Lake trout are known for retreating into cold, deep water during summer. But beneath winter ice, some swim surprisingly close to the surface while others remain far below.
A new study led by U of T Scarborough researchers, published in Proceedings of the National Academy of Sciences , suggests a simple explanation: the fish tend to seek water around 2°C, and a lake's shape helps determine where that temperature occurs.
The findings offer a new way to predict winter habitat for the iconic Canadian fish, filling a gap in understanding about how they live throughout the year and how they may respond to changing ice conditions caused by climate change.
"We know very little about what lake trout and many fish in general do over the winter," says Mathew Wells, a professor in the Department of Physical and Environmental Sciences at U of T Scarborough, who co-authored the study with postdoctoral fellow Lidi Shi.
"Understanding how species like lake trout use temperature to select their habitat is critical to understanding their vulnerability to climate change."
A winter temperature sweet spot
Summer habitat is relatively straightforward for lake trout. They move below warm surface water to find colder, more suitable conditions. Winter reverses this pattern. Water is densest at about 4°C, allowing relatively warm water to settle below colder water just beneath the ice.
But temperature differences depend on the lake. In wide, relatively shallow lakes, wind can stir cold surface water through much of the lake before freezing. In deeper, sheltered lakes, wind is less able to mix the water, allowing deeper water to retain more warmth.
As a result, lake trout seeking similar temperatures can occupy dramatically different depths when lakes freeze over.
In sheltered lakes, including those in Algonquin Provincial Park, the fish were generally found in shallow water. In Lake Simcoe, a large, wind-exposed lake, lake trout generally occupied deeper water.
"What's exciting is that there are patterns lake trout follow in the winter and those patterns can be understood by looking at the shape of the lake," says Shi, who led the study.
The relationship held even after researchers accounted for how much water was available at different depths, strengthening evidence that lake trout were selecting a particular temperature zone.
Wells says the temperature preference was unexpected, and why lake trout favour it remains an open question. One possibility is that it helps the fish conserve energy, but he adds further biological research is needed to explain it.
Listening beneath the ice
The work brought together new technologies that allow researchers to track fish through a season that has traditionally been difficult to study.
The research drew on measurements from 11 Canadian lakes. Lake trout were fitted with small electronic tags that transmit acoustic signals to underwater receivers, identifying individual fish and recording their depth. Temperature sensors suspended at different depths provided a picture of the surrounding water.
Combining those measurements allowed researchers to connect fish behaviour with physical conditions beneath the ice.
"We know where the fish were every five minutes to within about half a metre horizontally and within 10 centimetres vertically, so the information is very accurate," says Wells, who studies how water moves and temperatures change in lakes.
Understanding pressures on lake trout
Lake trout are an iconic predator in Canada's cold-water lakes, supporting Indigenous, recreational and commercial fisheries while serving as an indicator of lake health.
The findings suggest some lakes offer the fish less space during winter.
Because lakes generally narrow toward the bottom, a deeper preferred temperature zone can concentrate fish into less space, potentially increasing competition for food.
Using lake shape to model conditions across Canada and Alaska, the researchers estimated that 73 per cent of roughly 66,500 known lake trout lakes have warmer winter conditions associated with shallower habitat. Changing wind patterns and freeze-up timing could also alter winter temperatures and habitat.
The approach could help resource managers assess how warming and changing ice conditions may affect winter habitat and identify pressures that summer-focused assessments overlook.
"This serves as a starting point to help us understand the pressures that fish might face in different lakes at different times of the year," says Wells.
The research received support from the National Sciences and Engineering Research Council of Canada.
