Rusty crayfish an exotic pest

by KEVIN STRAUSS

People who live on or visit lakes may have noticed a new creature in the neighborhood. It is a larger, more aggressive crayfish with dark red markings on its body. It is a rusty crayfish (Orconectes rusticus).

These 2-4 inch long, aggressive crayfish have been known to pinch swimmers and destroy aquatic plant beds in Midwestern lakes. Because of these problems, wildlife officials are working to keep rusty crayfish out of northern lakes.

Rusty history

Researchers believe that this invasive species may have arrived in northern Minnesota rivers and lakes years ago as out-of-state anglers brought them in as bait and then released unused crayfish into Minnesota waters. It is also possible that some of the rusty crayfish that biological supply companies sold to schools may have found their way to local waterways as well-meaning teachers or students “freed” them into the wild.

The main problem with rusty crayfish is that they are bigger and more aggressive than native species. Although this species is native to parts of Illinois, Indiana, Ohio and Kentucky, rusty crayfish have spread to many northern lakes and streams where they cause a variety of ecological problems. Invading rusty crayfish frequently displace native crayfish, reduce the amount and kinds of aquatic plants and invertebrates (insects and other small organisms), and reduce some fish populations. Researchers haven’t been able to develop environmentally-sound ways to eradicate or control introduced populations of rusty crayfish. The best way to prevent further ecological problems is to prevent or slow their spread into new waters.

Regulations in both Minnesota and Wisconsin now make it illegal to introduce rusty crayfish into any waters. In Minnesota, it is illegal to sell live crayfish as bait and the Department of Natural Resources requires a permit to commercially harvest or raise crayfish.

The rusty menu

Rusty crayfish are considered “opportunistic feeders” meaning that they eat whatever is most abundant. They feed on a variety of aquatic plants, sometimes snipping them off with their claws. They also feed on aquatic invertebrates like snails, leeches, clams, aquatic insects, and crustaceans, detritus (dead animals and plants), fish eggs, and small fish.

Rusty problems

Rusty crayfish may cause a variety of negative environmental and economic impacts when introduced to new waters. They are an aggressive species that often displace native or existing crayfish species through a combination of direct competition for the same foods and increased fish predation on the less aggressive native species. As the larger rusty crayfish take over the best hiding places, native crayfish find it harder to hide from hungry fish.

Perhaps the most serious impact that rusty crayfish have on lakes is their destruction of aquatic plant beds. Researchers have documented cases where rusty crayfish have destroyed aquatic plant beds.

“There are places on lakes where (rusty crayfish) have removed large portions of the submerged vegetation in the water,” said Department of Natural Resources Tower Area Fisheries Supervisor Joe Geis. “In Wisconsin (rusty crayfish) have removed all the submerged vegetation in lakes.”

Submerged aquatic plants are important habitat for aquatic insects and fish and reduce erosion by slowing down wave action. Although other crayfish eat aquatic plants, rusty crayfish eat even more because they have a higher metabolic rate and appetite. In addition, crayfish populations grow faster than native populations, increasing their impact on lakes. Researchers also believe that a rusty crayfish could eat twice as many mayflies, stoneflies, midges, and side-swimmers as a native crayfish.

While there is no definitive proof that rusty crayfish cause declines in fish populations, observations and circumstantial evidence gathered by Wisconsin fishery managers suggest that bluegill and northern pike populations frequently decline following invasion of rusty crayfish. Smallmouth bass in Lake Lenore and largemouth bass in Pounsford Lake, Ontario also seemed to decline following introduction of rusty crayfish.

Watch out for pinchers

But rusty crayfish aren’t only aggressive toward other crayfish species, they can also pinch people. Some cabin owners on heavily-infested northern Wisconsin and Minnesota lakes have even stopped swimming because large numbers of rusty crayfish occupy their favorite swimming areas throughout the day. They fear stepping on them and getting pinched by the large-clawed "rusties." Other crayfish species, even if abundant, are usually hiding during daylight hours.

Dealing with the problem

Once rusty crayfish get established in a lake, it is difficult to dislodge them. Some fisheries managers advocate increased gamefish stocking in the hopes that gamefish will feed on more of the rusty crayfish. Others hope that human harvests could help to control the population. Anglers can trap up to 25 pounds of crayfish with a fishing license. But to date the DNR hasn’t provided residents with recipes for crayfish.

Experts contend that the best way to control this invasive species is by keeping them from spreading to new lakes. New laws that make it illegal to sell live crayfish bait and illegal to release rusty crayfish into Minnesota and Wisconsin waterways. Policies geared to stemming the spread of other invasive species like Eurasian milfoil and zebra mussels can also help prevent the spread of rusty crayfish, according to Geis.

“People need to make sure that they drain their livewell before they leave a lake and clean off their boat and trailer so they don’t transport invasive species to new lakes,” said Geis.

Insects like backswimmers remind us how productive our area lakes are this time of year. Be sure to take time to notice all of the life at your local lake.