The Effects of Zebra Mussels (Dreissena polymorpha) on Freshwater Drum (Aplodinotus grunniens) Diet and Growth in the South Basin of Lake Winnipeg

Abstract

The freshwater drum (Aplodinotus grunniens) is a large-bodied fish found throughout the Lake Winnipeg watershed. Little is known about freshwater drum diet and ecology in Manitoba, as they are not a fish of commercial interest or recreational popularity. However, freshwater drum may be a primary predator of zebra mussels (Dreissena polymorpha), which established in Lake Winnipeg in 2013. In this study, the diets and growth of 51 freshwater drum collected in 2019 (6 years post-zebra mussel invasion) were compared to the diets and growth of 64 freshwater drum sampled in 2000 (pre-zebra mussel invasion); all fish were collected from Lake Winnipeg’s south basin. Benthic insect larvae were the dominant food item in both sample years. Although mollusks occur in high densities in Lake Winnipeg, they were rarely consumed by freshwater drum in either sample year. Zebra mussels were not a frequent prey item in 2019 as they were only consumed by four of the sampled freshwater drum. Stable isotope analysis of white muscle tissue yielded similar δ13C and δ15N values in both sample years, which were consistent with a benthic, insectivorous diet. Length-at-age data derived from otolith aging revealed that the 2019 population had an equal or higher growth rate than the 2000 population. Weight-at-length data suggests that fish condition was greater in 2019 than in 2000, which coincides with increased benthic macroinvertebrate density in Lake Winnipeg. Based on these findings, Lake Winnipeg freshwater drum feed continue to feed preferentially on insect larvae over zebra mussels and were therefore unlikely to have consumed zebra mussels at the onset of invasion.