Battling the tench fish invasion: ‘Like Game of Thrones under water’

We don’t know if they flew economy or business class but Eurasian tench first arrived in Canada wiggling around in a picnic cooler on a commercial flight from Germany in 1986. Intent on farming them as a food source, the brazen Quebec businessman lugging the 30 freshwater fish declared his illegal cargo at Canadian customs. After some perfunctory checks, the customs agent waved him through even though he lacked an import licence. How much harm could a few six-centimetre-long fish cause?

The tench farm, in St-Alexandre, 60 kilometres southeast of Montreal, proved a disaster. The fish had low growth and survival rates — and marketing trials showed Quebecers didn’t want to eat them. In the early 1990s, the businessman drained the ponds and some of the tench escaped into nearby agricultural streams and into the Richelieu River. 

The rest is aquatic-invasive-species history. 

The tench took hold in the river and then spread through much of the St. Lawrence River, threatening local fish such as the copper redhorse and perch, and disrupting sports and commercial fisheries in Quebec. Now, the invaders are moving west toward an even juicier target: the Great Lakes.

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Native to some parts of Europe and Asia, tench have been eaten by humans in some parts of the world since the Middle Ages, though a taste for them never developed in North America. In Quebec, consumers prefer Salmonidae (the family of fish that includes salmon and trout) and saltwater species.

Eurasian tench are a highly invasive bottom-dwelling freshwater fish that belongs to the minnow family. They have characteristics that worry biologists. They’re voracious eaters that compete with native fish for food. They carry parasites, reproduce profusely, damage water quality — and can survive up to a day out of water.

The aquatic intruders are complicating battles already raging among species.

“It’s like Game of Thrones under water — there are wars for space, wars for territory, wars for food,” said McGill University invasive-species biologist Anthony Ricciardi, who is supervising an experiment this summer focused on the tench.

He’s convinced the tench “invasion of Lake Ontario is imminent” and that they will “almost certainly” spread to the other Great Lakes.

“For many years, (the species) didn’t seem to be doing very much (in the Richelieu River), they just kind of hung around in low densities,” said Sunci Avlijas, a PhD candidate at McGill University’s department of biology whose thesis focuses on the tench.

The species have spread on the St. Lawrence as far downstream as Quebec City, and upstream toward the Great Lakes. Last fall, a single tench was found in Lake Ontario — near Belleville, Ont., in the Bay of Quinte, not far from the head of the St. Lawrence River.

Slow-motion incursions are common with invasive species. “They begin really localized and people have a false sense of security,” Avlijas said. “People think, ‘Oh, it’s not really going to go far, it’s just kind of an accidental capture.’ But quite often there’s a lag at the start phase of invasions.”

Before they expand further, it’s important to understand the effect tench have on native fish. That’s the goal of an outdoor McGill University experiment being conducted by some of Ricciardi’s students, including Avlijas. They’re studying tench at a research facility at the base of Mont-St-Hilaire, about 40 kilometres north of where the species is believed to have first made its way to freedom in the Richelieu River.

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On a cloudy afternoon in mid-July, fishing net in hand, Christophe Benjamin was crouched over a bathtub-shaped blue bin from which he had just trawled out a slimy 35-centimetre, one-kilogram fish.

“It has fine scales, brown-olive colour, kind of red eyes and small barbels” at the corners of its lips — sensory filaments that allow it to detect its prey, said Benjamin, a master’s student in McGill’s school of biology.

He was giving visitors a tour of the outdoor tench experiment he is working on at McGill’s Gault Nature Reserve in Mont-St-Hilaire, about 50 kilometres east of Montreal.

Researchers want to measure the tench’s impact on a native fish — the shorthead redhorse, chosen because it’s part of the same family as the copper redhorse, a threatened species found in only a few rivers in southwestern Quebec, including the Richelieu River. Like the tench, redhorse are bottom-dwellers that feed on invertebrates.

Quebec has declared the tench to be a direct threat to the copper redhorse. The tench shares the same food sources and habitat and carries parasites that are harmful to the redhorse.

Eighteen tench and 18 redhorse were captured in Montreal-area rivers for the experiment. Researchers set up 21 bins, each holding about 700 litres of water, on a plot of land about 15 metres by 25 meters. Some of the bins will hold tench and shorthead redhorse together, others will hold only tench or only redhorse.  

The fish are being fed mollusks, including mussels and snails, in somewhat limited supplies to stimulate competition. Before the experiment began, the fish were weighed and their lengths noted. At the end of the operation, in October, the measurements will be taken again to gauge the tench’s impact on the redhorse. The fishes’ cortisol hormone levels may also be checked at the conclusion to determine how stressed the species are in each other’s company.

The researchers have learned from last year’s unsuccessful crack at the experiment when all the redhorse in the bins died during an extended heatwave. This year, to keep the bins shaded, a beige mesh tarp is draped over them. And a network of pipes is now used to add cool water if the bins get too warm.

Last year, all the tench survived the abnormally hot weather. That’s because, unlike some other fish, hardy tench can survive when water heats up and holds less oxygen.

“The tench is a very tough fish and very, very tolerant of harsh environmental conditions — that’s part of why he’s dangerous,” Benjamin said.

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Epic battles are being waged within Canada’s calm lakes and rivers, as native and invasive species vie for space and food.

Even among various types of intruders, competition is common and fierce, Ricciardi said. Take the quagga and zebra mussels. “They are like motorcycle gangs fighting for territory in the St. Lawrence. The zebra mussels are very good at coming in after a disturbance. But the quaggas are good at holding on once they get in.”

The two mussels were introduced to the St. Lawrence and the Great Lakes via ballast water from transoceanic vessels from Europe in the 1980s. They damage ecosystems, crowd out native mussels and have caused billions of dollars in damage to infrastructure such as water supply lines.

At least 180 invasive species are known to have invaded the Great Lakes. “They’ve all jockeyed for position,” Ricciardi said. “Some have waxed and waned. But I think they’re all still there or most of them are. So what happens when you keep adding invasive species? Is there a cumulative effect? Is there a synergistic effect?”

Another worry: climate change, which could bring hidden dangers to the surface.

“I’m concerned about all of those (invasive) species in the Great Lakes that have piled up over time,” Ricciardi said. “What about all those species that are considered to be low to moderate impacts? Will climate change trigger them to become triggered highly invasive?

“In other words are they like sleeper cells — waiting for something to trigger them? This is one problem with accumulating a lot of species. You’ve got a lot of potential there. You can’t assume they’re always going to stay this way.”

Which makes researching invading fish such as tench so important, he added.

“Tench can change the rules of existence for (other fish) in certain places,” Ricciardi said

He said the experiment could help answer some key questions: “What is the potential impact of tench on threatened or endangered species? And how will that impact change as the environmental context, especially specifically temperature, changes?”

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In some parts of the world, tench are an asset, not a nuisance.

They’re a popular game fish for anglers in the United Kingdom, for example. And they’re a delicacy on some Italian menus.

Avlijas tried some at a restaurant near Milan this summer. Poached in broth, “it had a very mild taste, not very ‘fishy’ and the filet was soft, but not flaky,” she said. “People have described tench as sometimes tasting ‘muddy’ but various Italian restaurant and recipe websites explain this is only the case with tench that live in areas with algae blooms or small ponds. … The ones that come from rivers or larger lakes are tastier.”

Savoury or not, when they invade new territories, tench are a menace.

In Quebec, commercial fishers on the Richelieu River looking to catch brown bullhead, for example, are collecting more and more worthless tench, Avlijas said. “For them, it has been a very large annoyance because they’re spending all this time trying to catch fish that they can sell but there’s no market for tench, which are crowding out the other fishes that they do want.”

Between 2007 and 2014, Quebec’s fisheries department monitored captures by commercial fishers in Lac St-Pierre, part of the St. Lawrence River between Sorel-Tracy and Trois-Rivières. The invasive-species-detection project found no tench in the first two years, but in the ensuing years, the number of tench caught increased exponentially.

Experts fear that in Lac St-Pierre the tench will compete with yellow perch, a once-popular commercial and sports catch in decline due to overfishing and habitat loss. The tench “just compounds the stress on the perch so they’re trying to get the population to bounce back and now a (new) competitor may undermine all that,” Avlijas said.

The tench is one of six fish on the Quebec government’s list of invasive species “of concern or potential concern.” In addition to being a threat to the copper redhorse, tench compete for food with perch and catfish.

“Tench has a harmful potential for many native species because of its high fertility rate and its adaptability, even under adverse conditions,” according to a compendium compiled by Quebec’s ministry of forests, wildlife and parks. “Tench tolerate extremely low oxygen levels and can even survive a day out of the water. This extreme tolerance allows it to colonize places too hostile for most other species.”

Quebec is also troubled by the tench’s ability to survive out of water. 

“One of the dangers is if people use tench as a baitfish,” Avlijas said. “If they leave it in the bottom of their bucket, even overnight, even in fairly little water, and then go to a different lake the next day and dump their bait bucket, the tench are going to survive.”

In Quebec, the use of any live baitfish has been illegal since 2017. The use of dead tench as bait is also prohibited, as is buying or selling live specimens of the species.

Adjacent jurisdictions are also nervous.

“Female tench may lay up to 600,000 eggs annually,” warns the Lake Champlain Basin Program, which aims to protect the lake that straddles Quebec, New York and Vermont borders. Tench were first noticed in the lake at the beginning of the 2000s.

“The tench (also) has a tendency to cloud the water where it lives by stirring up the bottom sediments. These fine sediments can suffocate the eggs and newly hatched fish of native species such as pike, perch or crappie.”

The Ontario government, for its part, has warned “tench compete with native minnows, bullheads and suckers for food and eat large quantities of snails and insect larvae. By feeding heavily on snails, which graze on algae, tench may (also) contribute to algal blooms. (And) aggressive feeding by tench stirs up sediments, leading to cloudy water.” It’s worried tench will be mistaken for other species and brought to Ontario as live baitfish.

Invasive species are a longstanding problem for the Great Lakes, where fisheries have been ravaged by zebra and quagga mussels and fish such as the sea lamprey, alewife and round gobies.

“Tench could be a bigger threat to the Great Lakes (than the St. Lawrence basin) because the Great Lakes may have more suitable habitat for tench to thrive in,” Avlijas said.

“The habitat tench prefer is wetland areas, and the Great Lakes have a fair amount of them and many of the fish species, including ones that are important for sports fishing, reproduce in wetland areas. Their young hatch in the wetland areas and so you have bass and walleye and other fish starting off their life in there.

“Tench when they become dense, they can become really, really dense. The danger is if they kind of take over and then start using up resources that other fish need, including juveniles of some of the larger fishes that people like to fish.”

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Can anything be done once an invasive species such as the tench become ensconced?

“It is very difficult to get rid of an established fish entirely,” Avlijas said.

But their impact can be eliminated or reduced by removing enough of the fish “to an acceptable level,” she added. “In the case of the tench, (that would likely) limit spread into new areas (because) tench seem to move out into new areas only when they start crowding each other out.”

Efforts can be made to stop the tench from invading the Great Lakes, she added.

Shipping locks and dams could be used to stop tench from going further upstream in the St. Lawrence. That would require testing and installing devices that discourage fish from swimming through locks, Avlijas said.

Anglers should be better educated about what they can do to help stop the spread of tench by not using them or any other invasive species as live bait, she added.

Tench can also be “fished down” in densely populated sites such as Lac St-Pierre or the Richelieu River, Avlijas said. “This could be done in collaboration with commercial fishermen who currently capture them as by-catch and re-release them into the river alive.”

What you need to know about invasive species

Invasive species can wreak havoc on ecosystems. Here’s what you should know.

What are invasive species?

They are species of animals or plants that “become established in areas outside their natural range and are capable of causing significant harm to our environment, the economy or to society,” according to the federal environment department. They are usually introduced by humans, either intentionally or accidentally. “In the absence of natural predators, (they) kill, crowd out or otherwise devastate native species and their ecosystems,” says the Ontario-based Invasive Species Centre.

Why should you care?

Invasive species are the second-biggest global threat to biodiversity, after habitat destruction, notes Quebec’s wildlife department. Invaders cause ecological damage, degrading ecosystems and causing upheaval among native animals. On the economic front, it costs $7.5 billion annually in Canada to control invasive species and deal with the damage they cause. The intruders also put at risk jobs related to the exploitation of native species.

How big is the fishing industry?

Commercial fishing and aquaculture is a major industry in Quebec. It employs about 8,000 people and the province exported almost $500 million worth of fish and seafood in 2017, Quebec government statistics show. In the Great Lakes, commercial and recreational fisheries are collectively valued at more than $7 billion U.S. annually and support more than 75,000 jobs, according to the Great Lakes Fisheries Commission. The five Great Lakes hold one-fifth of the world’s fresh surface water and are home to almost 200 species of fish.

Can invasive species be eradicated?

Once an invasive species is established, “it becomes virtually impossible to dislodge it and very expensive to control it,” Quebec’s wildlife department says. That’s why “prevention, early detection and early intervention are important to … effectively fight against alien species.”

What can the public do?

Anglers should clean, drain and dry their boats to avoid spreading invasives species. Using live bait is prohibited in Quebec, as is the use of some species, including tench, as dead bait. It’s also illegal to buy or sell tench in Quebec. Tench, known as “tanche” in French, resemble carp. Many websites can help you identify them, including the Quebec environment’s Sentinelle website, where the public can report sightings of invasive plants and animals.

ariga@postmedia.com

Additional reading

Eurasian Tench (Tinca tinca): the next Great Lakes invader, by Sunci Avlijas, Anthony Ricciardi and Nicholas E. Mandrak

Intentional Introduction of Tench into Southern Quebec, by Pierre Dumont, Nathalie Vachon, Jean Leclerc, Aymeric Guibert

La tanche, Quebec ministry of forests, wildlife and parks

Tench, Ontario government