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Great Lakes Article:

Disappearing Act
By J. Michael Kelly
The Post-Standard
11/22/2002

Diporeia, shrimp-like creatures that fattened the bellies of Great Lakes forage fish for thousands of years, virtually have disappeared from Lake Ontario during the last decade.

The quick fade-out has ominous implications for anglers because the species that depend on diporeia for calories are in turn eaten by Ontario's tackle-smashing trout and salmon.

Scientists aren't sure if other organisms can fill diporeia's niche in the food chain.

"There used to be between 5,000 and 7,000 (diporeia) per square meter of lake bottom," said Bob O'Gorman, a researcher with the U.S. Geological Survey's Lake Ontario Biological Station in Oswego. "Now we can't find any of them in most of our samples."

Scientists aren't sure why diporeia have vanished, but many believe the phenomenon is related to the 1990s invasion of the Great Lakes by two species of mollusks native to Europe, the zebra mussel and the quagga mussel. Transported to North America in the ballast tanks of cargo ships, mussels now cover the bottom of Lake Ontario's shallows and mid-depths.

"Why diporeia are suddenly gone is the $64 million question, and a lot of people are spending a lot of money to figure it out," O'Gorman said.

One possibility is that newly arrived mussels infected diporeia with some as-of-yet unidentified disease. Other scientists suspect mussels have crowded diporeia out of preferred habitats or eaten their share of the food supply. Diporeia dine mainly on microscopic plants called diatoms. Mussels ingest diatoms along with a variety of plankton.

O'Gorman, who monitors Lake Ontario's aquatic community by hauling samples aboard the trawling vessel Kaho, said the lake has become home to "incredible numbers" of mussels.

In October 2000, the Kaho found an average of 19,000 mussels per square meter at a depth of 180 feet off the lake's south shore, between Olcott and Oswego. Ninety-five percent of those mussels were the thumbnail-size quaggas.

Diporeia are only about one-third of an inch long, but their loss could have a huge impact on Lake Ontario's ecology. Historically, the burrowing amphipods were a dietary staple for rainbow smelt. Alewives also gorged on them at certain times of the year.

"Diporeia are the double bacon cheeseburgers of the fish-food world," O'Gorman said. "They're very high in lipids."

Like diporeia, Lake Ontario's smelt have gone into a steep decline in recent years. The slender, tasty bait fish used to be so abundant that sportsmen were able to scoop buckets of them from lake tributaries each spring. Now Ontario smelt are at their lowest ebb in at least 25 years, according to O'Gorman's records.

Alewives still number in the billions in the lake, but those sardine-size fish also are showing signs of stress.

Every year, the Kaho's crew nets alewives at various points in the lake to determine their relative abundance. The fish are weighed and measured and their stomach contents are tallied.

This year's surveys revealed that the Ontario alewife population is trending downward, after a late 1990s resurgence. Bumper crops of alewives hatched in 1998 and 1999 have been thinned out by salmon and other aquatic predators, and the 2000 spawning run "was a total bust," O'Gorman said.

However, the 2001 hatch appears to have been a strong one.

"We'll know better about that next year, when those fish reach adulthood," said O'Gorman. "The survival of alewives from age 1 to age 2 is quite variable, depending on the number of larger adults competing for the food supply, the overall productivity of the lake and the impact of predation."

At this point, O'Gorman is less concerned about the total number of alewives in the lake than he is about the slender shape of individual specimens.

"The condition of adult alewives this fall was the second worst we've ever seen, and we have records going back to 1976," he said. "You would think, with the number of alewives being low, that individual fish would be fat, but instead they're skinny."

That's most likely because of a diminished food supply, he said.

O'Gorman frets that the loss of diporeia will put intolerable pressure on other components of Lake Ontario's food chain. He's especially worried about the mysis shrimp - sometimes called possum shrimp - that are relished by young lake trout, as well as smelt and alewives.

Canadian researchers recently began a new study of the lake's mysis population. Relatively little is known about mysis because they spend daylight hours in deep water and migrate toward the surface after dark.

"I don't know what we'd do if the mysis disappeared, too," O'Gorman said. "God help us if that happens."

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