The bluegill, Illinois' state fish, has an average life span of five to six years. The average bluegill weighs one-fourth pound and is eight to 10 inches long. A dark flap extends from the gill cover. Its pectoral fin is long and pointed, and three spines are present in the anal fin. The body is deep and flattened side-to-side. A black spot may be seen at the rear of both the dorsal and anal fins. A typical bluegill has a green back, green sides and a white to yellow belly. Dark bars are sometimes visible on the sides. The breeding male has a blue head and back, a red-orange breast and belly and black pelvic fins.
The bluegill lives in lakes, ponds, swamps, creeks and rivers with warm, clear water and plenty of aquatic plants. This species moves in groups of about 20 to 30 fish. It is active in the evening and early morning. The bluegill reaches maturity at an age of one year. Spawning occurs May through August. The male builds a nest by fanning his fins over the bottom. The female deposits 2,300 to 67,000 eggs. Several females may spawn in the same nest, and a female may deposit eggs in more than one nest. The male guards the eggs, which hatch in five to 10 days. Small males that have been unable to establish their own territory may still spawn by sneaking into the territory of a larger male. The small male can fertilize eggs when the larger male is occupied with something else or by behaving like a female to fool the larger male. The bluegill eats insects, small fishes, crayfish and snails.