By ERIC FREEDMAN
Smallmouth bass chowing down on invasive fish. Image: Michigan Sea Grant
Can invasive species be good news — rather than bad — for native fish in the Great Lakes?
That sounds counterintuitive, but a new study shows that the invasive round goby has become an important food source for several native species, especially smallmouth bass, but with benefits also for yellow perch and walleye.
Even so, there are still unknowns, including whether the round goby transports contaminants up through the food chain, said Derek Crane, the lead author and a research associate at Lake Superior State University.
The study calls the round goby “one of the most successful aquatic invaders” in the Great Lakes.
It’s a bottom-dweller originating in Eurasia. Locally, it arrived in contaminated ballast water. It was first identified in Michigan’s St. Clair River in 1990 and spread to all five Great Lakes, according to the Environmental Protection Agency (EPA).
The EPA reflects prevailing attitudes about invaders when it says, “The Great Lakes sport and commercial fishing industry, valued at almost $4.5 billion annually, is at risk due to growing numbers of non-indigenous mussels and fish, such as the zebra and quagga mussels, sea lamprey, ruffe and round goby. Populations of native fish including lake trout, walleye, yellow perch and whitefish are threatened by the establishment of these exotic species.”
However, a Michigan Department of Natural Resources (DNR) fisheries expert says long-term experience with the round goby illustrates that such widely held fears don’t always prove true.
The round goby does exhibit some traits that other invaders share, such as quick reproduction, dense populations and adaptability to a variety of food sources and habitats, said Nicholas Popoff, the manager of aquatic species and regulatory affairs in DNR’s Fisheries Division.
But 25 years after their discovery in the Great Lakes, “we’re not documenting specific harms from gobies,” Popoff said, referring to feared environmental, economic and human health concerns.
In fact, there are indications of possible benefits from their presence, he said. For example, “we are seeing amazing smallmouth bass,” as well as some “amazing walleye,” while lake trout have modified their diets from sculpin to round gobies.
One possible exception, according to Popoff, is a decline in sculpin population as documented in Lake Michigan’s Grand Traverse Bay because they compete with round gobies for space and food. However, scientists haven’t determined whether the lake’s overall sculpin population is down or whether they’ve merely moved to deeper areas with fewer round gobies.
And in Calumet Harbor, the U.S. Geological Service has reported the absence of mottled sculpin nests since the round goby was established in that part of Lake Superior in 1994.
The newly published study about lakes Erie and Ontario by Crane and researchers from the New York Department of Environmental Conservation and Syracuse University reflects similar findings.
Their study published in the journal Freshwater Biology acknowledged, “Invasive species are one of the greatest threats to freshwater ecosystems globally and have the capacity to rapidly change the ecosystems they invade.” Adverse effects can include transmission of contaminants and impaired reproduction of native species.
However, the study found the round goby is now a widely available food source for many native fish because of its “extreme abundance, tolerance to a variety of habitat conditions and relatively small size.”
In lakes Erie and Ontario, round gobies accounted for 75 percent of the smallmouth bass diet, Crane said. If all other species have maintained stable populations, that means the bass are putting less pressure on other food sources.
His team examined trends in body condition after the round goby invasion, as reflected in the relation between fish weight and length. Heavier weight relative to length indicates a better nutritional state and more energy resources, according to Crane. Among the key findings:
· Body condition of smallmouth bass generally improved.
· Body condition of yellow perch also improved, but not as much.
· Body condition of longer-length walleye generally improved in Lake Ontario, although not necessarily because round gobies are available as a food source. Body condition of Lake Erie walleye generally didn’t change.
· As for burbot, there’s been moderate improvement in body condition only for smaller-length fish, with minor decreases for longer burbot.
Crane said other research shows smallmouth bass that feed on round gobies grow faster than they did before the invasion, with the greatest growth rate increase among 2-to-4-year-olds.
But that’s not automatically good news, he cautioned. “The flip side is that fish that grow faster tend to not live as long. They have a higher natural mortality rate.
“Live fast, die young,” he said.
In addition, he said there’ve been localized disappearances of some sculpin and darter populations in areas with lots of round gobies.
Round gobies are “bait stealers” and don’t appeal to sports anglers, he said, and “if you’re fishing for perch, they’ll be an annoyance. If you go to a clear pier like Charlevoix in the spring, you’ll literally see them everywhere.
“From that perspective, they take up space, they eat food, but when you pull all the effects out, the tough question is whether they’re more harmful or beneficial.”
While the round goby has proven relatively benign over the past quarter-century, Popoff said, “one thing is for certain: The Great Lakes have been decimated by invasive species.”
Michigan is focused on preventing other invaders from entering the state. “Absolutely 100 percent we don’t want them here. It’s better not to have these animals coming in,” he said.
“If we could go back 25 years and prevent gobies from entering the Great Lakes, we would absolutely do whatever it takes. But, now that they are so far down the invasion curve, control becomes less of a reality unless the harm outweighs the cost,” Popoff said. “And for gobies at this point the harm does not outweigh the cost.”