U.S. Water News Online
BLIND RIVER, Ontario -- The volume of prey fish in Lake
Michigan has dropped by half in the last year, according to an annual
survey done by the federal government.
And some scientists say a possible reason is the ballooning
population of mussels that eat plankton upon which the lake's fish
species directly or indirectly depend.
"The lake just can't support the mass of fish that it used to,"
said Tom Nalepa, a biologist with the National Oceanic and
Atmospheric Administration's Great Lakes Environmental Research
Laboratory. "The energy that used to go into the lake is now going
into mussel populations rather than other biological components."
The estimated biomass of prey fish in 1989 was about 450,000 tons,
compared to 30,000 tons today. That's down from about 60,000 tons the
year before, which was a record low since the annual surveys began in
The lake's overall invasive mussel numbers have increased 16-fold
in the past five years. The old culprit, zebra mussels, discovered in
the 1980s, typically dwelled in water 75 feet deep or less. But those
are being replaced by the quagga mussel, a slightly larger but much
hardier invasive mussel that isn't just clinging to the edges of the
Quagga mussels have been found at depths exceeding 500 feet and
can thrive on both rocky and sandy bottoms. Zebras mussels do most of
their filter feeding in the warm summer months; quaggas strip
plankton from the water year-round.
Russell Cuhel, a senior scientist at the University of
Wisconsin-Milwaukee's Great Lakes WATER Institute, estimated that the
quagga range on the bottom of the lake is 50 times greater than that
of the zebra mussel, and unlike the zebras, they are doing year-round
damage to the lake's plankton community.
Cuhel said the result is that the plankton or energy that would
normally be suspended in the water column is being sucked straight to
the lake bottom, and a corresponding decline in fish that depend on
that suspended plankton is what would be expected.
But U.S. Geological Survey biologist Chuck Madenjian, who is in
charge of the annual prey fish surveys, acknowledges an "explosion"
in quagga mussels.
"But I'm not ready to put the blame on mussels" for the plummet in
the prey fish, he said.
Madenjian said other factors that could be at play include
increased pressure from salmon and lake trout, and natural population
fluctuations of the prey species, which include alewives, chubs and
He also noted that prey fish species have historically fluctuated;
fish biomass tripled from 1973 to 1989, though they have been on an
uneven decline since.
It is quite likely that the quagga mussel explosion will be
followed by a collapse, a classic phenomenon in invasion biology, he
"I'm not ready to say the (mussels) are going to eat up the whole
lake," Madenjian said.
Madenjian expects a rebound of some of the prey fish populations,
particularly the chubs, which commercial fishermen harvest and sell
to the public as smoked fish. But he is making no predictions.
"We'll see," he said. "It's interesting times."
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