The Effects of Turbidity on Prey Selection by Piscivorous Largemouth Bass

—Turbidity in aquatic systems can change rapidly, affecting the visual ability of predators. Increased turbidity is known to reduce the reactive distance and foraging success of some planktivores and insectivores, leading to decreased growth rates. However, little is known about the effects of turbidity on prey selection by piscivores. We examined the interactions between largemouth bass Micropterus salmoides and their prey in 1.8-m-diameter tanks (58 cm deep) at four turbidity levels (0, 5, 10, and 40 nephelometric turbidity units [NTU]). Prey selectivity was significantly affected by turbidity. At lower turbidity levels (0 and 5 NTU), largemouth bass consumed mostly gizzard shad Dorosoma cepedianum and bluegills Lepomis macrochirus and had negative selectivity for northern crayfish Orconectes virilis. At 10 NTU, all three prey types had similar selectivity, presumably because the largemouth bass had more difficulty in capturing rapidly moving fish prey as their reactive distances decreased. At 40 NTU, the overall foraging rate was much lower and bluegills were selected significantly more often than the other prey types. Low light levels at the bottom of the tanks combined with reductions in visual clarity from clay sediments probably made it difficult for largemouth bass to feed effectively on virile crayfish at higher turbidities. Our results suggest that trophic interactions may be altered as turbidity levels change. Turbidity often varies on a seasonal basis in aquatic systems (Nellis et al. 1998; Dirnberger and Weinberger 2005) and can fluctuate rapidly owing to changes in phytoplankton density, sediment additions, or sediment resuspension (Chow-Fraser 1999; Anthony and Downing 2003; Parkos et al. 2003; Cozar et al. 2005). Because turbidity affects the ability of aquatic organisms to find prey (Vinyard and O’Brien 1976; Gregory and Northcote 1993), it has the potential to affect predator–prey interactions. Contrast degradation theory predicts that organisms that feed on large prey types (such as piscivores) should be more affected by increases in turbidity than organisms that feed on small prey types (such as planktivores) (Utne-Palm 2002; De Robertis et al. 2003). Despite this, most turbidity research has focused on planktivorous and insectivorous fishes, relatively little being done on piscivorous species. In particular, how prey selection by piscivores is affected by changes in turbidity is not known. The effects of turbidity on the foraging success of planktivorous and insectivorous fishes vary by species (Bonner and Wilde 2002). For many species, increased turbidity leads to reduced foraging return (Gardner 1981; Johnston and Wildish 1982; Barrett et al. 1992; Gregory and Northcote 1993; Benfield and Minello 1996), presumably because of decreased reactive distance (Vinyard and O’Brien 1976; Gregory and Northcote 1993). However, foraging return appears to be unaffected in some species (Sweka and Hartman 2001; Rowe et al. 2003; Granqvist and Mattila 2004) and may even increase with moderate increases in turbidity in other species (Boehlert and Morgan 1985; Rowe and Dean 1998). Species that experience reduced foraging return at higher turbidity levels often exhibit lower growth and survival (Buck 1956; Sigler et al. 1984; Sweka and Hartman 2001). Other fish compensate for reduced foraging return by increasing their activity levels, thereby increasing their encounter rates with prey so that foraging return is unchanged at higher turbidity levels (Gradall and Swenson 1982; Sweka