Temporal and spatial patterns in pelagic trawl fish catches in Lake Winnipeg

To better understand patterns of temporal and spatial variation of fish assemblages in offshore waters of Lake Winnipeg (Manitoba, Canada), midwater trawl tows were conducted near lakewide monitoring stations from 2002 to 2008. Trawl samples collected during spring, summer, and fall from the south basin, channel, and north basin were used to study effects of season and geographic region within the lake on species biomass estimates. Within each region, effect of trawl depth was explored. Greatest biomass in trawl catches was from the south basin. Of the most commonly caught species, across all seasons, greater biomass of emerald shiner (Notropis atherinoides) and cisco (Coregonus artedi) were found in the south basin and the channel, compared to the north basin. Biomass of the non-native rainbow smelt (Osmerus mordax), by contrast, was greater in the north basin compared to the south basin or the channel. Yellow perch (Perca flavescens) and introduced white bass (Morone chrysops) biomass varied temporally and spatially, with greatest biomass captured in the summer in the south basin. Estimated biomass of walleye (Sander vitreus) was greatest in the south basin, followed by the channel, and the north basin. Patterns in species distribution in Lake Winnipeg probably are influenced by a combination of factors, including species interactions and differing temperature and light conditions in the lake. As no lakewide pelagic trawl studies have been reported for this lake, these data form a baseline against which effects of changes, such as lake trophic state or establishment of non-native species, can be assessed.