Montana Field Guide

The rusty crayfish is a large (up to 6 inches), aggressive species of crayfish which is native to the Ohio River basin. Their carapace usually has a pair of rusty-colored spots and claws often have black bands at their tips. The rusty crayfish has not been found in Montana, but it has been transplanted to new waters in other nearby states (WY, CO) resulting in viable populations. Only three species of crayfish are thought to be native in Montana, Orconectes virilis, Pacifastacus leniusculus and P gambelii. O. imnunis, which has been found in the south central region of the state, is an introduced species (Dr. W. Gould, Professor Emeritus, MSU, Bozeman, MT). The native populations could be seriously impacted by introduction of a non-native species of crayfish, such as the rusty crayfish O. rustucus.

The rusty crayfish is larger (up to 6 inches) than Montana's native Orconectes, and usually has a pair of rusty-colored spots on the sides of their carapace and over-sized claws often have black bands at their tips.

Rusty crayfish are able to survive in many different aquatic environments, so once they are introduced it is easy for them to proliferate. It only takes one introduced female who has sperm stored to start a new population. Once introduced into an area, rusty crayfish can spread via connected waters.

Permanent water bodies (lakes, streams, wetlands) with cover of rocks, logs or trees. Rusty crayfish do not burrow like other crayfish.

Rusty crayfish are voracious eaters, consuming 2-3 times as much per day as native crayfish. Rusty crayfish eat whatever is available, including plants, snails, clams, insects, other crayfish, fish eggs and small fish.

The rusty crayfish has eliminated native Orconectes species in waterbodies of other states (Olden et al 2006) and has had serious negative impacts on macrophyte populations in some states by destroying aquatic plant beds, affecting the fish and organisms that require this habitat. Native crayfish are also susceptible to a variety of bacteria and viruses, which could be introduced with non-native crayfish. A life span of three to four years is typical for a rusty crayfish (Durland Donahou et al. 2018).

Rusty crayfish can mate in late summer, early fall, or early spring. The male’s sperm is stored in the female until the eggs are ready to be fertilized. Eggs and stored sperm are released at the same time and external fertilization takes place. The fertilized eggs then attach to the bottom of the females tail section. Eggs hatch in three to six weeks. A female can lay anywhere from 80 to 575 eggs. Once hatched, the young will continue to cling to the female’s tail section for further protection until they are large enough to be on their own. When the young become about 1 3/8 inches long they have reached maturity (Durland Donahou et al. 2018).

The rusty crayfish has not been found in Montana, but it has been transplanted to new waters in other states resulting in viable populations. Rusty crayfish can easily survive overland transport from the Midwest to Montana while in live wells or bait buckets. The rusty crayfish is a prolific competitor with great potential to disrupt aquatic ecosystems that it is introduced.

Once established in a waterbody, there are chemicals that will selectively kill only crayfish that have not been registered or labeled for crayfish control. Unfortunately, the chemicals do kill all species of crayfish, and are not specific to rusty crayfish . Manual harvest for human consumption is an effective control strategy due to the large size of rusty crayfish. However, this strategy is only useful in reducing the adult population. Once a population of rusty crayfish is introduced to a body of water, it is very difficult to completely eradicate them. Therefore, the best control strategy is to try to prevent . Therefore, the best management action to prevent any further spread of the rusty crayfish is to inspect boats with any water in live wells or bait buckets from entering Montana's waterways and the crayfish's release and establishment.

Contact information for Aquatic Invasive Species personnel:
Montana Fish, Wildlife, and Parks Aquatic Invasive Species staff
Montana Department of Natural Resources and Conservation's Aquatic Invasive Species Grant Program
Montana Invasive Species Council (MISC)
Upper Columbia Conservation Commission (UC3)

Once established in a waterbody, threats to the rusty crayfish are minimal, other than reservoir drawdowns and river dewatering which will leave the crayfish prone to desiccation. Since the rusty crayfish will assume a defensive position when threatened by predators, they are not eaten as much as other crayfish that flee when attacked.