Bluegills: Biology and Behavior

4: Competition


Foraging is ultimately energy-driven, and animals are presumed to forage more or less efficiently. For growth and reproduction to occur, the energy value of food consumed must exceed the energy expended to locate, capture, and ingest it. In addition, animals are presumed to control, within certain limits, their own foraging activities, moving to more productive areas as resources become scarce and the effort to obtain them too great. Foraging is always at the mercy of seasonal cycling and population peaks of prey organisms.422 As shown later, bluegills are in competition for food with fishes that use the same resources, mainly centrarchids and especially other bluegills.

Bluegills of age-0 living in vegetation have wide diets, ingesting mainly invertebrates smaller than predicted by the OFM (Chapter 3).423 Such size selectivity was evident in stomach analyses of fish from Devil’s Lake, Wisconsin, in August.424 Most prey items ranged from 0.3 to 1.5 mm, and the majority of organisms less than 1 mm were amphipods, cladocerans, copepods, ostracods, snails (hydrobiids), and hydracarinians; larger prey like trichopterans were 1–2 mm (Table 4). The experiments in Lawrence Lake (Chapter 3) indicated that age-0 bluegills stayed in the vegetation in late June foraging on increasingly limited prey while populations of profitable cladocerans were becoming abundant in open water. At Devil’s Lake, bluegills averaging 39.4 mm SL displayed a wide diet, foraging on more than a dozen organisms (including zooplankters) among macrophytes having stem concentrations of 352–2,585/m2. However, the average mass of prey in the stomachs of these fish declined with increasing stem concentrations of Canadian waterweed Elodea canadensis and decreasing submarine illumination.