Keeping an Eye on Tench, an Invasive Fish That’s Crept Into the Great Lakes

After finding the species in Lake Ontario last fall, scientists and authorities are trying to get ahead of a possible invasion before it’s too late.

Blickwinkel/Alamy Stock Photo

Several dozen fish called tench swam against a gentle current flowing in blue plastic bins covered by a tarp and chicken wire (to keep out raccoons). The fish—which drifted above sand or rocks or nothing at all—had been plucked from the wild to live strange new lives in a wooded corner of McGill University’s Gault Nature Reserve near Montreal. It was all part of an experiment conducted last fall by biologist Sunci Avlijas to see whether the tench, an invasive species from Europe and Asia, might reveal their preferred habitats. The goal: to learn whether the Great Lakes would fit the fish’s bill.

First brought to Quebec in the 1980s, tench, with their red eyes and green scales, have spent the past 30 years infiltrating the St. Lawrence River. A month after Avlijas wrapped up her research, this past September, tench were found in Lake Ontario for the first time, having pushed past the locks and dams meant to contain the species in the river. Tench are now officially among the ranks of hundreds of Great Lakes invaders. But it remains unknown if this fish will flourish and wreak havoc, as the zebra mussel and sea lamprey have.

Adult tench can grow as long as 27 inches, a size that would make them difficult prey in Great Lake ecosystems. And as bottom feeders that munch on large invertebrates like mollusks, they might compete with native fish like yellow perch for access to food. Stomach dissections have shown tench also eat fish eggs, which could put a dent into certain local populations. To boot, the fish are known to carry multiple parasites.

Sunci Avlijas

It’s a worry, but not a direct threat just yet. Avlijas emphasizes that the Lake Ontario discovery this past October doesn’t prove the fish are well established at this point in time. “But it’s likely we’ll catch more in the coming years,” she warns. And depending on how many more, Tinca tinca could end up being a major problem for the Great Lakes.

Predicting the scale of this invasion has been the focus of Avlijas’s graduate career. She’s observed tench in other nonnative habitats, such as South Africa, and traced the species’ global journeys back to the 1500s, when fishmongers began carting the fish around Europe. Today the species lives on every continent but Antarctica. Even so, tench don’t take off in every habitat.

The fish’s long history of introductions to the United States, for instance, has been hit-and-miss. Stocked in watersheds from Maryland to Oregon as a sport fish by the U.S. Fish Commission in the late 1800s, carried live from Italy to California in 1922, and brought to many other states (including Illinois and Ohio in 1891), tench have either quickly died out or thrived.

“It’s a really interesting species because it has such a varied effect,” Avlijas says. “In some places it doesn’t manage to establish; in other places it booms.”

Booming is what happened in Quebec. A farmer transported 30 of the fish to the province in 1986 with the idea of establishing an aquaculture operation. No one was buying, so he drained his ponds five years later. Tench, however, have an impressive ability to survive in low-oxygen conditions with little water—all they need is a few inches to splash through. They floundered into the Richelieu River, a tributary of the St. Lawrence, and spread. Fishers participating in an early detection program in nearby Lake St. Pierre went from catching no tench in 2004 to catching nearly 10,000  in 2014.

Sunci Avlijas

“It was predictable that as tench became more abundant they were going to spread further and further,” says Anthony Ricciardi, an invasive species biologist at McGill University who has coauthored papers with Avlijas. But for Ricciardi, tench are just another symptom of a much larger problem.

Around 200 invasive species currently occupy the Great Lakes. Most have found their way to the freshwater ecosystem via human folly, such as through lax ballast water regulations or intentional bait releases. According to Ricciardi and other scientists, prevention is the best strategy when it comes to biological invasions, and policymakers should follow a protocol of early detection and rapid response.

Some are already doing so. Last year, the Great Lakes–St. Lawrence Governors and Premiers, a coalition of leaders on both sides of the U.S.–Canada border, added tench to their “Least Wanted” list, indicating the fish poses a significant threat. Organizations like the St. Lawrence–Eastern Lake Ontario Partnership for Regional Invasive Species Management (SLELO-PRISM) are also jumping in with educational workshops to train anglers and boaters how to identify tench. Fishers are instructed to dispose of any tench they happen to catch, rather than releasing them into the water.

Protect Clean Water

“Tench will threaten our resources, the aquatic food web, and outcompete other species for food and shelter,” says Megan Pistolese, the education and outreach coordinator at SLELO-PRISM in New York. “We’re trying to raise awareness in the community to let them know this fish is out there.”

When tench made their way into Lake Champlain, on the border of New York and Vermont, groups like the Lake Champlain Basin Program listed tench as a species “of particular concern” and provided instructions on identifying them. The species is already on the radar of public officials in Michigan, where it has been highlighted as a potentially devastating invasive species.

Some of the strategies for slowing the invasion include instructing fishers not to dump unused bait, as they might mistakenly be using tench minnows; asking those who catch tench to take a photo or deliver the actual fish to a SLELO-PRISM office; and hosting workshops on the latest scientific research.

As for Avlijas’s research, while the results have yet to be published, she was surprised by one observation. In the past, tench had been reported to thrive when they could feed in soft sediment. But Avlijas discovered the fish did just as well when feeding in rocks. “It’s not conclusive, but it’s new,” she says. It might mean the fish can expand across more habitats.

Avlijas is spreading the word about her findings, but she worries local governments might wait too long to take action. “That’s the tragedy of invasive species,” she says. “By the time you realize it’s bad, it’s too late. And the time when it doesn’t seem like a real threat is the only window of opportunity to do something about it.”

Reporting for this story was made possible by the Institute for Journalism and Natural Resources

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