Bluegill sunfish (Lepomis macrochirus) are common throughout the lakes and rivers of eastern North America. They belong to the Centrarchid family of fishes along with the crappies, freshwater bass and other sunfish. Bluegill are not a 'trophy' fish species, but they remain a popular game fish because they respond to most baits, often nibble before biting, and put up a fight when they are hooked.
Natural history The size and lifespan of bluegill varies across populations, with the southernmost bluegill populations maturing earlier and growing larger than the more northern bluegill populations. The population that I study is found in Lake Opinicon, Ontario, which is near the northern edge of bluegill's range. Most of the qualitative characters I describe in bluegill will apply to all populations, but most numerical characters, such as ages and sizes are specific to my population.
Lake Opinicon is a shallow lake that is part of the Rideau Canal waterway. Among the fish found in Lake Opinicon are bluegill sunfish, pumpkinseed sunfish, northern pike, largemouth bass, smallmouth bass, yellow perch, black crappie, and bullhead catfish. Bluegill are one of the more abundant species in Lake Opinicon. They are generalist feeders, eating zooplankton, larval fish and benthic invertebrates (but, notably, do not eat zebra mussels).
Bluegill males are characterized by a discrete life history polymorphism. "Parental" males mature at age 7, and have several reproductive years before they die. These males use their caudal fins to construct bowl-shaped nests in colonies that may be as small as 10 nests, but may have more than 100. Parental males then court females and defend and care for the young for the length of the care-giving period (7-10 days). These males are approximately 20cm long and have a mass of close to 200g. "Cuckolder" males, in contrast, mature at age 2. They do not provide care for the young, and instead fertilize eggs opportunistically using one of two different tactics. "Sneaker" males are small (7-10cm total length) individuals 2-3 years old. When a female has entered a nest to spawn with the nest-guarding parental male, sneakers swim very quickly into the nest, usually beneath the spawning pair, and release their sperm. "Satellite" males (also called "Female mimics") are larger (10-14cm total length) and older (4-6 years). Rather than sneak into the nest, satellites mimic the appearance and behaviour of females. They enter the nest as a parental male and a female are in the act of spawning, but release sperm rather than eggs, thus stealing fertilizations from the parental male. In approximately 90% of cases, parental males are alone with females in the nest. However, in the cases in which males are in competition for fertilizations, cuckolders fertilize the majority of eggs released. On average, approximately one-fifth of larvae in a parental's nest were sired by cuckolder males. Females provide no care to the developing larvae. They mature at age 4 and enter colonies as a shoal to spawn. A parental male will have multiple females visit his nest, and females will visit the nest of multiple males. In Lake Opinicon, bluegill breed in June, with some spawning bouts occuring in late May and early July. Most adults participate in multiple spawning bouts in a year.
Parental males provide care for their brood by defending it against predators (such as bullhead catfish and other sunfish) and by fanning the eggs with their caudal fin. Fanning increases water flow over the eggs, providing a fresh source of oxygen to the developing embryos. Care is energetically expensive, and males lose up to 10% of their body mass in this period. To protect themselves against caring for larvae sired predominantly by cuckolders, parental males have developed an ability to assess their level of paternity in the nest using odour cues that emerge after the eggs in their nest have hatched. If the level of cuckoldry is unusually high, parental males abandon their nest. If, however, the level of cuckoldry is not abnormally high, parental males continue to provide care until the larvae in their nest have developed sufficiently to leave the nest. When they leave the nest, larvae swim out to deep water to feed on zooplankton. Presumably, cooperation among nestmates is important during this time so that they may feed most efficiently and increase their growth rate.
The life history adopted by an individual depends on multiple factors, and probably relates to the environment experienced by larvae during their early development period. About 70% of the young sired by parental males adopt the parental life history, and about half the young sired by cuckolder males adopt the parental life history.
Suggested reading The bluegill natural history section above has drawn from many sources and is intended only to introduce the reader to the bluegill story. If you wish to learn more about bluegill, I recommend investigating three authors in particular. Mart Gross (1980, 1982) and Wallace Dominey (1980) were the first to describe the cuckolder life history in bluegill sunfish. My supervisor, Bryan Neff, has made most of his research papers available on his website. By navigating his website, you will be exposed to several exceptional pictures of bluegill in their natural habitat.
Dominey, W. (1980) Female mimicry in male bluegill sunfish - a genetic polymorphism? Nature. 284: 546-548. Back to the Research page.
Gross, M. R. (1980) Alternative male life histories in bluegill sunfish. Proc. Natl. Acad. Sci. USA. 77: 6937-6940.
Gross, M. R. (1982) Sneakers, satellites and parentals: polymorphic mating strategies in North American sunfishes. Z. Tierpsychol. 60: 1-26.
Timothy J. A. Hain
PhD student
Department of Biology
University of Western Ontario
London, Ontario, Canada