Fish Biology

More than half the state boundaries surrounding Illinois are water – to the west is the Mississippi River, to the south is the Ohio River, to the east is the Wabash River, and to the northeast is Lake Michigan. These drainage systems, as well as the numerous interior streams, kettle lakes, wetlands in northern Illinois, and cypress swamps in southern Illinois, help create a species-rich fauna.


There are 192 species from 30 families of fish native to Illinois, and they vary greatly in their size, appearance, distribution, and preferred habitats. Fishes vary in their movement as well. Some only move a few yards during their life, whereas others can migrate thousands of miles to complete their life cycles. Prior to 1800, humans probably had little effect on the fish fauna of Illinois. During the 19th century, land use in Illinois changed drastically when much of the Illinois landscape became dominated by row-crop agriculture. These changes included draining wetlands and channelizing streams, forever altering aquatic ecosystems hydrology and water quality. Surprisingly, however, only 10 species of fish have become extirpated, although 19 species are listed as state-endangered and another 12 are listed as state-threatened. As climate and aquatic environments change, fish assemblages could become less resilient to changes and more species are in danger of becoming extirpated.


IDOT surveys for fish require using multiple methods of sampling. Some fishes, such as the state-threatened eastern sand darter (Ammocrypta pellucida), live near the bottom of streams, and thus are most effectively collected with either a trawl or seine.  Other fishes, such as the state-threatened starhead topminnow (Fundulus dispar), live near the water's surface and can be captured with dipnets, similar to what one would buy at a pet store.  Yet other fishes, such as the state-endangered northern brook lamprey (Ichthyomyzon fossor), live around physical structures (e.g., woody debris or rock out-croppings) or in deep water, and are best collected by electroshocking methods.  These methods use small amounts electricity supplied from a boat or barge, which temporarily immobilizes fishes and allows them to be easily captured. Electroshocking typically collects the highest diversity of fishes. No single method captures all species present; however, when used together, the above methods collect a true representation of the fish assemblage present at a given site.