankton. Little change occurred during the two year sampling period in either the diet or the vertical distribution of the two species, an indication that the fish were not interactively segregated. However, the physical and biological characteristics of the lakes may have reinforced, at least initially, previous behaviour patterns. As a result, more time may be required to properly assess the final results. Laboratory results indicate that primarily Chaoborous larvae and secondarily Daphnia pulex were the preferred prey types to both fish predators, mainly because of their poor escape response when compared to that of Diaptomus kenai. The abundance of these prey types in the diet of lake-caught fish was generally in accordance with the laboratory preferences. These results also demonstrated that cutthroat were more efficient planktivores than were Dolly Varden.Science, Faculty ofZoology, Department ofGraduat