20 years after discovery, spiny water fleas thrive in Island LakeTwo 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.
By: John Myers, Northland Outdoors
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 immigrants 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 — up to 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 midsized walleyes and perch may be growing faster thanks to munching on spiny water fleas.
1,400 fleas found inside a single perch
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.
Spread is speeding
Branstrator has spent the last 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.
Island Lake, which is fairly deep yet fairly warm, was their first target, probably because of the amount of recreational boaters and anglers who also boat on Superior, inadvertently moving the tiny critters or their eggs.
The invaders quickly found Island Lake to their liking, said Branstrator, who works with fellow UMD researcher Lyle Shannon and several graduate students. Core samples of sediment in the lake, which the researchers now are studying, seem to show a rapid spiny water flea population increase in the 1980s with numbers stable in recent years. They have eliminated a similar, native species called Leptadora.
At night, when spiny water fleas are more likely to rise closer to the surface, researchers dip nets into Island Lake to check on the invader. In most lakes where they are found, spiny water fleas usually are found at levels of 10-20 animals per cubic meter of water. In Island Lake, they number more than 100 per cubic meter at peak times of year, usually late summer.
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. The darker, bog stained water also may make it harder for some fish to find deeper spiny water fleas. And Island lake is “top heavy” of sorts with bigger fish that eat fish and don’t bother with little crustaceans like spiny water fleas.
Spiny water fleas have also caused a dramatic reduction in the amount of biomass in the lake, the small “planktonic” critters that make up the base of the food chain. It’s still unclear, Branstrator said, what that means for the ecosystem in the long haul.
Lindgren said that more fertile lakes with high populations of pan fish haven’t had spiny water flea infestations.
“They were documented at Fish Lake at one point in the 1980s, but they are gone now. And they still aren’t in Boulder Lake” just upstream of Island Lake, Lindgren said. “I think the amount of pan fish in those warmer, more fertile lakes just gobbles them (spiny water fleas) up so they can’t take hold.”
Branstrator said it’s not likely spiny water fleas will suddenly become a problem in the future.
“I don’t think we’ll see any big changes 20 years from now because of” spiny water fleas, he said. “Every fish in Island Lake now has lived its entire life with Bythotrephes and they have done just fine.”
But 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.