Bluegills: Biology and Behavior

3: Foraging


The information in this chapter and the next two unavoidably intermingle, and the titles of all three are simply arbitrary divisions lacking clean edges. Foraging can be impossible to separate from competition and predation, predation is sometimes competitive, competition typically drives foraging, fishes often forage where the likelihood of predation is less, and so forth. Some things are more certain: organisms happen to be in water because they live there, fall from the sky, or topple in from the land, and a bluegill thinks of them all as food. While mulling over what bluegills eat it occurred to me that a list of what they reject might be shorter. For obvious reasons this species has been described as insectivorous, planktivorous, omnivorous, and piscivorous.155 Bluegills fit all these classifications. Different age-classes eat the same things, variation being in the proportions.156 The diet ranges from detritus to other fishes157 to the larvae and eggs of amphibians.158 Little else is passed up, even bryozoans.159 Ostracods occur commonly in bluegill stomachs, but whether they offer any food value is uncertain. Their shells are sometimes unopened, in which case they pass intact into the intestine. 160

Bluegills also eat plants. Stomachs of specimens caught in Illinois had 24% plant material by volume,161 Wisconsin fish (mostly adults) held 20%,162 a sample from Minnesota yielded 16%,163 and some bluegills from Ontario contained 26%.164 Algae constituted 50% of the stomach contents of 42 Mississippi bluegills,165 but whether algae alone can meet metabolic requirements is questionable.166 Of 81 specimens averaging 128 mm FL (94–175 mm FL) caught by angling in Wyland Lake, Indiana, 29 had consumed algae or dead aquatic macrophytes.167 At some locations plant seeds are a substantial part of the diet,168 especially for larger bluegills. Bluegills introduced into Lake Biwa, Japan, in 1963, have evolved into at least three trophic morphs, one specialized for feeding on aquatic vegetation.169

Aquatic insect larvae are staples, but adult insects qualify too, if only serendipitously. Flies, grasshoppers, ants, spiders, terrestrial mites—any small creature that wriggles or twitches—is at least examined for its gustatory possibilities. Floating woodchips are nibbled speculatively, but so are toes dangled from a canoe170 and the fingers and toes of bathers.171 Cannibalism? Of course. Even the eggs and young of its principal antagonist, the largemouth bass, are attacked indiscriminately,172 perhaps assuaging future regret at being treated in kind.