Invasive mussels may halt Asian carp spread

If huge, hungry Asian carp reach Lake Michigan, their long-dreaded invasion may turn out to be less ferocious than once expected, because a tiny competitor is gobbling up their primary food source, some Great Lakes researchers say.

The quagga mussel, a thumbnail-sized foreign mollusk first spotted in the lakes two decades ago, has devoured so much plankton in southern Lake Michigan that the entire food web is being altered, federal and university scientists report in a series of newly published articles. 

Mussels have "beaten the Asian carp to the buffet table," Gary Fahnenstiel, senior ecologist with NOAA's Great Lakes Environmental Research Laboratory, said Tuesday. "While the public has been worried about Asian carp and the Chicago canal, another invader has fundamentally changed the lake and made it inhospitable to the Asian carp."

Some biologists and government officials say if the carp get a foothold in Lake Michigan, they could spread to most of the Great Lakes and vacuum up enough plankton to threaten collapse of the $7 billion fishery. But Fahnenstiel and other researchers said the quagga mussel is a greater danger.

'[Carp] can eat other things besides plankton. They are very flexible fish.' — Duane Chapman, fisheries biologist

Some types of microscopic plants have declined more than 80 per cent with the mussel's arrival, they said, which probably explains a similar drop-off of a freshwater shrimp species that is a dietary staple for small fish pursued by prized sport varieties such as salmon and trout.

Other scientists and policymakers insist the carp could survive and even thrive in a plankton-depleted environment.

"They can eat other things besides plankton," said Duane Chapman, a U.S. Geological Survey fisheries biologist. "They are very flexible fish."

Bighead and silver carp — Asian varieties threatening to enter Lake Michigan through Chicago-area rivers and canals — are filter feeders that consume up to 40 per cent of their body weight daily.

The biggest can grow to more than a metre in length and weigh 45 kilograms.

But Fahnenstiel said that if carp evade electronic barriers and reach the lake, they'll probably find so little nourishment they'll either go back or starve.

Carp taste tests

Chapman is based at the Columbia Environmental Research Center in Missouri, where researchers are measuring the Asian carp's appetite for substances that will remain abundant in the Great Lakes even where plankton runs short. One example: bits of food the mussels spit out rather than digest.

Another is cladophora, a green algae that annoys beachgoers by washing ashore in stinky, rotting clumps. The cause of its resurgence in recent years is unknown but some believe it's linked to the mussels, which improve the water's clarity as they filter it, allowing sunlight to penetrate deeper.

"Chances are pretty good that Asian carp would do just fine eating that stuff, but we're going to test it to make sure," Chapman said.

Quagga and zebra mussels, believed to have hitched a ride from Europe to the Great Lakes in ballast tanks of freighter ships in the 1980s, have wreaked ecological havoc and done hundreds of millions in damage to all the lakes except Superior, where only isolated colonies have been found.

Fahnenstiel and Michigan Tech University biologist Charles Kerfoot were among co-authors of a series in the Journal of Great Lakes Research that described the quagga mussel's takeover of southern Lake Michigan this decade.

Quaggas — which unlike zebra mussels thrive in cold, deep waters — are a likely culprit in the disappearance of phytoplankton blooms that feed opossum shrimp, the scientists said. 

Those tiny invertebrates, crucial food for prey fish, have plummeted by more than 70 per cent, said Steve Pothoven, a biologist with the National Oceanic and Atmospheric Administration field station in Muskegon.

Scientists say whitefish and salmon, two of Lake Michigan's most popular species, have gotten smaller in recent years, a probable sign of malnutrition from a deteriorating food web.

"We are really getting a genuine collapse in the third-largest freshwater lake in the world," Kerfoot said.

The quagga population should outgrow its food supply and level off sometime. How soon that happens will determine how severely fish populations suffer, said Tom Nalepa, another NOAA researcher.