Winter 2008

Stephanie Schmidt digs through a vat of preserved fish searching for a particular specimen. Schmidt, a UW–Madison graduate student, worked with food web ­ecologist Jake Vander Zanden on a Sea Grant-funded project to piece together the historical Great Lakes food web.
UW Sea Grant Research

Looking Back to Look Ahead

By Elizabeth Katt-Reinders

Hidden away in back rooms of museums around the country sit thousands and thousands of jars of pickled fish. Some of them have been there for over 100 years, locking up the stories of historical aquatic life. Those stories are preserved in the tissues of the fish, and Jake Vander Zanden is discovering what they have to say.

Vander Zanden, a food web ecologist at the University of Wisconsin-Madison, and graduate student Stephanie Schmidt have been analyzing these preserved fish in a research project that is the first of its kind. The two scientists are using the chemistry of fish tissues to piece together the historical Great Lakes food web to which the fish belonged.

“We were the first people to take this approach of going to museums, taking out specimens, and using the chemistry of the tissue to try to infer what the food web was like in historical times,” said Vander Zanden.

By using a technique called stable isotope analysis, Vander Zanden and Schmidt separated out the carbon and nitrogen levels from tiny slivers of fish tissue that had been preserved for decades. These values compose a chemical signature for the fish and provide information that allows the scientists to discover the ecological roles that these fish historically filled.

“Isotopes can tell us where in the lake you’re feeding and how high up in the food chain you are,” said Schmidt. “You are what you eat.” The researchers have found that a fish’s carbon value indicates the depth in the water column at which the fish lived and ate. The nitrogen shows how many steps there were between the base of the food chain and the fish in question.

“Each provides different types of information,” said Vander Zanden. “The two together can help you really describe a food web.”

This information is important for scientists and fisheries managers because many fish that once thrived in the Great Lakes have disappeared. Knowing what roles various fish played in the past may help guide fisheries managers as they consider reintroducing native fish whose populations have been eliminated.

In their study, Schmidt and Vander Zanden focused on several species of fish known as deepwater ciscoes. These fish, which ranged in length from six to 16 inches, once were the main source of food for lake trout. But over the course of the 20th century, populations of ciscoes collapsed.

“Exotic species introductions totally rewired the food web,” said Vander Zanden. He also implicates overfishing, habitat destruction, eutrophication, and toxic pollution discharges in the decline of the ciscoes.

“It was just a crazy time for the Great Lakes,” he said in reference to the 1920’s through the sixties. “Basically what happened is the food web collapsed, and the deep water ciscoes were wiped out.”

If Vander Zanden and Schmidt can identify the ciscoes’ historical roles and functions, and whether those roles differed from species to species, they may be able to increase the likelihood of successful reintroduction.

“We’re trying to see if these fish were the same throughout all of these lakes such that if we reintroduce one species from Lake Superior into Lake Michigan, would it be successful,” said Schmidt, “would it serve the same ecological function as it did historically in Lake Michigan?”

Although their research is in the early stages, Great Lakes Fishery Commission Science Advisor Randy Eshenroder said it “will be of great interest to the management community.”

Eshenroder said the interest among managers is in diversifying the food web, and that ciscoes would provide a “more diversified prey community” for fish like lake trout and salmon. And in order to stack the odds for successful ­reintroduction, it’s important to know what ecological roles the fish played historically.

Vander Zanden agreed: “It can help you see where you’ve been in the system, and that’s pretty critical when trying to decide where you’re going.”

The Aquatic Sciences Center is the administrative home of the
University of Wisconsin Sea Grant Institute & University of Wisconsin Water Resources Institute.

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