It's a mystery. Invasive sea lamprey, the Great Lakes' biggest predator, primarily feed on lake trout, one of the lakes' most prized sports fish. When trout populations are high, researchers expect to see fewer lamprey-wounded fish, and more of those wounds when lamprey populations are spiking.
But that's not always what scientists are finding.
New research into what may be behind the discrepancies holds promise to improve how sea lampreys are controlled in the Great Lakes, protecting a $7 billion fishery. It could allow lamprey managers to examine whether they have switched to other fish species as food sources, or whether lamprey and trout numbers in particular regions of Great Lakes should be more closely reviewed - helping direct their limited resources to hold back the invaders.
"When we saw the lake trout population go up, we would expect to see the wounding rate go down, or, if we saw the sea lamprey numbers go up, we would expect the wounding rate to go up. And we did, for the most part, see those things on a year-to-year basis," said Jean V. Adams, a biology statistician with the U.S. Geological Survey based in Wisconsin.
But curiously, the numbers didn't always connect. Either the number of lake trout with lamprey wounds would remain high despite larger fish populations, or researchers wouldn't see as many wounded lake trout as they expected, given what they knew about sea lamprey populations that year.
"Every now and again, the wounding rate would change in a way that just didn't really make sense to us," Adams said.
"That's what we were trying to puzzle out."
A scientist with the Hammond Bay Biological Station near Huron Beach, Michigan, holds a female sea lamprey. The lamprey uses its disk-shaped mouth and sharp teeth to fasten onto fish and suck out their bodily fluids.
Sea lampreys look like something out of a science fiction or horror movie. The eel-like fish, which grows up to 20 inches long, features a round, suction mouth circled with rows of sharp teeth. Sea lampreys are predators, attaching to fish and feeding on their blood and body fluids.
Adams' study and its surprising findings were recently published online in the peer-reviewed scientific journal Fisheries Research.
Adams suspected the discrepancies would be explained by "noise" in their data - distorted information that was throwing off their findings.
"Biological data are really subject to noise," she said. "It's hard to get really precise measurements of animals that you typically don't see - that can be difficult to detect. And to do so in a sort of standard fashion, covering such a big area as the Great Lakes. And with big changes year-to-year in populations, too."
Working with researchers at Michigan State University, Adams ran simulation models with the lake trout and sea lamprey data, testing the findings with intentional measurement errors one metric at a time, to see if they could duplicate the puzzling findings. That's when they got a surprising result: Their data was fine.
"The answer came back saying no, there's something else going on," Adams said. "And those indicators can be used to help figure out what that something is."
The research has the attention of the Great Lakes Fishery Commission, an Ann Arbor-based agency created in 1954 under a treaty between the U.S. and Canada to combat invasive sea lampreys.
The sea lamprey has rows of teeth similar to a shark. Kurt Stepnitz, Michigan State University.
"They are saying it is quite possible there are other things going on that we in the past really had never taken into account," commission spokesman Marc Gaden said.
Sea lampreys entered the Great Lakes from the Atlantic Ocean through the creation of shipping canals and were first observed in Lake Ontario in the 1830s, according to Minnesota Sea Grant, a Great Lakes research and education partnership between the University of Minnesota and the National Oceanic and Atmospheric Administration.
Niagara Falls acted as a natural barrier preventing sea lamprey movement to Lakes Erie, Huron, Michigan and Superior. But when the Welland Canal, constructed to bypass the falls, was deepened in 1919, sea lampreys gained access to the rest of the Great Lakes. By 1938, they had invaded all of the Great Lakes, becoming their top predator.
"Here it is, an invader in our system, and it's completely shifted our whole Great Lakes ecosystem," Adams said. "It's a pretty impressive animal."
Sea lampreys' impact on the Great Lakes fishery was staggering. Before they entered the Great Lakes, the U.S. and Canada harvested about 15 million pounds of lake trout in lakes Huron and Superior each year. By the early 1960s, the catch was down to about 300,000 pounds, according to Minnesota Sea Grant.
Since the early 1970s, the Great Lakes Fishery Commission has operated sea lamprey barriers in strategic locations, in addition to relying upon thousands of hydro-dams and other barriers built for other purposes that effectively control lamprey as well. The agency also strategically applies lampricides to tributaries where lamprey in their larval stage live for about three years before changing into their adult form that preys on fish. The chemical treatments, while controlling sea lamprey larvae, are considered otherwise safe for people and the environment by the U.S. Environmental Protection Agency.
Adams and other researchers have a preliminary suspicion regarding what might be going on to explain the data discrepancies. Recent research out of Lake Ontario shows sea lampreys occasionally switch their preferred fish host from lake trout to chinook salmon, particularly if trout abundance is low, she said.
"There's no reason to think that that's the only place that's going on," she said. "It could well be going on in other lakes, and with other species of fish - lake whitefish, perhaps, in Lake Huron."
Other things to examine could include reevaluating how well lake trout are surviving wounds from sea lamprey, and how well juvenile sea lamprey are surviving.
The Fishery Commission must decide each year where to devote its attention to sea lamprey, Gaden said.
"What this paper tells us is that our observational data is actually pretty good," he said. "And if we dig a little deeper into what it tells us, we can maximize the control effort in a way that also maximizes the benefit to the fishery."
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