By Leslie Mertz
Ecologists agree that Great Lakes coastal wetlands are vital to a healthy fishery, but they haven’t been able to describe precisely how critical they are, according to Donald G. Uzarski, director of Central Michigan University’s Institute for Great Lakes Research.
Uzarski’s research group is reading information stored in the ear bones, or otoliths, of fish. They use it to track the fish’s movements and determine how many times and when it visited particular coastal wetlands.
“We’re basically trying to find out exactly how important these coastal wetlands are to the overall energy base or food web of the entire Great Lakes ecosystem,” said Uzarski, who is the director of the university’s biological station on Beaver Island, Mich.
These data are sorely needed. “One thing we always say — and we’ve been saying it for years — is that coastal wetlands are great fish-nursery habitats,” Uzarski said. But that isn’t enough, especially when it comes to protecting wetlands from development.
“I find myself before lawmakers quite often with arguments (pitting) development or beach grooming vs. keeping the coastal wetland, and when I say they’re important nursery habitats and give numbers of juveniles in that wetland, they’ll say, ‘So if we develop that area, or we groom and turn that into beach, won’t you just find them somewhere else? What’s the real impact? Does that impact really matter?'” Uzarski said.
“We’ve never had hard evidence to be able to say that yes, it does matter, and this is how much.”
This new otolith research is beginning to provide that hard evidence. CMU graduate student Lee Schoen analyzed 12 wetlands: five in Lake Huron’s Saginaw Bay, an area where 95 percent of historical coastal wetlands are already filled, drained or developed; four in northern Lake Huron off the Les Cheneaux Islands in the eastern Upper Peninsula; and three drowned river mouths in Lake Michigan at Grand Haven, Pentwater and near Saugatuck. Each wetland has an environmental fingerprint of small but identifiable concentrations of the elements strontium and barium.
Schoen also collected yellow perch from the wetlands and from the open water of the lakes. He removed their otoliths to see if they match the wetlands’ environmental fingerprints.
These ear bones are perfect record-keepers. They accumulate in layers, much like the rings in a tree trunk.
“The otoliths grow in daily rings that integrate the trace elements from the surrounding water column at the time the ring was deposited,” Schoen said. “And once it’s deposited, these otoliths are relatively stable so those trace elements aren’t going to leach out.”
A darker ring is also laid down each winter. That makes it easy to track the age of the fish at different points in the otolith.
To track the fish’s whereabouts, Jim Student, director of isotopic and elemental analysis labs in the CMU College of Science and Technology, runs the otoliths through a laser-equipped mass spectrometer. The device picks up, among many other elements, strontium and barium levels from the rings. It pinpoints when in its lifespan each fish visited specific wetlands.
Schoen’s data has already challenged conventional thinking about the well-studied perch.
Fisheries biologists, including Uzarski, had believed that just the juveniles use wetlands. That belief was based on fish sampling at 30 coastal wetlands since the 1990s and a $10 million project that sampled about 175 coastal wetlands throughout the entire Great Lakes basin for insects, plants, fish and water chemistry. Those studies found young-of-the-year perch, not adults.
“We were assuming that these were only nursery habitats, like you’ve heard over and over, because we almost exclusively catch juveniles,” Uzarski said. “But these new data are suggesting that’s not the case.
“Adults do, in fact, go there, which is telling us that coastal wetlands are important throughout a fish’s life, at least in the yellow perch (Schoen’s) studying.”
Schoen hopes to expand the study to include walleye and possibly other fish. Uzarski wants to extend the study to more wetlands and all five Great Lakes with the help of CMU’s new 38.5-foot research vessel, the RV Chippewa.
The data will give ecologists the solid proof of the benefits of coastal wetlands, Uzarski said. “It will show not only how much coastal wetlands are contributing to the base of the overall Great Lakes food web, but even what types of wetlands are contributing and where some of the most important wetlands are located. That’s down the road a bit, but what Lee is working on is the first step in getting there.”
Dave Clapp, director of the Michigan Department of Natural Resources’ Charlevoix Fisheries Research Station, said he and other Great Lakes fisheries managers are “very interested in results” of this work.
“One specific question of particular interest is: What is the contribution of fish populations in a drowned-river-mouth lake and the associated wetland complexes to Great Lakes fish communities?” he said. “Don’s work in this area will be very important to Great Lakes fisheries management efforts.”
The goal is to make better choices about the Great Lakes basin’s coastal wetlands.
“If we start losing our remaining wetlands, the next time lawmakers will come to me will be to complain about the fishery and tourism crumbling, and the jobs associated with them disappearing,” Uzarski said. “I think hard data like this will be really powerful to help with management decisions, restoration decisions and protection/conservation of these systems.”