Warming temperatures are affecting thousands of lakes across the province and making life difficult for species that live beneath the waves — including the iconic lake trout, one of Ontario’s most popular sport fish.

“It’s water temperatures and oxygen that really limit lake trout,” says Sapna Sharma, associate professor of biology at York University and director of the Sharma Laboratory. “With climate change, water temperatures are generally becoming warmer and oxygen levels are becoming lower, and that can stress lake trout populations.”

According to a 2009 paper in Ecology authored by Sharma and two others, lake trout populations in more than 1,600 lakes in Ontario could be vulnerable by 2050, based on climate-change projections. By 2100, there could be as many as 9,700 vulnerable populations across Canada.

And warmer water isn’t the only issue that lake trout have to deal with — the effects of climate change have also made the province’s lakes more hospitable to smallmouth bass, which can out-eat the lake trout, leaving the struggling fish without enough food to survive.

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Smallmouth bass are native to North America, but over the past 100 years, they’ve spread further north in Canada. While they lack the ability to jump from one lake to another, they can travel by river between bodies of water. “They’re moving northward at a pretty rapid rate,” Sharma says. “They’re voracious predators — they eat a lot of the minnows. They can eat a lot of things.”

One of the lake trout’s most important sources of food is the cisco, or lake herring, a coldwater species that’s also threatened by warming temperatures. Smallmouth bass consume cisco in large quantities, which means there are fewer available for lake trout to eat.

Lake trout are what researchers call a bellwether species — that is, when they’re in trouble, it’s an indication that the water quality of the lakes they live in is dipping, Sharma says.

The fish already occupy Ontario’s northernmost lakes, meaning that when warming temperatures push lake trout out of their habitats in the south, they have nowhere else to go. The lake trout’s range is rolling up like a carpet.

“There are very few populations left in the U.S., because it’s become too warm for them,” Sharma says. “Most of the lake trout populations are in Canada and a large portion of recreational fisheries and tourism. People come to Ontario and Canada to fish for lake trout. It contributes quite a lot socioeconomically.”