Arctic lake trout can't take the heat

Fish living in areas such as the central Canadian Arctic experience a wide range of temperatures. Colder temperatures, e.g. 0°C, can slow metabolism and heart rate, and the heart can be more sensitive to adrenaline. However, the heart needs to pump faster or with greater force in warmer temperatures, e.g. 18°C, and this can be triggered by adrenaline. The lake trout, Salvelinus namaycush, a common species in the Canadian Arctic region, is active in the winter and experiences major temperature fluctuations. Individual trout may have different adaptations in order to thrive in colder or warmer water temperatures as fish cannot maintain their body temperature like humans. Therefore, Emily Williams and colleagues from The University of New Brunswick, Canada, wanted to see whether trout that are used to winter temperatures and those used to summer temperatures would show differences in their natural and maximum heart rate.To do so, Williams and colleagues caught fish from lakes in Nunavut, Canada, at the end of winter and the end of summer. Next, to see whether there was a difference in heart rates between summer and winter trout, the researchers placed electrodes over each fish's heart to measure their heart rate and electrical activity, much like an electrocardiogram. They also wanted to see whether heart rate would change in warmer water, so they warmed the water the fish was in at a rate of 6°C per hour until the heartbeat became irregular, which is known as the maximal heart rate just prior to heart failure.The heart rates of cold fish are usually slower than those of warm fish, so the team expected to record this; however, they found no difference in heart rate between the summer and winter fish. This species does not alter its heart rate based on seasonal temperatures, unlike most other fish. Also, the team expected that the hearts of the summer trout could withstand higher temperatures, similar to those that they would face in their environment, and keep beating regularly to a higher maximal heart rate than those of winter trout, but this was not the case either. In fact, both winter and summer trout reached maximal heart rate at about 18°C, which, alarmingly, was similar to the peak water temperatures in Canadian Arctic lakes in 2023. However, the winter trout were more sensitive to the hormone adrenaline, which speeds up the heartbeat, and means that it was easier for them to raise their heart rates in warm water and may prevent the heart rate from getting too low in winter.In summary, summer lake trout barely increase their maximal heart rate when in hotter water temperatures, which is concerning, because they are facing increasingly hotter water temperatures as climate change continues. As fish hearts begin to fail at temperatures a little higher than 18°C, these trout may be vulnerable to warming of their native lakes caused by climate change. Ultimately, the heart of the matter is that Arctic lake trout can't take the heat.