Zebra mussels (Dreissena polymorpha) change adult bluegill sunfish (Lepomis macrochirus) diet and feeding behavior in inland Michigan lakes

We conducted an extensive survey of inland Michigan lakes to examine the indirect impacts of zebra mussels (Dreissena polymorpha) on the growth and diet composition of adult bluegill sunfish (Lepomis macrochirus). Fifty lakes with similar morphometries and nutrient concentrations were sampled in 2002-2003. Twenty-five lakes were infested with zebra mussels (invaded), and 25 lakes were zebra mussel free (uninvaded). Zebra mussels lowered light extinction coefficients by 17%, total macrozooplankton biomass by 38%, and Daphnia spp., the favorite prey item of bluegill, by 42%. However, adult bluegills grew 3-5 mm more each year in invaded lakes, despite the decreased Daphnia biomass. Stable isotope analyses were conducted on 31 of the 50 study lakes (17 invaded, 14 uninvaded) to determine potential mechanisms for increased adult bluegill growth. Preliminary data show that adult bluegills in invaded lakes have a 2 ‰ δ¹⁵N enrichment from zooplankton, while adults in uninvaded lakes have a 3.5 ‰ δ¹⁵N enrichment from zooplankton. These results suggest that adult bluegills have changed their diet and feeding behavior in invaded lakes. Understanding how zebra mussels affect adult bluegill diet and growth will help identify potential mechanisms by which other recreationally important fish are influenced by this invader.