Exotic water flea hops to inland
waters, threatening fish
By Susanne Quick
Milwaukee Journal Sentinel
Wisconsin's inland lakes are on the verge of being infested
by fleas. Not the kind that you find on dogs and cats,
but a European crustacean called the spiny water flea.
A University of Wisconsin-Madison graduate student found
the barbed bandits in Gile Flowage, a large lake in Iron
County near Lake Superior.
"We had a hunch they'd be there," said Pieter
Johnson, the UW graduate student in limnology, referring
to the presence of fleas on inland lakes in Wisconsin.
Although the exotic water fleas have been living in the
Great Lakes for more than 15 years, this marks their first
appearance in the state's inland waters.
The fleas are a concern because they compete with and
eat native zooplankton - the food of young sport fish,
like walleye and perch, and small foraging fish, like
"The water fleas decrease the diversity and abundance
of zooplankton available to fish," Johnson said.
How this might affect the food chain remains speculative,
he said. But "it has the potential to hurt fisheries
and reduce the number of game fish."
The spiny water flea looks something like an alien/helicopter
hybrid. With big black eyes, a bulbous sac sitting above
its head, and a long, barbed, whip-like tail, the flea
swims around attacking zooplankton and other aquatic delicacies.
According to Philip Moy, a non-indigenous fisheries specialist
with UW-Manitowoc and the Wisconsin Sea Grant Institute,
the spiny water fleas have been known to eat very young
and tiny fish, too.
The flea, which measures about a half-inch long, has
four sets of legs. The first two legs are extra-long and
enable the flea to catch its prey. The other three sets
are used to restrain the victim while it is consumed.
But it isn't their killing style that has resource managers
concerned; it's their appetite for daphnia - a type of
zooplankton - and their superior sexual skills. During
the summer, a water flea can produce as many as 10 offspring
And according to Moy, the combination of prolific reproduction,
a hearty appetite and a tail like a medieval mace makes
this tiny crustacean a formidable foe.
Not only are they voracious consumers of large species
of daphnia, but they also are inedible to fish that would
normally eat the native zooplankton.
"Even if a little fish could swallow them,"
Moy said, they'd end up with a belly full of tough, sharp
spines - a sure way to puncture and tear the intestinal
tract of a small fish.
The Minnesota Department of Natural Resources reported
its first inland flea intruders last week. Michigan and
Ontario have had them for a couple of years.
The scary thing is that once they are found it doesn't
take long for a whole region to be infested, said Jake
Vander Zanden, a UW-Madison professor of limnology, who
worked with Johnson on the water flea project.
The water fleas are adept at hitchhiking from lake to
lake in bait buckets, boats' live wells and bilges.
And although the fleas don't last long out of the water,
the proximity of Gile Flowage to the lakes of Wisconsin's
northern highlands has Vander Zanden concerned.
"It's in striking distance" of the lakes to
its south, said Vander Zanden, who believes it's just
a matter of time before the fleas settle into Wisconsin's
The other concern is that although individual fleas don't
fare well out of the water, they are capable of producing
something called a "resting egg" - a resilient
egg sac that can encase and keep alive a few young fleas
for years on end.
This could mean that even if someone kept a boat out
of the water for a couple of days - drying out and killing
any vagabonds - the resting eggs could survive. And it'd
just take re-immersion to wake up these dormant fleas.
In any case, said Vander Zanden, "once they've been
established in a lake," the fleas are pretty much
there to stay.
Indeed, they are already considered permanent members
of the Great Lakes, said Moy, the UW-Manitowoc fisheries
expert. But figuring out what kind of damage they have
caused on that ecosystem is hard to determine "what
with zebra mussels and everything else" already wreaking
Prevention is the key to halting the threat of spiny
water fleas, said Steve AveLallemant, Wisconsin DNR water
resources management specialist.
So signs and a determined effort to inform boaters and
anglers about the water fleas is already under way, AveLallemant
And Johnson is hopeful that with a determined communal
effort, the fleas can be contained.
"We're lucky," he said, "we caught them
early on in the invasion - they are in just one (inland)
AveLallemant agreed that introduction of the fleas from
the Great Lakes is not likely. But it's the fact that
they are inland - with people using the same boats to
move between inland lakes - that has him worried.
"There is increased potential for expansion,"
he said. "I guess we're at a kind of wait and see."