Round goby-eating smallmouth bass in Lake Erie and Lake Ontario are bigger since invasion

A new study gives weight to what anglers on Lake Erie and Lake Ontario have suspected for years: Smallmouth bass have made hay since the round goby invaded both lakes, and the predators appear to be fatter for it.

“The fishermen out there saw changes in the weight of the smallmouth bass they were catching,” said Derek Crane, study author and research associate at Lake Superior State University’s School of Biological Sciences. “These fish were looking plump and well fed.”

Managers had noticed the same thing, so Crane and his colleagues from the New York State Department of Environmental Conservation and SUNY-ESF put some numbers to it for a study recently published in the journal Freshwater Biology. They looked at nearly 20 years of data gathered by the DEC on weights and lengths of four species from before and after the goby invasion — one of the most successful invasions in Great Lakes history. Smallmouth showed the clearest post-invasion weight boost, followed by yellow perch.

The improved condition of smallmouth makes sense. The species has keyed in on gobies, which one study found made up 75 percent of Lake Erie smallmouth diet. Smallmouth bass like rocky habitat and feed on forage like crayfish. Round gobies also like coarse rocky areas, and they’re a benthic prey.

“I think it was a real natural switch for smallmouth,” Crane said. “Round goby were real abundant, so they just started chowing on round goby.”

In Lake Erie, younger smallmouth appear to have gained the most. On average, something like a 6-inch bass caught today would be around 17 percent heavier than one caught before the gobies arrived. But bass around 19 inches and longer would look about the same. On the other hand, 19-inch-and-larger bass in Lake Ontario showed the biggest gain, coming in at around 12 percent greater mass than pre-invasion fish.

A round goby from elsewhere in the Great Lakes, where the species had a historically successful invasion. (Credit: U.S. Army Corps of Engineers)

The researchers also looked at walleye weights and lengths, but didn’t detect an effect. The longer walleye in Lake Ontario appear to be a little fatter than they were before the goby arrived, but those big fish feed mostly offshore and are more likely to be benefiting from changes in other prey fish populations.

Though fatter smallmouth may sound like a good thing, the round goby may not be a long-term blessing for the fishery. For one, fish that grow faster tend to die younger, Crane said. That cuts down on the number of times a fish can reproduce and contribute to the population. Gobies also raid nests and gobble smallmouth eggs. But fitter smallmouth may also mature earlier and have better reproductive success in their shorter lives.

The big-picture effects of the invasion on the smallmouth fisheries on each lake are as of yet unclear. Crane said Lake Ontario holds some “very large” bass, but anglers are having a harder time catching them. Lake Erie is a different story.

“Right now the bass population, at least in Lake Erie, is doing well,” Crane said. “It’s probably, if not the best, then one of the best smallmouth bass fisheries in the world.”