Summary
Young (35-50 mm, standard length) bluegills Lepomis macrochirus are restricted to vegetated habitats by predation pressure. Vegetation provides refuge by hindering predator foraging success. In this study, we tested the ability of bluegills to actively perceive and select densities of vegetation where they are "safe" from predation. Bluegills were presented with two plots of artificial vegetation (cover plots) of different densities (1,000, 250, 100, and 50 stems/m2) in an experimental arena and then confronted with a predator, a largemouth bass Micropterus salmoides. Prey distribution in the arena before, during, and after exposure to the predator, as well as the predator's foraging activities, were recorded for each trial. Before predator introduction, bluegills in all combinations of cover plots spent most of their time in or around the thicker plot of vegetation in the arena. With the predator present in combinations with cover plot 1,000, prey chose this plot as a refuge when attacked. Cover plot 250 was selected as often as positions at the water-air interface, and plots 100 and 50 were ignored as refuges. After removal of the predator, only in combinations with cover plot 1,000 did prey remain around the thicker cover plot present. Predator success (number of prey caught/total number of attacks) was lowest in trials with cover plot 1,000 present. When confronted with a predator, young bluegills appear capable of perceiving and selecting plots of vegetation offering safety from predation.
Methodology
Largemouth bass (25-35 cm standard length, SL) and bluegills (35-50 mm, SL) were collected from Lake Opinicon, Ontario (40°30'N, 76°30'W), by seining and angling. Predator and prey sizes were selected to approximately match those used by Savino and Stein (1982) to justify labeling our artificial vegetation plots as safe or unsafe based on their experiments, and also because bluegills of this size range are restricted to vegetated areas by predation in natural systems (Hall and Werner 1977; Werner et al. 1977, 1983a; Mittelbach 1981, 1984). When not in use, fish were housed in flow-through aquaria (90 cm long x 60 cm wide x 60 cm deep) at a water temperature and light regime identical to that of the experimental tank. Both species were maintained on a diet of frozen fish.
Experiments were conducted in November 1984 and from July to November 1985. They consisted of placing two different cover plots into the arena (see Figure 1), introducing 10 prey to acclimatize to the situation for 24 h prior to testing, and then introducing a predator, previously starved for 24-h, for a 30-min trial. Prey were not fed during the 24-h period of acclimatization. All trials were run between 1400 and 1600 hours to reduce any time-of-day effects. Five-minute pretrial and posttrial observation periods, with the predator absent, bracketed the 30-min trial period.