Invasive fish species dumped in Manitoba water bodies disrupting ‘very delicate’ ecosystems: fisheries manager
Along the banks of Hunt Lake in the south of Whiteshell Provincial Park, the largemouth bass stalks in the water, threatening the survival of other fish in the ecosystem.
But this competitive, aggressive predator wasn’t always part of Hunt Lake. Anglers first spotted the freshwater fish there in 2023 — and unlike other species, the largemouth bass was neither native to the ecosystem nor brought to the lake through provincial fish farming programs.
Instead, it’s there because people have put it into the lake illegally, said Eric Mullen, a provincial regional fisheries manager responsible for eastern Manitoba.
Those kinds of illegally introduced species have had what Mullen calls an “adverse impact” on native fish populations, which have since declined in numbers and become less healthy.
And that effect, in turn, creates a problem for anyone who relies on those native fish species for economic, cultural or traditional reasons, Mullen said.
A rather small number of largemouth bass might not create problems immediately. But as those and other invasive species propagate, they turn into a source of competition for other species that had thrived in this ecosystem until then.
“All of a sudden, you introduce this new player to the game, and they have to carve out their own niche, their own food source,” he said. “But the base of food that exists in the system isn’t getting any bigger.”
In Hunt Lake, the survival of the bass has come at the expense of walleyes and lake and brook trout inhabiting the ecosystem, which have been falling behind now that there’s a more efficient predator in the food chain, Mullen said.
He said people might be motivated to illegally plant certain fish in Manitoba’s bodies of water because they want to be able to fish for certain species — even if that puts biodiversity and conservation at risk.
“There’s a balance that exists within those ecosystems and sometimes is very delicate,” Mullen said. “And throwing any new predator in there can completely mess that up.”
Provincewide problem
Over the past 80 years, anglers and biologists have been working with the province on fish stocking programs to carefully build the fish supply Manitoba has today, said Chase Dreilich, a recreational angling co-ordinator at the Manitoba Wildlife Federation.
However, some anglers and fishers across the province have taken stocking into their own hands, he said, introducing species to ecosystems that were not planned or prepared to host them in the first place.
“It is an increasing concern and an increasing issue across the province,” Dreilich said.
Recent data provided by the province, based on surveys and current knowledge on reported incidents, shows at least 21 lakes, ponds and pits have been illegally stocked in Manitoba recently.
A common characteristic among these water bodies is their easy access by vehicle and the lack of interconnectedness with other sites that might harbour popular sport fish species, the province said.
Mullen, who supplied the data, said given how widespread the problem has become, the true number of bodies of water harbouring illegally introduced species may be even higher — and it goes beyond just the largemouth bass.
Research into illegal stocking in Manitoba has also found the addition of fish like crappies and sunfishes, which do “very well” in warm water habitats. And as temperatures rise because of climate change, those kinds of invasive species are able to propagate in lakes and rivers where they didn’t before.
“When you throw the illegal stocking problem on top of that, you get these really big artificial range expansions,” Mullen said. “Species that only existed as far north as Winnipeg now exist as far north as Thompson.”
Bodies of water connected by rivers also allow invasive species to expand into ecosystems beyond the one they were originally planted in.
“We don’t want this to continue,” Mullen said, adding once invasive species are established in an ecosystem, it’s “extremely difficult” to suppress or control them.
Recreational fishing regulations prevent fishers in Manitoba from transporting live fish away from the water bodies where they were caught, except under a permit issued by the Fisheries Branch. However, angling co-ordinator Dreilich said, it’s hard for authorities to enforce these rules and sample all water bodies for illegal stocking across the province.
That means it’s on anglers to stop invasive fish species from making it into Manitoba’s bodies of water, by not introducing them to begin with and reporting any species not listed as part of an ecosystem when they spot them, Dreilich said.
“That knowledge of what’s going on is a big help,” he said.
While strategies can be created to “adapt” to these new species, Mullen, provincial regional fisheries manager, said the only effective solution to protect the balance and biodiversity of the existing ecosystem is by stopping invasive species from getting into the water in the first place.
“The number one cure is prevention,” he said.
A solution in Montana
One U.S.-based scientist said he’s come up with a way to do just that, by creating a system to guess which bodies of water are most likely to be targeted for the “growing” practice of illegal fish stocking so authorities can take steps to protect them.
Sam Bourret, a biologist with Montana’s Fish, Wildlife and Parks department and the author of two studies on illegal stocking, said he came up with the idea after discovering a state-run dataset with information on Montana’s illegal stocking incidents from over 500 water bodies.
By identifying the common threads between those incidents — which often involved bodies of water that were easily accessible because they were close to roads or at low elevations — Bourret said he was able to do what he called a “vulnerability analysis” to determine which 10 lakes were most likely to be the future site of illegal stocking in Montana.
That in turn led to work with law enforcement and public outreach to help prevent people from dumping invasive fish species into those waters, he said, adding Manitoba should create its own database of illegal stocking incidents so it can do the same thing.
Manitoba’s fisheries branch currently has a list of the lakes confirmed to have been illegal stocked with sport fish species.
When a site is suspected to have been illegally stocked, the province said stakeholder groups are alerted so its members can be informed on the consequences of this practice for the ecosystem and fisheries.
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