They are the most reviled creature in all of Japan. After escaping from a Japanese biological compound 50 years ago, they have wreaked havoc on the native fishes of this Asian country. They have wiped out Japan’s royal bitterling and pushed many other fish species near the brink of extinction. Former Emperor Akihito’s “heart aches to see it all turned out like this”. Back in the United States, this creature also lurks in the lakes of Colorado. To us, however, it’s an old native friend who’s found its place in the hearts of many anglers; this fish is the bluegill.
The bluegill is a great example of a species native to Colorado that is an invasive species elsewhere. It has established its ecological niche in our lakes, and populations are kept in check by fish like the largemouth bass, channel catfish, and by Colorado’s licensed anglers! When a species is taken from its natural habitat, and placed in another habitat suitable to its needs, it can sometimes outcompete the natives because no natural predators exist to remove the invasive species from the environment.
The bluegill gets its name from bright blue edging that’s visible on its gill rakers (a projection of the gill 'arch', and function to help retain food organisms). The easiest way to identify this fish is its flat appearance with a blue or black “ear,” called the operculum (the gill flap covering).
For decades this fish has been widely popular with anglers who remember it as the first fish they’ve ever caught. According to many of Colorado’s anglers, the flavor of a bluegill is very delicious, on par with walleye. This article, however, isn’t about catching and eating these little swimmers. You don’t have to be an angler to appreciate Colorado’s fish. Watching these fish can lead to a new appreciation of animal behavior!
Bluegills are colonial nesters, and build their nests in the shallow waters of lakes during the spring. To build the nest, they sweep dirt and rocks away to excavate a small crater. Because the nests are built in shallow waters, they are easily seen by wildlife watchers. Watchers can notice that the larger, dominant males make their nests in the center of the colony, with the smaller, subordinate males around the perimeter. The males in the center of the colony are three times more likely to reproduce than those in peripheral nests.
These fish usually swim in schools of 20-30 individuals, but when it comes to their nests, males can be quite territorial. In June, when bluegills begin to spawn, males take on a very bold coloration pattern. This coloration lets others know that it’s ready to breed. An interesting fact is that some males will try to avoid aggressive confrontations with other males by taking on the female coloration. These shallow-water colonies provide a great opportunity to watch some of Colorado’s fishy wildlife!
References used in research for this article include:
McCurry, J. (2006, November 26). "Japan in culinary offensive to stop spread of US fish" The Guardian.Retrieved May 08, 2008 from www.guardian.co.uk.world/2007/nov/26/japan.isreal
Willis, D. (2005, November/December) "Bluegill nesting" Pond Boss 14(3): 22-23.
Gross, M.R., and A.M. MacMillan (1981) Predation and the evolution of colonial nesting in bluegill sunfish (Lepomis macrochirus). Behavioral Ecology and Sociobiology 8(3): 163-174.