Spiny water fleas thrive in lake near Duluth 20 years after discovery

DULUTH -- Two decades after first invading Island Lake Reservoir north of Duluth, spiny water fleas have muscled their way ahead of native species and signaled that they are here to stay.

DULUTH -- Two decades after first invading Island Lake Reservoir north of Duluth, spiny water fleas have muscled their way ahead of native species and signaled that they are here to stay.

The European emigrants have pushed their crustacean relatives, native water fleas, out of the lake; appear to have eradicated spot-tailed shiner minnows; and have become more numerous in Island Lake than any other lake in North America -- as much as 10 times the average infestation -- according to University of Minnesota Duluth researchers.

Happy 20th anniversary.

Yet, despite their intense colonization of Island Lake, spiny water fleas (formal name Bythotrephes) don't seem to have caused any major catastrophe with the overall ecosystem, and certainly not with fish or fishing.

In fact, a 12-year study by a Minnesota Department of Natural Resources fisheries biologist suggests mid-sized walleyes and perch may be growing faster thanks to munching on spiny water fleas.


In 1987, when the invaders first showed up in Lake Superior, scientists were afraid they would be accidentally carried to inland lakes, displace native species and that local fish wouldn't eat them because of the spines.

Indeed, fish smaller than 2 inches long can't seem to tolerate the spines and won't eat spiny water fleas. But mid-sized fish are eating them.

"We found faster growth rates for length in Island Lake than in (nearby) Whiteface Lake where Bythotrephes isn't present," said John Lindgren, fisheries specialist for the DNR. "We don't have growth rates for Island Lake before Bythotrephes came in, so we don't know if it's actually faster growth now because of them. But it does seem Bythotrephes aren't hurting anything and may be helping."

Donn Branstrator, UMD biology professor and research scientist, said more than 1,400 spiny water flea spines have been found inside a single perch.

"Clearly, once the fish get over about (2 inches) they are being targeted as a food source," Branstrator said.

Smaller, spot-tailed shiners, however, can't seem to digest those spines and, with no similar-sized native fleas to eat, have been eradicated from the lake, Lindgren said.

"We're not sure what the impact of that is," he said.

Branstrator has spent the past nine years studying spiny water fleas, especially in Island Lake but all over Minnesota, including Lake Superior where they first were found in the region in 1987. Scientists knew they would spread into inland lakes but weren't sure which ones or how fast.


Spiny water fleas now have infested not just Lake Superior and Island Lake, but in the last few years have been found in Lake Mille Lacs to the south and more than a dozen lakes along the Minnesota-Ontario border -- from the Gunflint Trail on the east through Crane and Rainy Lakes to Lake of the Woods. The DNR lists 38 lakes and streams in Minnesota as infested with spiny water fleas.

Researchers think Island Lake's generally infertile ecosystem (few weeds) and low population of smaller pan fish such as crappies and bluegills is allowing spiny water fleas to survive.

While they haven't caused any major problems on Island Lake, that doesn't mean spiny water fleas couldn't wreak havoc in other lakes. Their impact on more sterile, northern lakes with fewer overall species could be harsher, Lindgren noted. And the introduction of other invasive species, development, polluted runoff and warming water because of a warmer climate already are affecting lakes here.

Some lakes may be able to handle the extra load. Others may not. That's why researchers are joining resource agencies and Minnesota Sea Grant and DNR in urging people not to spread invasive species like spiny water fleas.

Adding spiny water fleas to Minnesota lakes "is another incremental change, like temperature and other issues we layer on," Branstrator said. "We don't know which one might turn out to be that straw" that broke the camel's back.

The Duluth News Tribune and the Herald are Forum Communications Co. newspapers.

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