It's hard to do research when your test subjects keep getting eaten.
Scientists studying the movements of smaller fish in the Great Lakes increasingly use a technology called acoustic telemetry — small tags inserted in the fish that transmit a unique data code via sound waves that's later picked up at different receiver stations the scientists place along a lake or river.
But there's a problem: The subject fish frequently get eaten by larger fish without the researchers knowing it, and they then collect data on the movements and behaviors of the wrong species.
"Different predator species, they could carry those tags for days or even weeks" after eating a smaller, tagged fish, said Scott Colborne, a research biologist with Chicago's Shedd Aquarium.
"We could have a long period of time where we aren't tracking what we think we are."
Colborne and other Shedd researchers, working with scientists at the Great Lakes Institute for Environmental Research at the University of Windsor, recently published a study on a new technology that helps solve the problem.
It's a specialized fish tag that sends out a different signal when it comes in to contact with a fish's stomach acids, letting researchers know that the small fish they are studying apparently became a bigger fish's lunch, and to question any further data coming from that signal.
It opens up a new level of understanding about fish movement and behavior, particularly with the smaller fish that serve as a key food source for the larger fish prized in Michigan's $2.3 billion fishing industry.
"It's nice to be concerned about walleye ... but walleye need to eat something," said Aaron Fisk, a professor of aquatic ecology at the University of Windsor's Great Lakes Institute.
"So we need to better understand what's going on with the prey-fish, the smaller fish in the system, so that we can properly manage it."
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The use of acoustic telemetry to study Great Lakes fish started expanding about a decade ago, with larger fish such as lake trout and sturgeon receiving the sound-emitting tags. And fish that size, at the top of the aquatic food web, don't get eaten, Fisk said.
"The new acoustic telemetry technology is producing really small tags now," he said. That's prompted "a real interest in understanding small fish behaviors and movement, particularly in the Great Lakes."
With older tag technology, researchers would detect a tag that stopped moving near a telemetry receiving station, or one that went away and was never detected again, and surmise that the tagged fish had died. Sometimes, if a tagged fish was eaten by a larger predator — a salmon, muskellunge, pike, lake trout, largemouth bass or other species — researchers could tell by sudden changes in the received data that were significantly unlike the other tagged fish's movements.
"So there would be times where you suspect predation, so let's remove this from the data set, to be conservative about what we are saying," Colborne said.
With the new tags, developed by Boston-based InnovaSea Systems Inc., a biopolymer — a substance created from the cells of living organisms — covers a portion of the fish tag. The substance dissolves away in a fish's stomach acid, eventually exposing a small magnet that falls away.
"When it comes out, it permanently switches the code of that tag," Colborne said. "So for the rest of the time that it's transmitting, it's putting out a new identification code" that lets researchers know that fish was apparently eaten.
Colborne and Fisk did some of the early lab-testing of the new tag at the University of Windsor, feeding largemouth bass captured on the Detroit River and kept in an aquarium in the laboratory with small fish tagged with the new devices.
In their recently published study, the testing was taken to the field — on the Detroit River near Fighting Island.
"The Detroit River is an amazing aquatic system," Fisk said. "There are almost 70 species of fish in there. It's hard to find a water environment anywhere else in the world that has the types of species and number of predators in that system."
The researchers tagged 60 small, juvenile yellow perch. Over five months, looking just in the area of north Fighting Island, the tags sent signals of 19 predation events — almost one out of every three of the test fish was eaten.
"For some of them, you could see major changes to where the fish were in the river, once they had triggered predation," Colborne said. "In the past, other studies would have looked at that data and said, 'That looks like predation, so we will remove that fish.'
"But we also saw a large number of our 19 predation events where the movement didn't change very much. That's because a lot of predators, like largemouth bass in that area, reside in the same area as those yellow perch all summer. They could have slipped through in prior studies because there wasn't an obvious change in habitat."
The predation tags are likely to go into wider use by small-fish researchers, Colborne said. His work at Shedd includes study of the round goby, a small, invasive species fish creating major changes in the Great Lakes.
"I'm very interested in how they are being integrated into the food web here," he said. "Part of that would be tracking the habitat use and getting an idea of the predation levels they face."
Acoustic telemetry is changing the way ecosystems and fisheries are managed in the Great Lakes region, Fisk said. "It's probably been the most influential new thing in maybe the past 50 years," he said.
Predation tags open up much more information for scientists and habitat managers.
"The smaller fish don't really live as long; they tend to be more sensitive to environmental change," Fisk said.
And as the climate in the Great Lakes region continues to warm; as the Great Lakes continue to break high water level records and invasive species continue to enter the system, adds a necessity to the research, he said.
"Being able to see what the smaller fish are doing, being able to manage them, is really important right now."
Contact Keith Matheny: 313-222-5021 or kmatheny@freepress.com.
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