Written By
A component of the Aquatic Invasive Species (AIS) program within the U.S. Fish and Wildlife Service (Service) is to monitor range expansions of non-native species. One such species, tench, has been identified by the Great Lakes and St. Lawrence Governors and Premiers as one of their “least wanted” aquatic invasive species invasive species
An invasive species is any plant or animal that has spread or been introduced into a new area where they are, or could, cause harm to the environment, economy, or human, animal, or plant health. Their unwelcome presence can destroy ecosystems and cost millions of dollars.
Learn more about invasive species because it presents a real threat to the ecology of the Great Lakes and St. Lawrence River systems. Thus, this invader species is also a priority for the Service’s Lower Great Lakes Fish and Wildlife Conservation Office (FWCO) AIS monitoring and surveillance efforts.
Tench is a freshwater fish, native to Europe and western Asia where they are an edible and popular game fish. They are a somewhat stocky, unique looking, and football-shaped fish with skin as slippery as an eel. They thrive in shallow lakes and rivers in areas of thick carpets of vegetation. Their olive green to bronze coloration allows them to blend in well in backwater habitats. In some areas, they bury themselves into mud bottoms to overwinter.
Tench have tiny red-orange eyes and rely on their barbels (whisker-like organs) or super sensors, to feel out tasty snacks and to help them navigate the dark, muddy environments. They have an incredibly diverse diet and eat fresh or decaying plant matter, algae, fish eggs, mosquito larvae, snails and other benthic invertebrates. They root around for food in the mud, increasing water turbidity which damages native fish spawning habitats. Tench also reproduce at rates much faster than native species. Spawning can begin in late spring with females releasing eggs every 15 days or so until mid-June. In one season, as many as 300,000 eggs may be produced by a single female. Thus, tench can have a devastating impact on local aquatic ecosystems by outcompeting native species for food and habitat, and swamping the local native fish population with their abundances.
Like carp, they have been used as a biocontrol of snails and aquatic vegetation in farm ponds and in aquarium tanks. Indeed, tench can tolerate water with low oxygen concentrations, even in areas where carp cannot persist. Unfortunately, tench made its way to Canada several decades ago, and to the United States.
A flooding event in the 1980’s allowed tench to escape from its stocked pond into the Richelieu River in Québec, Canada. The Richelieu River is connected to the larger waterways of Lake Champlain and the St. Lawrence River. These aquatic connections also allowed the species to spread into Lake Saint-Louis near Montreal, Quebec, Canada. Now, in the many backwaters of the St. Lawrence River around Montreal, the biomass is almost exclusively tench, a testament to their ability to take over and disrupt the ecosystem balance.
While conducting an electrofishing survey near the Wiley-Dondero Canal (hereafter Canal) in 2017, Saint Regis Mohawk Tribe staff uncovered an adult tench. This was the first detection of tench in New York waters of the St. Lawrence River, and confirmed tench was at Lake Ontario’s doorstep. The Canal is the last (upstream) structure structure
Something temporarily or permanently constructed, built, or placed; and constructed of natural or manufactured parts including, but not limited to, a building, shed, cabin, porch, bridge, walkway, stair steps, sign, landing, platform, dock, rack, fence, telecommunication device, antennae, fish cleaning table, satellite dish/mount, or well head.
Learn more about structure in the St. Lawrence River separating tench from unimpeded access to Lake Ontario. A subsequent finding of tench below the Moses-Saunders Dam in the St. Lawrence River, in the multijurisdictional waters of Canada and the U.S., led the Service’s Lower Great Lakes FWCO to become involved in the surveillance for tench at the invasion front.
The St. Lawrence River and all its pools is an incredibly expansive area to monitor; fortunately, locating the needle in a haystack is much easier with many sets of eyes. The above findings sparked the formation of the Binational Tench Working Group; members include the Saint Regis Mohawk Tribe, Fisheries and Oceans Canada (DFO), the Ontario Ministry of Natural Resources and Forestry (OMNRF), Québec Ministry of Natural Resources and Forests, McGill University, the New York State Department of Environmental Conservation, and the Service’s Lower Great Lakes Fish and Wildlife Conservation Office in Basom, NY and the Service’s New York Field Office in Cortland, NY.
Together, the working group members are surveying and monitoring tench movements annually below the Wiley-Dondero Canal, within the Canal, and in the St. Lawrence River system from pool to pool. The group shares strategies and best methods for detecting tench, shares data, and strategically fills niches across this area.
The Canal, located near Masena NY, is probably the most concerning to everyone monitoring the tench invasion because it not only allows ships to bypass the nearby dams such as the Moses-Saunders Dam, the Canal is also passable by fish and presents a direct route to Lake Ontario.
Ships up to 225.5 meters in length (twice as long as a football field), carrying up to 25,000 metric tons (2,296 school buses), are raised to heights as high as a 60-story building through the Great Lakes St. Lawrence Seaway lock and dam system, which includes the Wiley-Dondero Canal. Indeed, the Seaway and the canal system is an active marine point of entry for freighters as well as invasive species.
With the cooperation of the Great Lakes St. Lawrence Seaway Development Corporation, survey crews from the United States and Canada are able to work and sample within the Canal. Indeed, their cooperation is critical to the Tench Working Group effectiveness, and integral to surveying the very complex St. Lawrence River system of canals and pools. The Great Lakes St. Lawrence Seaway Development Corporation is an agency of the United States Department of Transportation that operates and maintains the facilities of the St. Lawrence Seaway.
The Service (Lower Great Lakes FWCO and New York Field Office) conducts monitoring and surveillance for tench in the United States and the DFO mirrors this effort in Canada. Additionally, every spring, the Saint Regis Mohawk Tribe and the Lower Great Lakes FWCO work together on the same vessel conducting surveys in and outside of tribal waters. The survey crews have been using boat-based electrofishing to collect invasive fish species and recently began collecting water samples for eDNA and metabarcoding analyses to help bolster detection of the elusive tench. Environmental DNA, or eDNA, can come from fish scales which are shed frequently, from feces, and other secretions. Each species has a unique DNA sequence, like a barcode on your cereal box. Metabarcoding allows scientists to identify fish species present in an area by looking at the DNA they leave behind.
Since 2018, the Service has conducted over 300 fish surveys in the St. Lawrence River near Massena, NY targeting tench. In 2022 and 2023, the Service also collected 130 water samples from the Canal system and found positive DNA detections below and within the Canal system. Then in 2022, the Lower Great Lakes office captured a tench downstream of the Canal during one of their electrofishing surveys, further evidence of fish just below the final lock and dam barrier to Lake Ontario. The office has since doubled the number of water samples they are collecting for eDNA and metabarcoding analyses to help monitor and track tench movement. This robust data set allows agencies to make informed decisions of where to target efforts in the future.
Surveillance efforts on the invasion front sheds light on the fish's distribution and allows for the removal of individuals that could establish populations. The vast backwaters and countless river miles that make up the St. Lawrence River highlights how necessary multi-jurisdictional collaboration is when it comes to invasive species management and control within these dynamic and large geographical areas. The cumulative support and efforts of all the working group partners has resulted in coverage of the river that no one agency could achieve alone. An important question the working group seeks to answer is have tench made it above or upstream of the Wiley-Dondero Canal? The answer we hope for is NO. The Lower Great Lakes FWCO and the Binational Tench Working Group, therefore, will continue to survey and monitor these areas because the earlier tench is detected, the easier it will be to remove and eradicate.
