Invasive silver carp may compete with unionid mussels for algae: First experimental evidence

1. Unionid mussels are imperiled throughout the United States, where their global diversity is highest. Silver carp (Hypophthalmichthys molitrix Valenciennes in Cuvier & Valenciennes, 1844), an invasive planktivorous fish, has spread throughout Midwestern rivers and currently threatens the Great Lakes. Because silver carp remove plankton and other particles from the water column, they may compete with mussels for food resources. This would be among the first examples of a direct competitive interaction between fish and mussels.

2. To examine the potential for competition, a 30-d tank experiment was performed with 2-yr old fatmucket mussels (Lampsilis siliquoidea Barnes, 1823) and age-1 silver carp in three treatments: fatmucket only, silver carp only, and fatmucket + silver carp. All tanks were given a commercial algal-based diet daily and dry mass of suspended particles (an estimate of available food) and NH4, NO3, and NO2 concentrations were quantified every 5 d to track food availability and changes in nutrients. Initial and final silver carp total length (mm) and mass (g), and fatmucket length (mm), height (mm), and surface area (mm2) were measured.

3. Survival was 100% over the test duration for both species. Fatmucket grew less in the fatmucket + silver carp treatment, while silver carp growth was low regardless of treatment. Fatmucket also exhibited increased movement in the presence of silver carp. Suspended particles did not differ among treatments. Dissolved nitrogen concentrations were higher in the silver carp treatments, suggesting that silver carp increase nutrient availability in aquatic systems.

4. Overall, the slower growth rates observed in the fatmucket + silver carp treatment compared to the mussel only treatment suggest exploitative competition between invasive planktivorous silver carp and fatmucket and this competition could contribute to additional stress on already imperiled mussels.