Effects of Zebra Mussel Invasion on Juvenile Steelhead Distribution, Diet, Growth, and Condition in the Muskegon River, Michigan

Abstract

Dreissenid mussels (zebra mussels Dreissena polymorpha; quagga mussel, D. bugensis) are an invasive species from the Ponto-Caspian region. Since their introduction, zebra mussels have drastically changed Great Lakes aquatic ecosystems by altering nutrient cycling, biomass and species composition of lower trophic levels, with consequent impacts on fish community composition, diet and growth. This study examined zebra mussels’ effects on the spatial distribution, diet and growth of age-0 steelhead (Oncorhynchus mykiss) in the Muskegon River, a major tributary of Lake Michigan. Densities, spatial distributions, diets, sizes and growth rates of age-0 steelhead were compared from 1998, before zebra mussel invasion, to the 2000-2003 period, post zebra mussel invasion. Before zebra mussel introduction, the majority of age-0 steelhead were found in the upper portion of the study reach in strata 1 and 2. After zebra mussel introduction, age-0 steelhead were found in strata 2 and 5 in the summer and stratum 2 in the fall. Following zebra mussel introduction, mean weights, lengths, condition and growth rates of age-0 steelhead decreased. Significant changes in the diet were observed in the pre- and post-zebra mussel periods (χ2=21.05, p=0.007). While age-0 steelhead consumed mostly Amphipoda (42%) and Trichoptera (27%) in 1998 before zebra mussel invasion, decreases in the abundance and availability of Trichoptera resulted in a post-zebra mussel diet shift to Diptera (51%) and Amphipoda (30%). Using bioenergetics modeling, it was determined that the change in growth of age-0 steelhead was mainly caused by the changes in diet following the introduction of zebra mussels, rather than differences in water temperature over the study period. Reduced growth of age-0 steelhead has negative implications for steelhead survival and potential recruitment if survival is size-dependent.