Rising river temperatures challenge fish survival

Finland’s rivers are getting warmer, with heatwaves becoming more frequent and more intense. According to recent study by Natural Resources Institute Finland (Luke) any of our fish populations are becoming stressed by the high temperatures, which directly impact the metabolic rate of individuals and leads to an increasing demand for oxygen. If fish cannot increase their uptake of oxygen as this happens, they experience physiological stress. This can cause them to stop spawning, growing, and feeding, and makes them easier for predators to catch. In severe cases, the stress can lead to death.

“Atlantic salmon are especially vulnerable to heatwaves as a cold-adapted species”, says Lucy Cotgrove, research scientist at Luke. Juvenile salmon may remain in rivers for several years before migrating to the sea. This means they are exposed to repeated heatwaves and prolonged warm periods during summer.

Juvenile salmon struggle during prolonged heat exposure

At Luke’s Laukaa Hatchery, researchers studied how juvenile salmon cope with longterm exposure to warmer water. Salmon were kept at 16 °C (typical for summer) and then exposed to a 10day heatwave reaching 22 °C. The results showed that juvenile salmon could not increase their maximal oxygen uptake enough to handle the heat. This suggests that they would not be able to meet the oxygen demands of different physiological processes, such as feeding or growth or recovery from stress, for which oxygen requirements increase with temperature. In fish farms, reducing the exposure of fish to extreme temperatures is also crucial, as warmer water causes stress, slows growth and feeding, and can increase disease risk.

Shaded rivers may protect future salmon populations

There is good news: river restoration work is increasing across Finland. Many of these projects will create more cold-water refuges where fish can escape rising temperatures. By allowing rivers to curve naturally, reducing nutrient runoff, and adding different sizes and kinds of substrate, we help form cooler pockets of water. Planting trees and leaving deadwood along riverbanks also provide shade, preventing the water from heating up in the sun.

Understanding fish physiology helps guide conservation

Studying fish physiology is essential for predicting how salmon will respond to future heatwaves. This knowledge helps guide environmental management and conservation efforts. “It is important to remember that climate change does not only involve extreme heatwaves that cause sudden fish deaths. Long periods of moderate warming can also harm fish, even if the effects are not immediately visible. Understanding these impacts now gives us time to protect salmon populations before the situation becomes critical”, Cotgrove says.