Rusty crayfish may cause a variety of negative environmental and economic impacts when introduced to new waters. This aggressive species often displaces native or existing crayfish species. Rusty crayfish displace other crayfish species through three primary mechanisms:
1.) Crayfish to Crayfish Competition
Rusty crayfish are better able to exclude other crayfish from shelters and better able to compete for limited food resources.
2.) Increased Fish Predation
Rusty crayfish can increase fish predation on native crayfish in a variety of ways. They force native species from the best hiding places. As the native crayfish try to swim away from a fish or rusty crayfish attack, this makes them more vulnerable to capture by fish. Rusty crayfish, on the other hand, assume a claws-up defensive posture that reduces their susceptibility to fish predation. Also, rusty crayfish are larger and have larger claws than most native species, which results in fish preying upon native species over rusty crayfish.
3.) Hybrdization
While rusty crayfish do not hybridize with Orconectes virilis, they do hybridize with Orconectes propinquus. This hybridization results in fertile and vigorous offspring, but ultimately results in the decline of Orconectes propinquus. The competitive superiority of the hybrids helps exclude genetically pure Orconectes propinquus faster than Orconectes rusticus would without hybridization.
The destruction of aquatic plant beds is perhaps the most serious impact. Rusty crayfish have been shown to reduce aquatic plant abundance and species diversity. This can be especially damaging in relatively unproductive northern lakes, where beds of aquatic plants are not abundant. Submerged aquatic plants are important in these systems for:
- habitat for invertebrates (which provide food for fish and ducks),
- shelter for young gamefish, panfish, or forage species of fish,
- nesting substrate for fish, and
- erosion control (by minimizing waves)
Although other crayfish eat aquatic plants, rusty crayfish eat even more because they have a higher metabolic rate and appetite. They also grow larger, hide less often from predators – and therefore feed longer – attaining high population densities.
Rusty crayfish are more likely to compete with juvenile game fish and forage fish species for benthic invertebrates than are native crayfish species. Displacement of native crayfish by rusty crayfish could result in less food for fish. Crayfish are eaten by fish, but because of the higher ratio of their thick exoskeleton (shell) relative to soft tissue, their food quality is not as high as many of the invertebrates that they replace. Less food or lower food quality means slower growth, which can reduce fish survival.
Rusty crayfish can harm fish populations by eating fish eggs, reducing invertebrate prey, and through loss of habitat (aquatic plants). Male bass and sunfish protect their nests until the eggs hatch and the advanced fry swim away. It was also found that total zoobenthos, larval midges, mayflies, dragonflies, and stoneflies decline as rusty crayfish populations increase. Walleye reproduction dropped after a rusty crayfish invasion.
Observations and circumstantial evidence gathered by Wisconsin fishery managers suggest that bluegill and northern pike populations frequently decline following the introduction of rusty crayfish. Impacts on other fish species are not as obvious. The cause of bluegill, bass, and northern pike declines is probably reduced abundance and diversity of aquatic plants. Reduced food (such as mayflies, midges, and stoneflies) and egg predation may also play a role.
Cabin owners on heavily infested northern Wisconsin and Minnesota lakes have even stopped swimming because large numbers of "rustys" occupy their favorite swimming area throughout the day. They fear stepping on them and getting pinched by their large claws. Other crayfish species, even if abundant, are less conspicuous during daylight hours.
