Great Lakes Fish Food Disappearing

C H I C A G O, Feb. 5, 2001 -- A primary source of food for young fish isquickly disappearing from the Great Lakes, according to scientistswho fear it could jeopardize decades of progress in restoring fishpopulations.

Diporeia, half-inch-long shrimp-like crustaceans, already areextinct in Lake Erie and declining at alarming rates from lakesMichigan, Ontario and Huron — a phenomenon scientists suspect islinked to zebra mussels, a Black Sea native that arrived in thiscountry in the late 1980s.

"It's one of those issues that is just so scary because … wehave seen such recovery [of fish species] in the 30 years since theClean Water Act was passed," said John Gannon, science coordinatorat the U.S. Geological Survey's Great Lakes Science Center in AnnArbor, Mich. "We had this wonderful success story running and thenone of the main food sources starts to disappear."

Base of the Food Chain

The demise of diporeia could have dire consequences for manytypes of fish, scientists said. Many fish that eat diporeia in turnare eaten by larger fish such as salmon and lake trout. The problemhas not affected such sport fish, but whitefish, which arecommercially harvested, have suffered.

Exactly what is causing diporeia, which live on lake bottoms, todisappear remains a mystery.

One theory is that zebra mussels, thumbnail-sized mollusks thatarrived in the ballast water of oceangoing ships, are competingwith diporeia for the same food — and winning, said Marc Tuchman,an environmental scientist in the EPA's Great Lakes NationalProgram Office in Chicago. Both dine on bacteria and algae, but themussels multiply rapidly and can filter vast amounts of water.

There also is speculation that there is enough food but musselsare extracting from it a nutrient essential to diporeia, thatmussels introduced a pathogen lethal to diporeia, or that amucous-like substance excreted by mussels is killing diporeia, saidThomas Nalepa, a research biologist at the Great LakesEnvironmental Research Lab in Ann Arbor. The lab is part of theNational Oceanographic and Atmospheric Administration.

Researchers have found no diporeia in Lake Erie since the early1990s — Tuchman said fisheries experts have reported a dramaticdecline in rainbow smelt and young whitefish there — and diporeialevels have collapsed in Lake Ontario and are disappearing fromshallow waters of southern Lake Michigan.

Their numbers have declined in Lake Huron, but have notdisappeared in any locations there yet, Tuchman said, and in colderLake Superior, where there are few zebra mussels, diporeia continueto thrive.

In the last few years, diporeia also have begun disappearingfrom deeper waters of northern Lake Michigan. In waters offManistique, in Michigan's Upper Peninsula, for example, researchersfound 10,000 diporeia per square meter in 1997, but none in 2000.

Gannon, with the Great Lakes Science Center, said that isparticularly alarming because whitefish, which feed on diporeia andare important commercially, have made a phenomenal recovery in theupper Great Lakes in the last 15 years.

Skinny Fish

Fish may develop a taste for other food, such as mysis, ashrimplike organism similar to diporeia. But that could put astrain on mysis populations, researchers said. The EPA plans tomonitor mysis and other fish food.

In areas of Lake Michigan where diporeia disappeared, whitefisheven began eating zebra mussels, which weren't very healthy forthem, Nalepa said. He said the fish, whose guts became packed withshell material, were so skinny that fishermen could no longer get afillet off them.

The diporeia decline has not yet affected sport fish such astrout and salmon because their food — including alewives and smelt&0151; still are finding alternatives to diporeia, researchers said.

But the lack of diversity — especially if bottom-feeding fishbegin to decline — eventually could affect popular species, theysaid.

"The fish that are affected now are not glamorous fish liketrout or salmon," said Randy Owens, a fishery biologist studyingLake Ontario for the U.S. Geological Survey in Oswego, N.Y. "It'sjust that, from a scientific standpoint, we will have big void inwhole food web out here.

"We would be getting down to a system where we're justdepending on a few fish to keep things going, instead of a widevariety of fish," he said. "If something happens to the alewivesand smelt, then it's crunch time on trout and salmon."

Scientists say they fear the diporeia disappearance is justbeginning, and say the area in which they are declining expandsevery year.

"It may be slow or it may be fast, but a decline is occurring,and we will see more and more impacts on fish," Nalepa said.

He said he believes the decline of diporeia is related to zebramussels, although he's not sure how and, in some cases, there is noobvious connection. In St. Joseph, Mich., where diporeiadisappeared within six months in 1992, researchers had never founda zebra mussel, he said.

But most disturbing is that there doesn't appear to be any wayto stop the disappearance, Tuchman said.

"Unfortunately, all we can do is keep eye on it, monitor it,note the decline and hope it reverses," he said. "There isnothing we can do."