Dietary overlap between two zooplanktivores in Lake Winnipeg, Manitoba: Rainbow smelt (Osmerus mordax) and cisco (Coregonus artedi)

Dietary niche overlap between invasive rainbow smelt (Osmerus mordax) and the native cisco (Coregonus artedi) of similar size in Lake Winnipeg was examined using stomach content analysis throughout the open water period in 2010 and 2011. Although smelt and cisco were caught together in the North Basin (n = 7 stations), rainbow smelt were exclusively caught in the North Basin (n = 39 stations), whereas cisco were predominantly caught in the South Basin (n = 31 stations). Substantial dietary overlap was evident when found together and apart, illustrating little dietary niche differentiation between the two species in sympatry and allopatry in the lake. Both smelt and cisco appear to shift from copepods in spring to large-bodied cladocerans in summer and fall. Although high dietary overlap suggests that these species have the potential to compete for prey, differential abundance in each basin of Lake Winnipeg suggests possible environmental niche divergence between these two species. Overall, this study provides important baseline information to examine the impact of another recent invasive planktivorous species (e.g. Bythotrephes longimanus, spiny water flea) on the smelt-cisco competition as well as the commercially important piscivorous walleye (Sander vitreus) population in Lake Winnipeg.