Temporal and Spatial Variation in the Pelagic Fish Assemblage in Lake Winnipeg: Implications of Night Sampling

Lake Winnipeg is the tenth largest freshwater lake in the world, by surface area, and the sixth largest lake in Canada. Compared to other lakes of its size, Lake Winnipeg is relatively shallow and does not typically thermally stratify. The lake is divided into two distinct basins with different morphological and physical characteristics which influence fish species distribution and abundance. Reliable estimates of fish biomass depend on sampling methods that account for temporal and spatial variation. Lakewide surveys of pelagic fishes have been conducted seasonally since 2002 using a midwater trawl towed at three depth strata, from surface to near lake bottom, during daytime. Studies from the Great Lakes have reported greater biomass estimates for some species from nighttime versus daytime surveys, such as rainbow smelt, an invasive species in Lake Winnipeg and the Great Lakes. The objectives of this work were to compare fish biomass estimates from two diel periods (day and night) sampled during summer 2011, and to describe how environmental variation influenced fish distribution temporally and spatially. Results from this study will be used to inform design of midwater trawl surveys in Lake Winnipeg to improve estimates of fish biomass and to help manage important subsistence, recreational and commercial fisheries.