Illinois Natural History Survey - University of Illinois

The Rusty Crayfish in Illinois

 Crayfishes, also known as crawfish or crawdads, are rapidly becoming one of the most destructive invaders of aquatic ecosystems in North America. When introduced into new habitats, non-native crayfish populations increase in size exponentially. Subsequently, the invaders will significantly reduce the size of native fish and amphibian populations by direct predation on their eggs. Non-native crayfishes can also affect fish populations by consuming the aquatic plants that juvenile fishes use for shelter. Perhaps the greatest effect of non-natives is felt by the native crayfishes. Through a variety of methods, invading crayfishes are extremely efficient at displacing natives species.

Unfortunately, one of the most destructive invaders, the rusty crayfish (Orconectes rusticus), has become firmly established in Illinois waters. The rusty crayfish was first collected in Illinois in 1973 from the Illinois River at Peoria and until 1985 was known from only nine locations (Fig. A). Since then the species has rapidly spread throughout the northern half of the state. A field study conducted by INHS biologists from June 1994 to October 1995 found the rusty crayfish at 39 sites in Illinois (Fig. B). Not only has the rusty crayfish expanded its range in Illinois, but it has done so at the expense of our native crayfishes. The rusty crayfish was the only crayfish found at 14 of the 39 sites collected in the 1994-1995 study. Prior to the introduction of the rusty crayfish, the native virile crayfish (Orconectes virilis) and northern clearwater crayfish (Orconectes propinquus) occurred commonly at most of these 14 sites.

The rusty crayfish, Orconectes rusticus.


The rusty crayfish is a large, aggressive species compared to other Illinois crayfishes and as such is able to force native species out of habitat that provides refuge from predation. Deprived of this habitat, species such as the virile and northern clearwater crayfishes are either consumed by fish or mammal predators or are forced to move to other areas. A second component influencing the success of the rusty crayfish is the type of habitat present. The rusty crayfish and most of Illinois' native species need shelter from predation. This shelter is usually in the form of rocks or boulders on the stream or lake bottom. Crayfishes will crawl under or between these rock and boulders for protection and it is this habitat that the rusty crayfish takes solitary control of. Rock or boulder substrate is an excellent predictor of where the rusty crayfish will most likely invade. This type of habitat occurs throughout much of the Kankakee, Vermilion, Fox, and lower Rock River drainages. The species is already known to occur in all of these river systems (Fig. B) and it is only a matter of time before it becomes more widespread in them.

Crayfishes are often used as fishing bait and as such unused individuals are often dumped into lakes and rivers at the end of the day. This practice has led to the establishment and rapid spread of the rusty crayfish in Illinois and other states. In recent years state legislation has been passed that bans the possession and use of the rusty crayfish as bait; however this might be a case of too little too late. The species is already well established in the northern half of the state and there is currently no known means to eradicate it. At a minimum, strict enforcement of this legislation will help slow the spread of the rusty crayfish. Additional actions should include the continued education of anglers on the negative effects of rusty crayfish and the undesirability of dumping unused bait.

To prevent the introduction of additional non-native crayfish species into Illinois, new policies and legislation are needed. Wisconsin, a state that has seen a large portion of its native crayfish population eradicated by the rusty crayfish, has passed laws that ban the use of any live crayfish species as bait. These measures have had very little, if any, effect on the recreational fishing industry and serve as excellent means to prevent additional introductions. With two species of crayfishes known to occur only in Illinois and several others with small native ranges, it is obvious that the state would benefit greatly from similar legislation. If we are to protect our state's native crayfish diversity, we must take proactive steps to prevent introductions and not ignore what we have learned from the rusty crayfish.

Christopher A. Taylor, Center for Biodiversity

Illinois Natural History Survey

1816 South Oak Street, MC 652
Champaign, IL 61820

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