In the first analysis of how vulnerable the world's freshwater and marine fishes are to climate change, researchers at the University of Washington (UW) have found that sensitivity to temperature changes varied greatly between ocean-dwelling and freshwater fish.

　　Using physiological data to predict how fish species living in oceans and rivers will respond to warming water temperatures in different regions, the researchers reported in Nature Climate Change that marine fish in the tropics and freshwater fish in higher latitudes of the Northern Hemisphere were the most at risk when water temperatures warmed.

　　With years of data and relying on the fact that many fish species are taxonomically related and tend to share the same thermal limits, the researchers were able to predict the breaking-point temperature for close to 3,000 species. Regional patterns then emerged when those data were paired with climate-model data predicting temperature increases under climate change.

　　For example, fish in the tropical oceans are already living in water that is approaching the upper range of their tolerance, according to the researchers. They might not have much wiggle room when temperatures increase slightly. By contrast, in freshwater streams in the far north, fish are accustomed to cooler water temperatures but have much less tolerance for warming waters.

　　Since the effects of climate change are acutely felt in high latitudes, this doesn't bode well for fish in those streams that have a small window for survivable temperatures.

　　Fish will either migrate, adapt or die off as temperatures continue to warm, the researchers explained.

　　Given past evolutionary rates of critical thermal limits, it is unlikely that fish will be able to keep up with the rate at which temperatures are increasing, said senior author Julian Olden, a UW professor of aquatic and fishery sciences. The ability to move, then, is imperative for fish that live in the most critical areas identified in this analysis.

　　Dams and other infrastructure may block fish from getting where they might need to be in the future; fish ladders and other means to allow fish to circumvent these barriers could be more readily used, although the effectiveness of these structures is highly variable. And actions to restore vegetation along the edges of streams and lakes can help shade and reduce water temperature for the benefit of fish.

　　"Nowhere on Earth are fish spared from having to cope with climate change," Olden said in a news release from UW. "Fish have unique challenges - they either have to make rapid movements to track their temperature requirements, or they will be forced to adapt quickly."