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Spiny Waterfleas Found in Rainy Lake

Matt Gunderson holding a walleye caught on Rainy Lake.

Matt Gunderson found more than walleyes in Rainy Lake this summer.

A Duluth teenager vacationing on Rainy Lake in mid-July found more than fish on his line. Matt Gunderson (18) caught an invasive species known as the spiny waterflea (Bythotrephes longimanus). More than a dozen collected on his fishing line while he was trolling for walleyes off Blueberry Island in Voyageurs National Park.

“I knew what they were as soon as the line came out of the water because I’d seen spiny waterfleas before on Lake Superior,” said Gunderson, who is currently a freshman at the University of Minnesota Duluth. He was taught from a young age to watch for invasive species by his father Jeff Gunderson, who works for Minnesota Sea Grant.

According to Steve Windels, terrestrial ecologist for Voyageurs National Park, this is likely a recent infestation because the Park Service periodically sampled Rainy Lake and other lakes in the park in cooperation with the U.S. Geological Survey through 2005 and did not find any spiny waterfleas. The agencies cooperated again to respond quickly to the sighting.

“Since we received the initial report, we’ve confirmed the presence of spiny waterfleas at seven different sites throughout the U.S. side of Rainy Lake,” said Windels. The finding has implications for how the National Park Service conducts operations in the park, specifically, taking extra precautions to ensure that staff and visitors do not spread the species to uninfested waters.

The discovery also has international implications as more than 75 percent of Rainy Lake is in Ontario, Canada, and will impact how the Ontario Ministry of Natural Resources operates in that area. The waterfleas now have the potential to spread downstream to Lake of the Woods (Minnesota’s largest lake) or upstream to Kabetogama, Namakan, Sand Point, and Crane lakes. The finding brings the number of infested lakes in Minnesota to ten (excluding Lake Superior).

Experts believe spiny waterfleas originally arrived in the U.S. from Eurasia in the ballast water of cargo ships when they were found in Lake Ontario in 1982 and Lake Superior in 1987. Boaters and anglers have most likely spread them to inland waters since then. Waterfleas collect in masses on fishing lines and downrigger cables. The masses can resemble gelatin or cotton batting with tiny black spots, which are the creatures’ eyes. Individual animals are difficult to distinguish without magnification because they are only 1/4 to 5/8 inch long.

Spiny waterfleas on a fishing line.

Spiny waterfleas can often collect in masses on fishing lines, allowing unaware anglers to spread them to other lakes.

“Spiny waterfleas can spread when boats and fishing gear become contaminated with egg-laden females,” said Doug Jensen, coordinator of the Minnesota Sea Grant’s aquatic invasive species program. Although the females might die between fishing trips, they could carry resting eggs that can begin a new infestation.

“It’s like dry soup mix,” said Jensen. “Just add water and you’ve got more spiny waterfleas. That’s why boaters and anglers need to be especially careful about draining water and cleaning their equipment before going from one lake to another.”

The Minnesota DNR is working to designate Rainy Lake as infested. The designation means it will be illegal to transport water or harvest bait from the lake, similar to zebra mussel-infested waters.


By Marie Zhuikov
October 2006

Return to October 2006 Seiche



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