Lessons in ecology from the humble bluegill: Part 2
Bluegill are known for their willingness to bite a baited hook, but ecologists have studied the selective feeding behavior of bluegill to learn more about how animals weigh the risks and benefits of foraging.
“Selective feeding” is a phrase more commonly heard from trout anglers trying to “match the hatch” than from bluegill anglers who find their quarry ready to attack a worm at a moment’s notice. Even so, the bluegill does exhibit selective feeding in some circumstances. It has even served as the test subject in ecological experiments that have taught us much about when, where, and what fish choose to eat.
Bluegill can and do eat almost anything they can fit in their small mouths, including small fish and fish eggs, amphibian larvae and eggs, and even algae and aquatic plants. Invertebrates are most often on the menu, though. Grasshoppers, spiders, ants, dragonfly and damselfly larvae, midges, snails, scuds, water fleas, and a long list of other insects, zooplankters, and bottom-dwellers are fair game. Bluegill are capable of eating a very wide range of prey, but in a lake full of potential prey how does a bluegill determine how to forage?
One of the best-known models of foraging behavior is Optimal Foraging Theory. The logic behind the model makes intuitive sense; predators try to maximize the amount of energy they consume and minimize the amount of time and energy they spend searching for and processing their food. Some food items require more search time than others. Damselfly naiads hiding in thick weeds may be harder to find than water fleas suspended in open water. Other foods, such as snails or zebra mussels, require a longer handling time due to their protective shells.
Different foods offer different payoffs in terms of energy, too. Plants and zebra mussels don’t offer many calories despite their abundance. Even energy-rich prey like water fleas might be too tiny to bother with unless they are extremely abundant and search time is low.
All of these tradeoffs influence the foraging behavior of bluegills, but the Optimal Foraging Theory doesn’t go far enough to explain the actual feeding patterns of bluegill. One of the simple predictions of the theory is that bluegill should select relatively large prey because of the higher energy content offered by large prey. Even when the small mouth of a bluegill is considered, small bluegill regularly key in on smaller food than the theory predicts.
One reason for this is that small bluegill are confined to shallow, weedy habitats that offer less food than open water does during late summer. The vegetation offers protection from predators like largemouth bass, and it is more important for small bluegill to avoid predators than to forage effectively. When food of all types is relatively rare, bluegill feed on virtually anything that comes their way. It is a survival strategy that makes sense: why be picky when the pickings are slim?
Other resources on bluegill:
Bluegills: Biology and Behavior (American Fisheries Society book)