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Spiny Waterfleas Found in Rainy Lake

July 28, 2006

A Duluth teenager vacationing on Rainy Lake last week 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 on July 18 while he was trolling for walleyes off Blueberry Island in Voyageurs National Park.

Gunderson immediately recognized the mass of tiny animals because he’s been educated to watch for invasive species most of his life. His father works for the University of Minnesota Sea Grant Program, which funds research and conducts public education programs about aquatic invasive species.

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 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 might be carrying 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."

Spiny waterfleas can foul angling gear, causing anglers to lose hooked fish. Several other traits make this species of zooplankton particularly vexing:

  • They compete with small fish for the same zooplankton.
  • Small fish generally do not eat them due to their long spiny tails.
  • They can shift the natural balance of a lake's plankton community, disrupting food web dynamics.

To combat the spread of harmful aquatic invasive species, Voyageurs National Park will step up efforts to promote a campaign called Stop Aquatic Hitchhikers! (www.protectyourwaters.net), which features a logo and prevention messages targeting boaters and anglers. Staff will post water accesses and distribute brochures and identification cards to boaters and anglers to inform them how to prevent the spread.

Before leaving the water access, boaters and anglers should:

  • Learn to recognize spiny waterfleas (call Sea Grant or the DNR for identification cards? see phone numbers below)
  • Inspect and remove aquatic plants and animals, including gelatinous or cotton-batting-like material from fishing lines, downrigger cables, or anchor ropes
  • Drain water from livewells, bait containers, and bilge
  • Dispose of unwanted live bait in the trash
  • Spray with high pressure or hot tap water (above 104 degrees F or 40 degrees C)
  • Report new sightings (note location and put specimen in a sealed container with rubbing alcohol; call either Minnesota Sea Grant, (218) 726-8712 or the Minnesota Department of Natural Resources (DNR) Invasive Species Program in St. Paul, 1-888-MINNDNR or (651) 259-5100.

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


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