Too much mussel: Mollusk explosion could do great harm to Lake Michigan sport-fishing industry
Written by Muskegon Chronicle   
Tuesday, 03 February 2009 21:14

Below the steel blue waters of Lake Michigan, a giant sucking sound is transforming the world's sixth largest lake in ways that scientists never thought possible.

An estimated 330 trillion quagga mussels carpet vast areas of Lake Michigan's underbelly. The foreign mollusks literally are sucking the aquatic life out of the water and depositing it on the lake bottom, according to new scientific data.

"It's unbelievable, the changes that are happening," said Gary Fahnenstiel, a senior ecologist at the National Oceanic and Atmospheric Administration's Lake Michigan Field State in Muskegon. "The quagga mussel population has exploded and is taking over the lake bottom."

Quagga mussels are the slightly larger and far more disruptive cousins of zebra mussels. The filter-feeders have been linked to increased algae blooms that have fouled Great Lakes beaches and botulism outbreaks that have claimed more than 70,000 fish-eating birds and countless fish.

The dime-sized mollusks that ocean freighters unintentionally imported to North America in the late 1980s are turning Lake Michigan and its twin -- Lake Huron -- into biological mirror images of Lake Superior.

That's good news for people who enjoy clear water. But it is potentially disastrous for the Lake Michigan's billion-dollar sport fishery. The reason: Quagga mussels hog vast quantities of tiny plankton and other organisms that are the foundation of the lake's fish food chain.

Some fish species have resorted to eating quagga mussels just to survive, a situation analogous to humans dining on pretzels instead of steak or fish.

Each quagga mussel can filter up to a liter of water through its tiny body every day. The hundreds of trillions of quaggas in Lake Michigan have derailed the natural distribution of carbon and plankton -- the fuel that powers the lake's entire ecosystem.

Instead of carbon and other food energy cycling through the food chain -- from tiny bottom-dwelling organisms up to sport fish -- quagga mussels have turned the lake bottom into a sink for critical nutrients. Much of the energy that normally flows through the food chain is being tied up in quagga mussel shells, according to new research data.

Signs of a looming biological crisis are mounting, according to government data.

Nalepa authored a soon-to-be-released study that concluded quagga mussels have seized control of the Lake Michigan ecosystem. He said the mussels have displaced native amphipods that fish eat, trapped vast quantities of critical nutrients on the lake bottom, and displaced nearly all zebra mussels in the lake.

"We project that previously observed impacts on fish populations will continue and become more pronounced as the (quagga mussel) population continues to expand in deeper waters," Nalepa said in a study to be published in an upcoming edition of the scientific journal Freshwater Biology.

Lake Michigan charter boat captains have reported near-record salmon catches the past three years on Michigan's side of the lake. But the fish are smaller than in the past.

• The abundance of lake diporeia, the shrimp-like organism that once was the main source of fish food, has dropped 96 percent since 1995.

• The abundance of mysis, another important source of fish food, is declining.

• The volume of prey fish in the lake -- the small fish that whitefish, salmon and lake trout eat -- has dropped 94 percent since 1989 and hit an all-time low for the third straight year in 2008.

• Salmon and whitefish are shrinking.

• The mass of quagga mussels in the lake is nearly four times the volume, by weight, of all prey fish species, according to a study set for release in March.

"It's astounding to me that there is almost four times more dreissena mussels (by weight) in the lake than prey fish," said Tom Nalepa, a NOAA research biologist.

State data shows the average size of a chinook caught in the lake has decreased from 16 to 12 pounds over the past decade. Similar trends have been recorded in lakes Huron and Ontario, where quaggas also have multiplied like cockroaches.

The U.S. Geological Survey, which tracks prey fish abundance and the volume of zebra and quagga mussels in the Great Lakes, estimated there were 500 million pounds of the foreign mollusks in Lake Michigan in 2007.

The annual USGS bottom trawl survey recorded a 95 percent drop in the volume of quagga mussels in the lake in 2008. But that data was at odds with more precise sampling conducted by scientists from NOAA's Great Lakes Environmental Research Laboratory.

Nalepa said NOAA's tests showed the quagga mussel population has leveled off in waters less than 100 feet deep but continues to multiply in deeper waters. At a monitoring site offshore of Muskegon, in water 300-feet deep, quagga mussel densities reached 3,500 mollusks per square meter last year.

Though the evidence remains circumstantial, Nalepa and other scientists believe quagga mussels are to blame for the near disappearance of diporeia and the precipitous drop in prey fish abundance.

One government scientist believes other factors are at work.

Chuck Madenjian, a USGS research fishery biologist, said he believes an excess of predatory fish in the lake -- salmon, lake trout and whitefish -- are the reason prey fish populations are dwindling.

"I'm not sold on the idea that quagga mussels are devouring the food chain," Madenjian said. "It's too early to say the bottom has fallen out of the food chain."

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