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Focus on Freshwater Fish

Although the West Branch of the DuPage River flows right through the City of Naperville, freshwater fish are animals that many of us never see unless we hook one on a fishing line. They stay hidden under the surface of their watery habitat where they spend their whole lives. Like us, fish have a backbone and are a type of vertebrate. But unlike us, fish are ideally suited to their life underwater.

Most fish have laterally compressed bodies, making them flattened from side to side and highly streamlined. Their bodies are covered in scales, which protect their bodies, and most fish possess a slimy layer of mucus which also protects and insulates their bodies. When you catch and release fish, be sure to get your hand wet before handling the fish. A dry hand removes this layer of slime exposing the fish to diseases and parasites when you place it back in the water.

Instead of lungs fish have feathery gills that take oxygen out of the water and release carbon dioxide. The gills are found under the operculum, or gill cover. To move, fish have a variety of fins that help propel them forward, steer and their maintain position. The caudal fin, or tail fin, propels them through the water. Top fins, called dorsal fins, help to keep fish from rolling, especially during sudden stops and quick turns. Most fish have one dorsal fin, but some have two or three. Bottom fins, near the fish’s’ tail, are called anal fins and also stabilize the fish. On each side of a fish are pectoral fins, usually right behind the gills, which allow the fish to make fine adjustments of their position and steer left or right. Below the pectoral fins, on the underside of the fish, the pelvic fins can be found. These fins help fish move up and down, and turn and stop quickly.

In order to see underwater, the lens of a fish’s eye is more spherical than our own, making it bulge out. Like us, fish can see color and some can see ultraviolet light.  Fish have good vision although some species have lost their ability to see and depend upon their other senses to get around. Their nostrils, or nares, are used to smell underwater. They feel vibrations and can detect movement with a feature called a lateral line that runs along the length of a fish’s body. This special sense is what causes fish to dart away when someone taps on their tank; they think it might be a predator approaching so they swim for cover.

Despite the common features listed above, fish come in all shapes and sizes. Typical looking fish, like bluegills and bass, are found in the DuPage River, but so are Long-nosed Gar with their prehistoric-looking mouth full of teeth and Yellow Bullheads, a type of catfish, with whiskery barbels around their mouth that help them smell and taste. Where a fish lives and what it eats defines how a fish will look. The study of fish is called ichthyology.

To learn more about ichthyology and Illinois fish species, visit the Illinois Natural History Survey