There’s hope of recovery in Trout Lake as parasite causes major drop in invasive Rusty Crayfish population
On a cool, calm, and sunny morning, Cheyenne Stratton and Ashley Hrdina make their way down to the dock by UW Trout Lake Station in Boulder Junction.
Stratton explains the “calm” part of the equation has been missing most of this summer, making her research a little more challenging.
“If there's any amount of wind Trout Lake it’s hard to work on it. It’s so wavy,” said Stratton.
The only ripples on Trout Lake that morning are from Stratton lowering the boat into the water and steering it toward the shoreline east of the dock where she and Hrdina set crayfish traps the day before.
Hrdina lays on her stomach across the bow of the boat as Stratton lines her up with a plastic jug buoy.
Hrdina pushes herself up to standing as she pulls the line and the trap at the end of it into the boat.
It’s filled with dozens of rusty crayfish clicking away.
The trap looks full, but Stratton says it’s nothing compared to what she used to pull out of the lake five years ago.
“There wasn't room for a single other crayfish in the trap. They would just be stuffed to the brim. You’d get over 100 crayfish in a trap. So the numbers are still down this year,” she said.
Rusty crayfish used to be used as bait for anglers and were first found in Trout Lake around the early 80s.
Their numbers quickly grew and the impact on the lake was evident.
“They’re kind of like little lawnmowers. They go around just clipping the bases of all the macrophytes, not necessarily completely consuming them, but just removing them. Then that has a lot of implications for other invertebrates, specifically, snails. Snails really like to live and eat macrophytes, which is the aquatic plants in the lake,” said Stratton.
By taking out the base of the food web, Rusty Crayfish have impacted many species and lake health.
They’ve had such an impact on sports fishing that researchers estimate the rusty crayfish costs the fishing industry in Vilas County about 1.5 million annually.
The crayfish population continued to grow even after the species was banned in Wisconsin in 1983.
There are chemicals that can control rusty crayfish, but they impact all crayfish, even the native species.
Trying to physically remove the crayfish hasn’t seemed to work well as a population control over the years.
But in 2016, the population started dropping.
In 2019, Cheyenne Stratton began research on Trout Lake. She’s working on her Ph.D. dissertation at the University of Florida.
That year, the rusty crayfish population saw its biggest drop.
That drop is mostly attributed to a parasite.
“The parasite is a microsporidian, a spore-forming parasite. It is specific to the rusty crayfish,” said Stratton.
Stratton has been studying the outbreak for the last five years.
“We wanted to not only understand how it impacted the host, the Rusty Crayfish but also how the rusty crayfish has ecosystem impacts. We've been doing a lot of monitoring of Trout Lake as a whole looking at various communities like the aquatic plants or the macrophytes, the snails, and the fish to see how those communities may change over time. Then we've also been looking at various trait and density mediated impacts of the disease on the rusty crayfish,” said Stratton.
Stratton and Hrdina, a recent University of Missouri graduate who’s been helping Stratton with the field work the last two summers, are looking for signs of the disease in the crayfish they’re collecting.
Each trap load of crayfish gets dumped into a five-gallon bucket.
Stratton and Hrdina pull one crayfish one by one and carefully study their abdomens.
“You can visually see the infection if you look at the abdomen, underneath the tail, the muscle tissue will actually be white and opaque white color and compared to the clear muscle tissue of a healthy individual,” Stratton said holding a crayfish and pointing to the underside of the tail.
They didn’t find any infected crayfish in the five traps they pulled that morning. That means they’ll start diving to find infected ones by hand.
“Trapping isn't the most effective method for finding the infected individuals because this disease is actually a spore-forming parasite, it replaces the muscle tissue of a host,” said Stratton. “It causes lethargy, and ultimately death. When they're infected, especially heavily infected, they're not out seeking food.”
Stratton is trying to find 240 infected ones for this portion of her research that’s is looking into how the disease is transmitted.
“We're conducting a laboratory experiment back in Florida this fall to try to actually transmit the disease to healthy crayfish. We believe that it's transmitted through cannibalism because crayfish are actually highly cannibalistic, and there's evidence of closely related microsporidian being transmitted that way,” said Stratton.
Even with the piece of information still missing, Stratton says the implications of it give hope that ecosystems can recover from invasive species.
It’s something she’s seen firsthand in Trout Lake.
“Especially at the sites where we're hardly finding any Rusty Crayfish, we're starting to see a lot higher species diversity of the aquatic plants. A lot of aquatic plants we haven't seen in Trout Lake in decades, we're starting to see reappear. We're also seeing a higher species diversity of fish at those sites as well,” said Stratton.
It’s a sign that nature may be correcting course on its own.
“We expect to definitely see a very drastic shift in the aquatic plant biomass and species richness in the lake following this decline,” said Stratton.