Invasive Rusty Crayfish Appear to Be Dying Off and It's Not Clear Why

Populations of rusty crayfish, an invasive species in the lakes of Minnesota and Wisconsin, have seen a steady decline in some regions—and, for once, humans aren't responsible.

A study published in the journal Ecological Applications on February 11 found that over a 33-year period, the rusty crayfish population in several northern Wisconsin lakes had decreased naturally without human input, dropping to nearly zero in four of the lakes.

"It can be tough to get an actual population estimate because there's so many rusty crayfish in a lake," lead study author Danny Szydlowski, a Ph.D. researcher at University of Wisconsin-Madison's Center for Limnology, told Newsweek.

"For example, one study removed 90,000 rusty crayfish from a single lake over about 8 years of intensive trapping. Because of this, we usually measure populations as crayfish caught per trap we set. In the "high crayfish" lakes, we can catch up to 50 crayfish per trap, or up to 800 crayfish in a day! In the low crayfish lakes, we might only get a handful of crayfish total, or catch rates between 0-5 per trap. Crayfish in our decline lakes have thus declined roughly 90-95% from the peak of their invasion."

Rusty crayfish are freshwater crustaceans native to the Ohio River Basin (Ohio, Indiana, Kentucky, Tennessee). Anglers using them as bait are suspected of having introduced them to the northern lakes, according to the National Park Service (NPS).

They established themselves as invasive species in the lakes of Wisconsin and Minnesota, competing with native crayfish for food and shelter and reducing the native populations. They eat a number of native snails, and also clip and eat large amounts of water plants, which causes major shifts across the whole food web: Fish may lose shelter and substrate to nest upon, and sediment erosion may increase.

The researchers think that the reasons for the crayfish's natural decline is due to a combination of them destroying their own habitats and contracting parasites.

"Declines are likely caused because of their destruction of aquatic plants, exposing themselves to predation by fish in lakes without enough rock for them to hide in, or because of a microsporidian disease that is affecting them," Szydlowski said.

Essentially, it's possible that the crayfish caused so much damage to the native water plant species that they are no longer able to shelter themselves from predators. Alternatively, the crayfish may have been parasite-free when they arrived in the lakes but have since gained parasites, limiting their populations.

"One common hypothesis for why invasive species become successful (more abundant than native species, for example) is the idea of 'enemy release'," Eric Larson, co-author of the paper and associate professor of conservation at the University of Illinois, told Newsweek.

"That they might invade new ecosystems without some of their native range parasites or pathogens. But their native range enemies might eventually find them through subsequent introductions—crayfish introduced by bait buckets in the 1960s might have just not been infected by a parasite by chance, but a subsequent introduction of the same species in the 1980s or 1990s might have been infected. Or native parasites or pathogens might eventually adapt to the new, invasive species after a few decades."

The findings will be welcome news to government scientists as human schemes to control the rusty crayfish population have met with only limited success. Chemical controls can't be used for fear of harming native crustaceans, while trapping and using native predators such as bass or sunfish to keep down rusty crayfish numbers have had only "scattered successes" in places where rusty crayfish populations were not firmly established, the NPS said.

The researchers are, however, open to the idea that the driving force behind the crayfishes' decline may be something else altogether.

"We haven't done any population genetics or population genomics with this species, and invasive species sometimes lose genetic diversity over multiple decades due to things like genetic drift," Larson said. "I think in the future, we're going to really focus on both enemy accumulation and genomics explanations for these population dynamics for rusty crayfish."

These findings may have major implications on the field of invasive species control. If the crayfish really are responsible for their own demise, then it may be cheaper to control other invasive species populations by simply leaving them alone and waiting for the inevitable.

"These natural declines of invasive species (without human intervention) are understudied, but even more understudied is whether affected communities recover after. It could indicate that, where communities recover, managers won't need to spend resources actively restoring ecosystems," Larson said.

"However, there was still a ton of damage done during the peak of invasion, and we really stress that preventing the spread of invasives is still the best management strategy. Continued monitoring and research in this realm will also hopefully reveal how common such declines are, whether other communities recover, and how managers can take this information into account."

It is also possible that in the absence of the crayfish, the plant life of the lakes and snail populations will recover to their previous levels, providing habitats for the crayfish to make a comeback. Larson hopes that due to the additional influence of parasitic diseases, the crayfish populations won't entirely bounce back to their original levels.

"Even if rusty crayfish do persist in these lakes in the future but at low abundances, their ecological effects may be very weak. Remaining populations have some risk of booming again, but I don't necessarily expect that rusty crayfish have to be absent from these lakes for their ecological effects or impacts to be weak," Larson said.

Szydlowski agreed: "Whether or not those declines continue remains an open question, and there are still some lakes where rusties haven't declined. I'm optimistic that a combination of a disease affecting them and their destruction of their own habitat (aquatic plants) will be able to keep their numbers down, but continued monitoring is needed."

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