Zebrafish are unlikely to evolve fast enough to adapt to rising water temperatures caused by climate change, leaving them vulnerable to deadly heat waves predicted in the future.

A new study in the Proceedings of the National Academy of Sciences is among the first to investigate how quickly fish and other vertebrates adapt to heat. Its authors uncovered evidence of a hard limit to zebrafish’s ability to expand their heat tolerance, a trait found in some other animals by previous research.

The scientists captured about 2,300 of the freshwater, minnow-like fish from several sites near India. Zebrafish, an often-studied species native to South Asian rivers, was chosen because it lives only a few degrees below its heat limit in the wild, said Rachael Morgan, the study’s lead author and a postdoctoral researcher at the University of Glasgow.

“Zebrafish are already living very close to their maximum thermal tolerance in the wild already,” said Morgan, who studies the effects of rising temperatures on fish, “so it makes them a very good species to study whether or not they can evolve, because they're probably already reaching temperatures very close to those limits in the wild.”

Morgan and her colleagues at the Norwegian University of Science and Technology selectively bred the fish with the highest heat tolerance with each other. Six generations and more than 20,000 zebrafish later, the researchers found that their upper limit for high temperatures increased by about 0.04°C (0.07°F) per generation.

Although Morgan has said it is encouraging that the fish can evolve greater heat tolerance, it is most likely too slow to keep up with the more frequent and intense heat waves predicted for India and other regions of the world.

Her team also found that the change in the zebrafish’s heat tolerance slowed with each successive generation, implying that there is a strict biological “ceiling” to the temperature of waters they can inhabit.

Previous research has observed similar upper limits in other species. A 2016 study of European perch found that the fish living near a Swedish power plant that warmed nearby waters by 5-10°C (9-18°F) did not evolve any better heat tolerance than control perch. As for cold-blooded, terrestrial animals, a 2012 scientific review noted some evidence of heat-tolerance increases but acknowledged “limited potential” for these creatures to adapt to the global warming expected for the remaining century.

“There is likely to be genetic variation for heat responses in populations, but selection and heritability experiments suggest that upper thermal limits may not increase much,” its authors wrote.

Morgan said her team’s results cannot be directly generalized to other tropical fish but noted that some other species are also running up against their thermal limits in the wild. Marine heat waves have recently decimated the Great Barrier Reef and many other coral-reef ecosystems around the world, and spikes in temperature in Australia caused widespread deaths of bats alongside historic bushfires.

The insufficient thermal adaptation found in the zebrafish study is another reminder to reduce carbon-dioxide emissions as quickly as possible, according to Morgan. The heat stress from warming oceans will be exacerbated by other human activities, such as overfishing, pollution and deforestation in freshwater habitats, she said.

“These are all aspects which could make the effect of the temperature even stronger, or if they're already vulnerable, this is an additional stressor,” Morgan said. “So by removing some of those additional stressors, that could also help the species overall.”

Moving forward, the Glasgow postdoc said her lab is interested in investigating exactly how the fish’s genes and surroundings create a greater tolerance to heat. A similar experiment could also study how zebrafish adapt to the different stress of rising average temperatures, rather than extreme temperatures.

The article, “Low potential for evolutionary rescue from climate change in a tropical fish,” was published Dec. 29 in Proceedings of the National Academy of Sciences.

The authors of the study were Rachael Morgan, University of Glasgow and Norwegian University of Science and Technology; and Mette Finnøen, Henrik Jenson, Christophe Pélabon and Fredrik Jutfelt, Norwegian University of Science and Technology. The lead author was Rachael Morgan.