The Effect of Largemouth Bass Predation on Overwinter Survival of Two Size-Classes of Age-0 Bluegills

—Overwinter mortality is an important force structuring year-class strength of many fishes. Conventional wisdom is that overwinter mortality is primarily caused by starvation; however, recent research has demonstrated that piscivores continue to feed during winter and may also contribute to overwinter mortality of their prey populations. We conducted experiments in ten 0.04-ha earthen ponds in central Illinois to assess the effect of predation by largemouth bass Micropterus salmoides on survival of two size-classes (20–30 and 40–65 mm total length [TL]) of age-0 bluegills Lepomis macrochirus. Bluegills (400 fish/sizeclass) were stocked into each pond at the end of November. Half of the ponds also received five small (90–124 mm TL) and four large (166–192 mm TL) largemouth bass. Survival to the end of the 113-d experiment was higher for large bluegills than for small bluegills in all ponds. For both bluegill size-classes, the predator-present treatment had higher mortality than the predator-free treatment. Relative to the predatorfree treatment, mortality of bluegills in the predator-present treatment increased by 16% for the large sizeclass and 49% for the small size-class. Mean length and relative condition (K n ) of both bluegill size-classes increased by the end of the experiment in both treatments, suggesting that the observed mortality was not caused by starvation. Further exploration is needed to elucidate why bluegills in the predator-free treatment suffered overwinter mortality despite the increase in K n during the experiment. Length and K n of largemouth bass increased (large size-class) or stayed the same (small size-class), suggesting that at least some of the predators foraged during winter. Our results indicate that size-specific overwinter mortality of bluegills occurs at the middle latitudes of the species’ range. Further, winter predation can be an important component influencing size-specific overwinter survival and size-structured interactions between fishes. For many fishes, first-year overwinter mortality structures year-class strength (Post and Evans 1989; McCollum et al. 2003). Reduced survival during winter has been attributed to low food availability or consumption (Post and Evans 1989; Kirjasniemi and Valtonen 1997), metabolic–digestive inefficiencies (Schultz and Conover 1999), reduced swimming ability (Larimore and Duever 1968; Jonas and Wahl 1998), osmotic stress (Morris and Bull 1968; Lankford and Targett 2001), and increased vulnerability to disease or toxicants (Lemly 1993). Overwinter mortality of age-0 fish is often size specific, affecting smaller individuals to a greater extent than larger individuals (Post and Evans 1989; Miranda and Hubbard 1994a; Cargnelli and Gross 1996; Bernard and Fox 1997; but see Toneys and Coble 1979; McCollum et al. 2003; Santucci and Wahl 2003), presumably because large individuals have greater energy reserves and lower mass-specific metabolic rates (Miranda and Hubbard 1994a; Schultz and Conover 1999). Overwinter mortality is usually more dramatic in years with harsh (longer or colder) winter conditions (Fullerton et al. 2000; McCollum et al. 2003). However, warm winters can lead to higher mortality than cold winters when food availability is low (Sogard and Olla 2000), and winter severity may not affect all length-classes of age0 fish in the same way (Connolly and Petersen 2003). Predation may also cause size-specific overwinter mortality. Predation has traditionally been considered unimportant during winter, as piscivores were believed to be inactive and to cease feeding during that period (Johnson and Charlton 1960; Coutant 1975; Crawshaw 1984). However, recent studies suggest that piscivorous largemouth bass Micropterus salmoides do eat during winter (Miranda and Hubbard 1994b; Fullerton et al. 2000; Micucci et al. 2003; Ostrand et al. 2005) and that predation reduces overwinter survival of age-0 fish (Garvey et al. 2004). Reductions in prey species’ survival could be caused by direct consumption, increased energy expenditures by fish as they evade predation (Post and Prankevicius 1987; Miranda and Hubbard 1994b; Garvey et al. 1998), or both. These mechanisms could also cause size-specific overwinter mortality, as larger prey fish (1) may occupy a size refuge from smaller predators (Lawrence 1958) and (2) * Corresponding author: [email protected] 1 Present address: Department of Natural Resource Ecology and Management, Oklahoma State University, 008C Agricultural Hall, Stillwater, Oklahoma 74078, USA. Received February 19, 2007; accepted December 13, 2007 Published online June 19, 2008 1063 Transactions of the American Fisheries Society 137:1063–1071, 2008 Copyright by the American Fisheries Society 2008 DOI: 10.1577/T07-038.1 [Article]