Effects of Predation Risk and Foraging Return on the Diel Use of Vegetated Habitat by Two Size-Classes of Bluegills

—Little is known about nocturnal habitat selection by fishes under the risk of predation. Using a photoperiod of 15 h light : 9 h dark, we quantified the diel use of artificial macrophytes and open water by two size-classes of bluegill Lepomis macrochirus when the open water was empty (control), contained food, or contained both a caged predator and food. Small bluegills (6.2–7.7 cm total length) spent significantly more time in macrophytes in the predator and food treatment than in the control, followed by the food-only treatment. In addition, small bluegills spent significantly more time in macrophytes during the day than at night in all treatments. The frequency with which small bluegills were found in the same location in subsequent observations was significantly higher in the predator and food treatment during the day than in any other treatment and light combination. Large bluegills (10.2–13.0 cm total length) showed no difference in habitat use among treatments but spent significantly less time in macrophytes at night than during the day. There was no difference in the frequency with which large bluegills were found in the same location in subsequent observations among treatments or light levels. This suggests the potential for a diel littoral2pelagic habitat change by juvenile bluegills that would have important implications for the role of bluegills in lake food webs, including the possibility of nutrient translocation that could generate alternate stable states in lakes. Predation can have strong structuring effects on communities through direct consumption of prey organisms (i.e., top-down control; Hairston et al. 1960; Carpenter et al. 1985). However, predation can also cause indirect effects by altering the behavior of prey with respect to habitat selection, diet, feeding rate, foraging location, activity level, and reproduction (see reviews by Dill 1987; Mittelbach and Chesson 1987; Sih 1987, 1994; Lima and Dill 1990). These behavioral changes, in turn, can lead to changes in competitive interactions * Corresponding author: [email protected] 1 Present address: Illinois Natural History Survey, Kaskaskia Biology Station, Rural Route 1, Box 157, Sullivan, Illinois 61951, USA. Received October 2, 2001; accepted October 3, 2002 (Kotler and Holt 1989; Werner 1991; Abrahams 1994) and top-down control effects (Power et al. 1985; He et al. 1993), which ultimately may lead to changes in community structure. Organisms often realize reduced growth rates as a direct consequence of altering their behavior to lower predation risk (Mittelbach 1981; Gotceitas 1990a; Savino et al. 1992; Diehl and Eklov 1995). Predation-induced habitat changes are welldocumented in fish predator–prey systems, typically involving movement from open waters when predation risk is low into dense vegetated habitat when predation risk is high (e.g., He and Kitchell 1990; Diehl and Eklov 1995; Chick and McIvor 1997). Despite this wealth of research, most studies have only considered daytime habitat selection. However, many field studies suggest that some fishes forage in macrophytes during the day when predation risk is great and in open water at night when predation risk is reduced (e.g., Rodeheffer 1939; Emery 1973; Hall et al. 1979; Bohl 1980; Gaudreau and Boisclair 2000). Most of these studies used different types of sampling gear in vegetated versus open waters, making it difficult to determine whether diel differences were due to fish migration or diel variation in gear selectivity. Even studies using similar gear types in both habitats cannot rule out the possibility that there are two subpopulations, one that stays in open water and has a peak activity at one time of day, and one that stays in vegetation and has a peak activity at a different time of day. We know of only four studies in which diel effects of predation risk on fish habitat selection were examined in controlled experiments (Fraser and Cerri 1982; Schlosser 1988; Jacobsen and Berg 1998; Jacobsen and Perrow 1998), and they give conflicting results, indicating that diel habitat changes may depend on the specific predator–prey system considered. Bluegills Lepomis macrochirus and largemouth bass Micropterus salmoides are important game fish species that are widespread across the conti-