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 1973.
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 lake.
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 smelt.
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 said.
"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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