Thermal shelters help trout beat the heat in hot weather
What do fish do when the water gets too hot?
As climate change makes heat waves more common around the world, including here in Connecticut, understanding this question has become even more important.
Cold-water fish, such as trout, have a strategy to survive extreme temperatures: moving to thermal refuges. Thermal refuges are areas in a river where cold water drains into warmer stream water, creating a “cold water patch.” This can be caused by groundwater seeping into the stream in certain locations or even when a smaller, well-shaded stream joins a larger river, creating a plume of cold water.
Jason Faucon, professor and chair of the Department of Natural Resources and Environment in the College of Agriculture, Health and Natural Resources, Ph.D. Student Christopher Sullivan studies trout in thermal refugia along the Housatonic River to better understand this phenomenon.
Their latest findings were published in the journal Ecosphere and found that not all thermal shelters are created equal.
“What’s really interesting about thermal refugia is that a few years ago, people envisioned them as just cold-water areas,” Sullivan says. “But what we are finding is that their composition and other characteristics, as well as long-term weather patterns, tend to be important determinants of how (and how many) fish use them.”
The ideal thermal refuge is large, deep, and at a temperature well below the ambient water temperature. Using underwater cameras, the researchers observed that these refuges had the highest abundance of fish during the summer heatwave of 2022.
During the heatwave, main river temperatures reached 30°C (or 86°F) during the day. Trout comfortably tolerate water temperatures up to 17°C, or 63°F, but must use thermal refuges where temperatures exceed 23-24°C, or 73-75°F.
The temperature of the refuges was, on average, one to three degrees Celsius cooler than the surrounding river water, and ranged from half a degree to six degrees Fahrenheit. The roof areas of the shelters ranged from small to fairly large (11 to 352 square metres).
The researchers also found that river gradient, the extent to which the water surface elevation decreases within the main river near the thermal refuge, is also important. A river with a higher gradient creates more turbulent water, which may be less attractive to heat-stressed trout.
The study found that fish abundance inside thermal refuges fluctuated significantly during a heatwave, suggesting that fish were leaving the refuge more often. Given that thermal refugia tend to have very little trout prey, Sullivan explains that they may leave to forage in the main river.
“It supports the hypothesis that fish can and will leave the refuge even though prevailing temperatures are unsuitable,” Sullivan says. “The refuges are likely necessary for survival, but the fish may not be as trapped there as one might initially think. But over time with climate change, the proverbial noose may tighten.”
In another study published in the journal Ecology and Evolution, the team looked at which terrestrial predators visit and feed in refuges, which also affects how fish use the refuges.
Observed terrestrial predators include foxes, coyotes, cats, great blue herons, common mergansers, river otters, raccoons, bald eagles, and osprey, as well as people who fish, kayak, and visit refuges.
Researchers included humans as part of this list because, like other animals, we influence the ecosystems with which we interact.
In fact, there have been problems with poaching in these refuges. Even those who do not hunt can impact these important areas. Fish will leave the shelter if they believe there is a predator nearby. When people come to see the fish, the fish may interpret them as predators and flee the refuge, moving to warmer waters.
“People coming down and watching the thermoregulation of fish in the refuges could have some indirect effects,” Sullivan says. “My idea was that by measuring the rate at which people visit shelters, in addition to hunters, it could give an indication of the magnitude of these indirect effects.”
Sullivan says he was surprised by the number of people who visited the refuge to observe the fish, a fact that could enhance future outreach and education programs.
“This is a well-known phenomenon that can be used as a tool to understand the importance of fisheries management and habitat conservation,” says Sullivan.
Sometimes, hundreds of fish congregate in the shallow waters of the refuges, making them vulnerable to predation. However, researchers have observed very few instances of actual predation in refugia. Of 2,500 observations of terrestrial predators visiting refuges, there were only 22 predation attempts.
“For me, this is the really interesting part of the study,” Sullivan says. “There were some cases (of predation). But much less than we expected.”
Although not as many fish were eaten as expected, the consequences of having to leave the refuge repeatedly to avoid being eaten could be significant, Sullivan says.
These studies support the need to protect these refuges and conduct research on the effectiveness of protection strategies.
“We have known for some time that thermal refuges are important for trout survival in many Connecticut watersheds, but this project helps the fisheries community, managers and anglers alike, focus on understanding how to maintain this limited thermal habitat for trout in Our region. “The rivers are warming,” says Faucon.
This work relates to CAHNR’s strategic vision area that focuses on enhancing adaptation and resilience in a changing climate.