As tench continue to spread through the St. Lawrence River, a study from McGill University provides fisheries managers with guidance on how to detect the invasive species, an essential first step in preventing it from reaching new waters. The findings can inform efforts to contain and manage existing populations, helping agencies determine how and where to carry out removal or control activities.
The researchers, who compared the effectiveness of common sampling tools, found that gill nets and fyke (cylindrical) nets captured the largest number of tench, while gill nets and electrofishing had the highest probability of detecting the species in the shallow, vegetated habitats where they live.
Tench, a Eurasian species, was introduced to Quebec in the early 1990s. There are now dense populations in Lake St. Pierre, and the species is expected to continue moving toward the Great Lakes without coordinated early detection and containment measures.
Detecting the species at the "invasion front" has been a challenge for agencies on both sides of the Canada-U.S. border. Because managers need reliable ways to locate tench before attempting removal or long-term suppression, improving detection methods is a critical early step in developing targeted control strategies.
The study, conducted in collaboration with Fisheries and Oceans Canada, tested four types of fishing - with gill nets, fyke nets, seines and electrofishing - during daytime and nighttime sampling across two vegetated bays.
Lead author Christophe Benjamin, who completed his MSc in Biology in 2022 and now works with Fisheries and Oceans Canada (DFO), a federal agency as a aquatic science biologist, said the results offer managers clear, evidence-based options.
In areas with at-risk species, such as copper redhorse, fyke nets may be preferable, he said.
"Every time we're sampling we're impacting the environment, so what we try to do is actually find a balance," he explained. "With fyke nets, even though efficiency is a little bit lower, you actually reduce the overall impact."
The team also analyzed a second dataset collected along the St. Lawrence and Richelieu rivers to look for species that tend to occur where tench are present. They identified positive associations with six native fish, including emerald bowfin, pumpkinseed and largemouth bass, suggesting their locations could help guide where managers focus their efforts. This information can support the design of focused sampling and removal programs in places where tench may be newly establishing.
Together, these findings build on two other recent studies from the same lab. One examined how tench respond to different substrates and temperatures, showing the species can thrive in a wider range of habitats than expected. Another compared feeding efficiency between tench and native fishes, suggesting tench could compete with resident species in productive wetlands. All three studies point to a species that is adaptable, mobile and likely to continue spreading unless early detection and active management improve.
Benjamin said he hopes the work will support agencies as they plan for a potential arrival in the Great Lakes.
"Our goal was to help managers choose the right tools," he said. "If you're trying to monitor or contain tench, you need a method that actually works."
About the study
Optimal methods for sampling Tench (Tinca tinca) in its introduced range by Christophe Benjamin, Anthony Ricciardi, et al,, was published in Management of Biological Invasions
