Rainbow smelt (Osmerus mordax), a non-indigenous fish species first observed in Lake Winnipeg during late 1990, has a yet undefined impact on the structure of the lake’s food web topped by a commercially valuable walleye fishery. In other invaded lakes, the presence of smelt has caused dramatic disruptions in food webs via restructuring of the zooplankton community through selective foraging. This study addressed whether rainbow smelt show any dietary preference towards their zooplankton prey and how this changes with fluctuating seasonal zooplankton densities in the North Basin of Lake Winnipeg. Rainbow smelt (70-130 mm total length) and zooplankton (integrated vertical tows) were collected concurrently from pre-determined locations within Lake Winnipeg over the open-water seasons of 2010 and 2011. This allowed for direct proportional comparison between smelt gut contents and what was available in the environment. Proportions of six zooplankton prey groups were considered: Bosmina longirostris, Eubosmina coregoni, Daphniaspp., calanoid copepods, cyclopoid copepods and copepod nauplii.
Results/Conclusions
Gut contents of rainbow smelt (70–130 mm total length) revealed that dietary selection shifted from a preference for fast-moving copepods (calanoids and cyclopoids) during times of low total zooplankton density (spring) to slow-moving large cladocerans (Daphnia spp. and Eubosmina sp.) during times of high total zooplankton density (summer). In a time of intermediate total zooplankton density (fall), there was an increase in selection for Eubosmina sp. and a decrease in selection for Daphnia spp. This suggests a selective foraging strategy utilized by this invasive population, primarily dictated by differences in zooplankton escape capabilities. Even though smelt exhibit dietary selectivity, it is concluded that this is not strong enough to have been a major disruptor of the zooplankton community in Lake Winnipeg. This is contrary to other smelt-invaded lakes, and may be due to the warm, shallow nature of Lake Winnipeg providing a poorer quality habitat for rainbow smelt than that of a colder, deeper lake.