For the second year in a row, groups in Michigan are laying down barriers and traps to try and keep invasive rusty crayfish away from vulnerable lake trout and whitefish eggs.
The experiment is focused on keeping rusty crayfish from eating eggs along rocky reefs in Little Traverse Bay and Elk Rapids during the October-November spawning season, according to David Clapp, station manager. Research suggests egg predators are a major factor in the struggles both keystone species have faced in northern Lake Michigan, particularly from invasive species like the rusty crayfish and round goby.
Rusty crayfish were introduced into Lake Michigan from their native Ohio River habitat in the 1960s or early 1970s, likely as escaped bait or from school biology projects. Rusty crayfish, identifiable by their rusty coloration and large, smooth claws, are more aggressive than native species, outperforming and largely displacing them in the Traverse Bay area. The animals are considered invasive because they move onto the reefs each fall to feast on fish eggs during spawning seasons. Project researchers and collaborators (including the Little Traverse Bay Band of Odawa Indians and the Grand Traverse Bay Band of Ottawa Indians) were interested in seeing if they could give those eggs a better chance of survival. Thanks to a $550,000 grant from the US Environmental Protection Agency administered through The Nature Conservancy (and in-kind support from all project partners), the Michigan Department of Natural Resources (DNR) and its partners were able to begin the three-year experimental project in the fall of 2018 to keep the crayfish out.
Clapp said lake whitefish, beyond being important ecologically, also are an important commercial fish that are struggling with low populations due to environmental changes. Lake trout are an important native predator and source of recreational fishing. Historically, cisco also have reproduced on these reefs, Clapp said, with an small but expanding population that also could benefit from the project.
The strategy features two components. First, project members installed barriers up to and around the reefs made of vinyl and held in place with attached floats and chains. Crayfish can try and swim over these barriers, but they’ll risk being eaten by smallmouth bass. The DNR staff also perform “intensive trapping” both inside and outside of the barrier for rusty crayfish, Clapp said, actively working to remove crayfish from the area with lines of seven to 11 traps each. In 2018, this work was only done in Little Traverse Bay.
“We saw some effects immediately,” Clapp said. “The only thing we can measure is whether we see eggs disappearing … but we did see pretty good movement towards control of those crayfish.”
Compared to a control section of the reef, in 2018 researchers found fewer crayfish in the experimental section, Clapp said. Eggs were still eaten by gobies and to a lesser degree native species such as sculpin.
The 2019 effort, which expanded to include the reef in Elk Rapids, is still ongoing and results were not yet available as of this writing.
Clapp said the Elk Rapids location is much more popular among lake whitefish, most likely due to a slightly different habitat in terms of rock size and the space between them. This year, the DNR also has tagged some of the invasive crayfish to get a better idea of their population size and movements. This should help with any necessary tweaks to make the trapping strategy more effective.
If these efforts prove to have a positive effect on trout and whitefish populations by the end of the three-year study in 2020, Clapp said the next step would be to test it on a larger scale and see if it’s possible to get these same results without emptying the traps as often. And, he added, the partnership is also considering ways they might control round gobies at the reefs.
“There are lots of them, and they’re in habitat that’s not conducive to making it easy to control them, but we’re thinking of ways we might try to do that as well,” he said.