Some fishes can make themselves at home anywhere, with several species thriving in subzero Antarctica while others live in temporary tide pools. Within a single species, some populations are locally adapted to particular habitats based on what they've become accustomed to over the years. However, not all species show this flexibility. To make predictions about how fish populations will change with global warming, scientists are trying to understand which fish can adapt to the heat and why.
While rainbow trout are generally thought of as a cold-water fish of the Pacific Northwest, a specific population has thrived in the desert of western Australia for almost 50 years. Isolated from their original population of the San Francisco Bay area, these fish have been used to stock natural lakes, and have undergone a few traumatic heat events during their time in Australia. More than once, summer temperature extremes have killed many fish that couldn't handle the heat, unintentionally selecting for fish with impressive upper temperature limits. Olivia Adams from the University of British Columbia, Canada, with colleagues from the same university, Harvard University, USA, the University of Western Australia and Aquatic Life Industries in Perth, Australia, wanted to investigate if a lengthy exposure to warm temperatures would improve this population's already remarkable ability to withstand extreme heat.
To answer this question, the team raised groups of 1-year-old trout at six different temperatures ranging from 15 to 25°C for at least 1 month. Typically, to understand how fish cope with increased temperature, scientists focus on one or a few measures, such as growth or upper thermal limit, in a single study. But in the most comprehensive study to date, Adams and colleagues were able to look at how temperature affected the growth, energy it takes to digest a meal, physical fitness, upper temperature limit, ability to withstand low oxygen, and heart rate of these animals raised at 15, 17, 19, 21, 23 and 25°C.
The Australian rainbow trout showed off their skills, with just about every aspect of their performance peaking in the groups of fish raised at 17–23°C. At these temperatures, fish had the best growth, used the least energy to digest dinner and, when exercised, were the most physically fit. These fish could even endure temperatures up to 31°C before toppling over (which is how scientists typically measure a fish's upper thermal limits). Impressively, the fish raised at 25°C were still able to grow and could also handle temperatures up to 31°C before losing their balance, but they began to struggle with digestion and fitness, and couldn't tolerate low oxygen as well as the other groups. And when the scientists tested how the fish hearts handle a brief heat exposure, they actually started to fail at 27°C, well below the temperature at which they topple over. While fish raised at 25°C show some remarkable performance in growth and increased upper thermal limits, they are noticeably limited in other ways. By using a variety of tests, Adams and colleagues presented detailed evidence that this population optimally perform at 21–23°C, which is an incredible feat when rainbow trout are thought to do best below 20°C.
While clearly superstars, Australian rainbow trout aren't the only trout population that can handle the heat. Their cousins, redband rainbow trout, are geographically isolated in the desert region of the Pacific Northwest, where they regularly encounter summer temperatures between 19 and 29°C. Adams and colleagues suggest that rainbow trout may naturally carry the genes that they require to tolerate high temperatures. With a combination of artificial and natural selection, populations like the Australian rainbow trout have been able to tap into that latent heat-tolerant talent and give us a bit of hope for the future in a warming climate.