Yet another invasive fish species is headed for the Great Lakes

A Eurasian tench swimming in green-hued water By Vladimir Wrangel/Shutterstock

Earlier this fall, American anglers on their side of the St. Lawrence River reeled in an unwelcome catch: Eurasian tench. The foot-long members of the minnow family are invasive on both sides of the border.

Tench were widely introduced across the United States in the late 1800s, and for many years to follow, for food and sport fishing, with varying  degrees of success. Today, only a fraction of those areas that were once stocked are still rife with tench. But in Canada, the fish found far greater success in establishing themselves. It all started when a farmer in Quebec imported 30 tench from Germany in the late 1980s to propagate in his pond, explains Sunci Avlijas, Anthony Ricciardi and Nicholas Mandrak  in a paper on the invasive fish for the Canadian Journal of Fisheries and Aquatic Sciences. Once the farmer recognized there was no commercial value for tench, he drained the pond and released the remaining fish into a tributary of the St. Lawrence River, the Richelieu. Since then, these tench has been gaining ground—or water. A group of researchers, including Mandrak, are working on genotyping these tench, to determine whether they’re related to those that became established across the U.S. And, Avlijas is looking at whether the physical environment—the water flow, depth, aquatic plants, and all—or the makeup of the local fish communities is helping or hindering the spread of tench. To underscore the threat this species poses, the paper two worked on in 2017 was titled Eurasian Tench (Tinca tinca): the next Great Lakes invader. Unfortunately, the threat was imminent, there was actually a single tench captured in 2016 in Lake St. Francis, and no management strategy has been applied to prevent passage of Tench upstream,” Avlijas says.

Canada’s pesky tench were first recorded in the St. Lawrence in 2006, and the Quebec government has kept a close eye on them with regular surveys. Since 2009, the population has grown rapidly, according to the paper. Tench have continued downstream in the St. Lawrence, establishing a healthy population in the popular cottaging destination Lake Saint-Pierre. In 2016, the Lake St. Francis catch, just at the the Quebec-Ontario border, showed its push downstream and now, as the early fall catch just off Cornwall Island proves, tench are venturing further upstream toward Lake Ontario—an entry-point to the Great Lakes.

Concern over this European and western Asian invader comes down to its insatiable appetite and high tolerance. Not only will tench—a larger relative of the minnow family, usually 20 to 25 centimetres but growing up to 70 centimetres long in full maturity—outfeed its native cousins, it could also feast on snails that, according to Ontario’s Invading Species Awareness Program, graze on algae and help keep algal blooms at bay. And they eat vegetation that trap nutrients released in water, say from fertilizer, which can also lead to algal blooms—something no cottager or angler wants to see.

As well as happily eating most organic matter, tench can live pretty well anywhere. They can survive in low oxygen environments, meaning they’re easily transportable and can live in areas like shallows and wetlands where other fish cannot due to the warm, low oxygen environment, though it’s well stocked with food. “This could give them an edge over other fish and allow them to grow and reproduce faster than if they had to directly compete for food with other species,” says Avlijas. They also tend to be a bit of a mess: stirring up settlement as they feed on the lake and river bottoms, reducing water clarity, and harbouring parasites that are not native to the Great Lakes. “Most of these impacts are only a concern when the tench reach high population densities,” says Avlijas. But, as well as its spread towards Lake Ontario, she says they are seeing steady population growth of tench in the St. Lawrence (specifically in Lake Saint-Pierre) and the Richelieu River.

So, what can citizens do to prevent the spread and help track tench in Canadian waterways? Learn how to identify tench and notify the local ministry of natural resources or environment if any are caught or spotted. A quick way to recognize tench is by its thin, deep, and golden body and small, bright red-orange eyes. It has darker fins than body and single whiskers hang from either corner of the fish’s mouth. Also, because of its minnow-relation, tench can be mistaken for a bait fish—but do not use tench as bait. Remember, these tolerant fish can survive in a bucket with little water for days, so using tench as live bait is a quick way to introduce them to new waters, says Avlijas. And, don’t be like that person in 2014 who took a note from the Quebec farmer’s book and tried to start their own tench farm in a pond within the Humber River watershed west of Toronto. We know tench are likely to thrive, so best not to give them the opportunity.


To report a sighting or illegal importing, breeding or distribution of tench, call: 1-877-TIPS-MNR (847-7667)