A primary source of food for young fish is quickly disappearing from the Great Lakes, according to scientists who fear it could jeopardize decades of progress in restoring fish populations.
Diporeia, half-inch-long shrimp-like crustaceans, already are extinct in Lake Erie and declining at alarming rates from lakes Michigan, Ontario and Huron — a phenomenon scientists suspect is linked to zebra mussels, a Black Sea native that arrived in this country in the late 1980s.
"It's one of those issues that is just so scary because … we have seen such recovery [of fish species] in the 30 years since the Clean Water Act was passed," said John Gannon, science coordinator at the U.S. Geological Survey's Great Lakes Science Center in Ann Arbor, Mich. "We had this wonderful success story running and then one of the main food sources starts to disappear."
Base of the Food Chain
The demise of diporeia could have dire consequences for many types of fish, scientists said. Many fish that eat diporeia in turn are eaten by larger fish such as salmon and lake trout. The problem has not affected such sport fish, but whitefish, which are commercially harvested, have suffered.
Exactly what is causing diporeia, which live on lake bottoms, to disappear remains a mystery.
One theory is that zebra mussels, thumbnail-sized mollusks that arrived in the ballast water of oceangoing ships, are competing with diporeia for the same food — and winning, said Marc Tuchman, an environmental scientist in the EPA's Great Lakes National Program Office in Chicago. Both dine on bacteria and algae, but the mussels multiply rapidly and can filter vast amounts of water.
There also is speculation that there is enough food but mussels are extracting from it a nutrient essential to diporeia, that mussels introduced a pathogen lethal to diporeia, or that a mucous-like substance excreted by mussels is killing diporeia, said Thomas Nalepa, a research biologist at the Great Lakes Environmental Research Lab in Ann Arbor. The lab is part of the National Oceanographic and Atmospheric Administration.
Researchers have found no diporeia in Lake Erie since the early 1990s — Tuchman said fisheries experts have reported a dramatic decline in rainbow smelt and young whitefish there — and diporeia levels have collapsed in Lake Ontario and are disappearing from shallow waters of southern Lake Michigan.
Their numbers have declined in Lake Huron, but have not disappeared in any locations there yet, Tuchman said, and in colder Lake Superior, where there are few zebra mussels, diporeia continue to thrive.
In the last few years, diporeia also have begun disappearing from deeper waters of northern Lake Michigan. In waters off Manistique, in Michigan's Upper Peninsula, for example, researchers found 10,000 diporeia per square meter in 1997, but none in 2000.
Gannon, with the Great Lakes Science Center, said that is particularly alarming because whitefish, which feed on diporeia and are important commercially, have made a phenomenal recovery in the upper Great Lakes in the last 15 years.
Fish may develop a taste for other food, such as mysis, a shrimplike organism similar to diporeia. But that could put a strain on mysis populations, researchers said. The EPA plans to monitor mysis and other fish food.