Impacts of invasive species and restoration on recreational fisheries: Zebra mussels and common carp in a shallow hypereutrophic lake

Many multiple-use aquatic ecosystems of the Midwestern United States suffer from eutrophication due to external nutrient loading from land use, and internal nutrient loadings caused by invasive common carp (Cyprinus carpio). Clear Lake, Iowa is a recreationally important system undergoing a combination of management actions to reverse eutrophication and improve water quality, including dredging and intensive harvest of carp. The potential impact of a recent invasion of zebra mussels (Dreissena polymorpha) and actions to reverse eutrophication on primary production that supports the recreational fishery of Clear Lake is uncertain. An ECOPATH model was used to quantify trophic dynamics early in the zebra mussel invasion based on field data and empirical relationships. Predicted impacts of zebra mussels on sport fishes and fishery under nutrient reduction scenarios designed to emulate the effects of restoration were evaluated using ECOSIM. Predicted decreases in sport fish forage were larger in nutrient reduction scenarios than losses attributed to zebra mussels. Predicted effects of nutrient reductions and common carp in the food web were stronger than zebra mussel effects. Similarly, predicted impacts of zebra mussels on the sport fishes and the fishery were minor relative to nutrient reductions associated with ongoing lake restoration.