The energetic impact of overwinter prey assemblages on age-0 largemouth bass, Micropterus salmoides Kenneth G. Ostrand a , Steven J. Cooke b,d , James E. Garvey c , & David H. Wahl a,b a Center for Aquatic Ecology, Illinois Natural History Survey, Sam Parr Biological Station, 6401 Meacham Road, Kinmundy, IL 62854, U.S.A. (e-mail: ostrand@uiuc.edu) b Department of Natural Resources and Environmental Sciences, University of Illinois and Center for Aquatic Ecology, Illinois Natural History Survey, 607 E. Peabody Dr., Champaign, IL 61820, U.S.A c Fisheries and Illinois Aquaculture Center, Department of Zoology, Southern Illinois University, Carbondale, IL 62901, U.S.A d Current address: Center for Applied Conservation Research, University of British Columbia, 2424 Main Mall, Vancouver, British Columbia, V6R 2J3 Canada Received 19 June 2003 Accepted 12 July 2004 Key words: Micropterus salmoides, swimming performance, growth, forage, temperature Synopsis We compared survival, growth, and swimming performance of two size classes of age-0 largemouth bass, Micropterus salmoides, in the spring after being fed diets of bluegill, Lepomis macrochirus, fathead min- nows, Pimephales promelas, or invertebrate prey during the winter. Regardless of prey assemblage, survival was uniformly high and independent of size. Length, wet- and dry-mass, and condition was also similar among treatments for both size classes. However, variation in individual performance differed, with the lowest variability in growth occurring among small age-0 largemouth bass in the invertebrate only treat- ment. Absolute and length corrected swimming speeds of largemouth bass were highest for invertebrate prey assemblages, intermediate for fathead minnow prey, and lowest for bluegill prey. The patterns in growth and spring swimming performance likely reflect the varied nutritive quality of different prey, the ability of largemouth bass to capture different prey, and competition with the piscine prey. Introduction At southern and mid-temperate latitudes winter temperatures and prey assemblages vary among lakes and years. Age-0 largemouth bass forage during the winter at these latitudes when tempera- tures are high enough to advance growth coupled with abundant prey (Garvey et al. 1998, Fullerton et al. 2000, Micucci et al. 2003). Because activity levels and metabolic rates are low in the winter, small amounts of exogenous energy have the potential to provide substantial benefit to fish (Garvey et al. 1998, Wright et al. 1999, Fullerton et al. 2000). However, these benefits may not simply be influenced by the quantity of prey but also the quality of prey because different species of prey exhibit dissimilar caloric values, levels of winter activity, and habitat use (Davis & Boyd 1978, Lud- sin & DeVries 1997). In addition, these effects may differ with the size of age-0 largemouth bass (Ludsin & DeVries 1997). For example, larger fish should ingest more energy per gram than smaller counter parts (Ludsin & DeVries 1997). Differences in prey consumption, including predator–prey encounter rates and prey quality, should affect survival, growth, and energetic performance patterns among age-0 largemouth bass through winter until early spring and these relationships may vary with size. Environmental Biology of Fishes (2005) 72: 305–311 Ó Springer 2005