On the radar: Hunting invasive crayfish in Michigan rivers and streams
In the summer of 2013, Michigan anglers began noticing something strange: carcasses of large crayfish, blood red in color, on the banks of Lake Macatawa in West Michigan’s Ottawa County. The crustaceans were identified as red swamp crayfish, a particularly aggressive species hailing from the southeast United States that has steadily invaded every continent except Antarctica.
Their presence has left a path of local ecosystems in turmoil. Fearing Michigan might be next on the crayfish’s advance, the Michigan Department of Natural Resources (MDNR) turned to the Michigan State University (MSU) Department of Fisheries and Wildlife for assistance. Red swamp crayfish are the most widely consumed crayfish in the world, which is in part to blame for their worldwide expansion. Exported alive across oceans and international borders, some invariably escape their confines and take up residence in streams and wetlands, wherever they happen to land.
This brings with them complications for the wildlife, environments and human communities that already call those places home. Known for rapid reproduction and aggressive burrowing behavior, red swamp crayfish pose serious threats to wetlands, which they can drain; dykes and dams, which they can undermine; and native crayfish species, which they can supplant. In many cases, red swamp crayfish disrupt local food webs because, being larger than native species, they are unsuitable as a food source for predatory fish.
In 2014, when Kelley Smith began his master’s degree program at MSU, crayfish were not on his professional radar. That changed when MDNR put out a call for proposals from researchers to execute a statewide survey of Michigan’s rivers and streams in search of evidence of the red swamp crayfish. The southern Ohio native had just completed two and a half years as a contractor for the U.S. Geological Survey’s fisheries division when Michael Jones — Peter A. Larkin professor of quantitative fisheries in the MSU Department of Fisheries and Wildlife and MSU AgBioResearch assistant director of natural resources programs — approached him about a crayfish research project.
“Growing up, I was always mucking around in creeks and catching crayfish, so I was already aware of them and the issues they pose,” Smith recalled. “Professionally, fieldwork and survey design are my strongest suits, which all came together to make this project really appealing to me.”
Once Smith began his work, he was joined by Brian Roth, MSU AgBioResearch ecologist and invasive species expert.
“We knew from talking with other crayfish researchers that they had already made a foothold in Ohio, in the Sandusky Bay area on Lake Erie,” Roth said. “That’s close enough for concern, so we knew we had to develop a better grasp of the crayfish situation here in Michigan.”
Jones stayed on the project as well. As the co-director of the Quantitative Fisheries Center, he is familiar with statistical models and the uncertainty that comes with estimating wildlife populations across large expanses of terrain. The last statewide survey of crayfish in Michigan was conducted in the 1930s, with the most recent data coming from a partial survey conducted in 1975. The last 40 years had seen numerous shifts in Michigan’s environmental and ecological makeup, however, and that data was simply no longer sufficient for the task at hand.
In order to find statistically valid evidence of red swamp crayfish in Michigan, assess the risks they pose and craft an accurate picture of the state of all crayfish populations throughout the state, the team decided to conduct their own statewide survey. The task would take two years, but the results would inform Michigan’s natural resources policy for much longer.
WALK SOFTLY AND CARRY A BIG NET
“All it takes is a five-gallon bucket, a pair of waders and a net on a long pole,” Roth explained how the team collected data on crayfish. “Kelley and his field technicians waded the streams, overturning rocks on the bottom and scooping with the nets to see what they could find.”
This method, called dip netting, was adopted because it could be implemented across the diversity of Michigan river and stream systems, allowing for consistent results. Many streams in the state, for example, feature coarse substrates like boulders or stone cobbles that would render other techniques unusable. While data collection may have been simple, selecting the streams for sampling was not. Using the MDNR stream database, the agency’s collection of information on every river and stream in the state, the team randomly selected streams, and segments of those streams, in every region of Michigan.
This gave a statistically valid estimation of crayfish populations throughout the state without having to tackle the task of surveying each and every stream. In the first year, the project only had funding to survey the Lower Peninsula, but the MDNR was so satisfied that they extended it for a second year in order to cover the Upper Peninsula as well. Gauging wild populations of crayfish was only one aspect of their work, however. In order to fully assess the risk of invasion Michigan faced, the team had to look for other ways crayfish could arrive.
One of the team’s hypotheses regarding the origin of the first red swamp crayfish carcasses found by anglers was they had been purchased for live bait and escaped. As this is similar to how the crayfish had first become established in other regions, they decided it was worth investigating. At the time, it was legal under Michigan law and MDNR policy to import live red swamp crayfish for personal consumption, and there was concern they were also being brought in for other purposes, not only for bait, but as pets or classroom science projects.
Smith and his team identified and visited numerous bait shops, pet stores and seafood markets to assess the potential for red swamp crayfish to enter the state. They also surveyed 157 public school science teachers regarding use and disposal of live crayfish.
GOOD NEWS AND BAD NEWS
After two years and surveying hundreds of stream sections in the Upper and Lower Peninsulas, Smith’s team did not find any live red swamp crayfish in the wild, nor did they find that stores or classrooms presented a particularly serious invasion gateway.
“In general, we didn’t find that there was one pathway that was by far the most likely through which an invasion could occur,” Jones said. “Instead, we found that we have to pay attention to all of them.”
In response, MDNR has placed new restrictions on importing any non-native live crayfish, including red swamp crayfish, in order to limit potential invasion.
“Red swamp crayfish are still a concern, because the Ohio population is still located very close to Michigan, but we were relieved to find out they weren’t already here,” Roth said. “Your best opportunity to control any invasive species is very early, before they get firmly established, and we know now that we have that chance to take preventive action.”
The team did uncover a more comprehensive picture of the crayfish population in Michigan than had been available since the first statewide survey 80 years ago. Having such a complete data set will allow MDNR officials to make sound, science-based, natural resources management decisions that will impact anyone who relies on Michigan’s ecosystems, from recreational anglers to commercial fishermen to anyone buying local seafood at the store.
The lack of red swamp crayfish was certainly welcome, but not all of the insights derived from the project were so optimistic. Smith discovered that another invasive species, the rusty crayfish, has spread significantly in the last 40 years.
“They’ve gone from being in just a few counties to spreading across the entire Lower Peninsula and into the Upper Peninsula,” Jones said. “From our findings, we can say that rusty crayfish are present in about 60 percent of our watersheds. Of equal importance is what that means for our native species.”
While not all of the data has been processed, Smith said he learned from similar research conducted in Wisconsin that rusty crayfish frequently outcompete native species, such as northern crayfish and northern clearwater crayfish, pushing them into less desirable habitats where they are exposed to greater fish predation. By cutting vegetation with their pincers, they have also threatened important fish populations like bluegill and bass, whose young rely on vegetation for shelter.
In addition to finding the extent of rusty crayfish expansion, the team was able to describe the most desirable habitat conditions. In streams with beds of rock or stone cobble, for example, rusty crayfish spread rapidly and displace nearly all native species. In streams with more malleable beds of mud or sand, however, native species have proven resilient despite their new neighbors.
THE IMPORTANCE OF CRAYFISH
Crayfish, though ubiquitous, have not historically received the same level of research interest or funding as other common aquatic organisms. Smith, Roth, Jones and their team hope the findings will help people recognize the importance of crayfish to the state’s ecology and perhaps drum up more research funding.
“Crayfish are ecosystem engineers,” Jones said. “Like beavers, they’re among a small subset of organisms that can actually create their own habitat. They can cut vegetation with their pincers, move rocks, burrow into riverbanks and otherwise reconfigure their surroundings. That, in turn, affects the other organisms sharing that habitat by changing what resources are available.”
Roth, who has studied crayfish – rusty crayfish in particular – since his doctoral work at the University of Wisconsin, is already seeing growing interest in the freshwater crustacean.
“If you look around the state and talk to natural resource managers and conservation officers, they know crayfish are important parts of the ecosystem,” Roth said. “The thought of non-native species coming into Michigan is concerning to a lot of people, and our study highlights the changes that have already taken place in the last decades. There needs to be repeat monitoring and studies on how they impact food webs, and we hope this work serves as a springboard for that.”
For Smith, the project was more than an important step in improving natural resources management.
“I basically got funding to camp for three summers and see a lot of backcountry streams that a lot of people just don’t get to,” Smith said. “Crayfish are a keystone species, and having healthy populations – meaning not too many and not too few – benefits every part of the ecosystem. I learned a lot about the importance of collaboration in science and I loved being able to apply my skills toward helping solve real problems.”
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