Invasive Eurasian Tench Threatens Lake Ontario

By Kevin Bunch, IJC

invasive Eurasian tench
The invasive Eurasian tench was introduced to North American waterways as a game and food fish in the late 1800s and early 1900s. The fish has recently been found in the St. Lawrence River. Credit:

Eurasian tench, an invasive species found in Canada and the United States, has been rapidly expanding its range into the St. Lawrence River in recent years. Its upstream spread has reached as far west as Lake St. Francis in southeast Ontario Great Lakes researchers, scientists, and resource managers are concerned the tench could wreak havoc on native fish and their habitat if it enters the Great Lakes.

Tench are native to Europe and western Asia, and were introduced to North America by the U.S. Fish Commission in 1877 for use as a food and sport fish, according to the US Geological Survey. That effort continued into the 20th century, but in most areas where the fish was introduced, it did not become established. However, a population introduced illegally to the Richelieu River by an unlicensed fish farm in 1986 has spread rapidly to the St. Lawrence River and Lake Champlain, according to McGill University Ph.D student Sunci Avlijas, who has studied the tench.

Ever since the fish were first detected in the St. Lawrence River in 2006, Avlijas said, a monitoring program run by the Quebec government and commercial fishermen has been in place. The population has grown exponentially every year between 2009 and 2014. They’ve also spread downstream on the St. Lawrence toward Quebec City and upstream toward Lake Ontario.

“We’re concerned about it moving toward the Great Lakes since the tench prefers slow-moving waters in wetland areas, and there are many such habitats in the Great Lakes,” said Avlijas, whose findings were presented at the International Association for Great Lakes Research conference in June 2017. “(Once) tench enter the Great Lakes there’s the Bay of Quinte, which is even better habitat than we find in the St. Lawrence.”

Once established in an ideal environment, tench form dense populations. Avlijas said tench will eat a variety of macroinvertebrates – zooplankton, mollusks and mussels, insects, and crayfish – mainly from the water bottom, but in calm waters they’ll even go to the surface for food. They also tend to kick up mud and sediment, reducing water quality. Aside from direct competition with native fish for food, tench also carry non-native parasites that aren’t known to be present in the Great Lakes, Avljias said, making them potential disease carriers for native fish. Tench also are known for eating zooplankton that can keep algae in check, potentially worsening the amount and size of harmful algal blooms.

What’s more, they can survive in low-oxygen environments, and cover themselves in mud to survive outside of water for a limited period, allowing them to be introduced into new water bodies, Avlijas said. There have been documented cases of tench being mailed in wet sacks and arriving alive a day later.

“They’re a prime candidate for being transported by people,” she said.

Tench compete with native fish for food and habitat in nearshore regions, and can cause water quality issues as they dig through mud
Tench compete with native fish for food and habitat in nearshore regions, and can cause water quality issues as they dig through mud. Credit: Sunci Avlijas

While tench are eaten by native fish like walleye, northern pike, smallmouth bass, largemouth bass and bowfin, once they grow longer than about 12 inches (30 centimeters), they become too large for most predators to consume. Avlijas said this has happened in Lake St. Pierre, where the fish are abundant.

The extent to which tench could impact the Great Lakes is still debated, but it’s predicted they could become established here, said Jeff Brinsmead, senior invasive species biologist with the Ontario Ministry of Natural Resources and Forestry.

While most Great Lakes states don’t ban tench, Wisconsin has a prohibition on the species dating back to when its own invasive species rule went into effect in 2009. Under the rule, the transportation, possession, transfer and introduction of Eurasian tench is illegal in the state. According to Joanne Haas, a Wisconsin Department of Natural Resources public information officer, tench had been stocked in some lakes in the past, and has been known to exist in surrounding states like Ohio, Indiana, Illinois and Michigan – albeit with few reproducing populations. Wisconsin is still concerned about reproductive potential, however, and sees tench as a potential competitor to minnows and native sportfish.

Michigan also has laws making it illegal to transport live specimens of tench, with civil fines up to $10,000. It’s a prohibited species in the state.

While tench are not regulated as an invasive species in Ontario, rules that apply to all fish species in the province also apply to the tench: a fish can only be released into the water body it was found in unless the releasing person or organization has a license. The use of tench as a baitfish is also illegal in the province, and residents are asked to alert the Ministry of Natural Resources and Forestry if tench are found in the wild by calling the Invading Species Hotline at 1-800-563-7711, or going online to www.EDDMapS/Ontario. Illegal activities involving tench can be reported to the ministry’s enforcement branch at 877-TIPS-MNR (877-847-7667). More information can be found on Ontario’s Invading Species Awareness Program website.

Once an invasive species becomes established in a new environment, it is very difficult, if not impossible, to eradicate. However, it may be possible to slow or block the spread of the species. Education and outreach are critical to ensure that people are aware of the rules that apply to moving live fish. Brinsmead said that since tench are related to Asian carp, it’s possible that similar techniques could be effective in containing the spread of tench, like electric barriers. However, testing specific to tench hasn’t been done yet, and Brinsmead noted that other species – like the endangered American eel – travel through the St. Lawrence River too, so any measures to block tench would need to keep the passage of these species in mind.

Avlijas suggested that to limit the spread, people throughout the lakes follow provincial and state regulations.

“People just consider it non-invasive because after its (legal) introduction it was not spreading,” she said. “It was ignored for a long time.”

adult tench 27 inches
An adult tench can grow up to about 27 inches (70 centimeters) long. Credit: Sunci Avlijas

Kevin Bunch is a writer-communications specialist at the IJC’s US Section office in Washington, D.C.

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