Abundance and seasonal shifts in vertical and horizontal distribution of lake whitefish (Coregonus clupeaformis) in a western United States reservoir

A series of studies have been implemented on Banks Lake, Washington, in recent years with the overall objective to maximize the fishery by determining the factors that limit the recruitment of various sport fish. When sampling commenced, it became apparent that the lake whitefish (Coregonus clupeaformis) was a major component of the system. The purpose of this study was to estimate the species composition, distribution, and abundance of the limnetic fish community of Banks Lake using hydroacoustics and gill nets. Surveys were conducted in May and July to test for differences in seasonal distribution and efficacy of the survey gear to establish a time period for conducting annual surveys for trend monitoring. Whitefish dominated the limnetic gill-net species composition by weight (95% both months) and number (83% July and 88% May). In May, relatively few fish were distributed deeper than 8 m at the south end of the reservoir, and low densities were common for the deepest strata (16 24 m). In July, most fish were in the deepest strata available, 16-24m throughout most of the reservoir. Reservoir-wide mean fish density was not significantly different between May and July, but acoustic target distribution varied by depth and location within and between surveys. In July, fish were concentrated near the North Dam where cooler water was being pumped in from Lake Roosevelt and in Devil's Lake embayment where deeper bottom depths offered a cool water refuge. These areas represent cool water refuge for salmonids and will be key spatial considerations for future fish management actions.