When ocean temperatures rise, tropical fish which struggled to adapt would rather pack up and move than adapt.

As climate change causes warmer ocean temperatures, temperature-sensitive coral reef fish tend to migrate to a cooler area if given the chance, according to a recent international study led by University of Copenhagen’s Adam Habary and supervised by UT Marine Science Institute postdoctoral fellow Jacob Johansen. 

This study is the first to explore fish movement related to global warming, Johansen said.

“The question then arose, instead of sitting in a habitat where you’re essentially struggling to survive, why wouldn’t you just move?” Johansen said. 

The researchers studied a species of damselfish, called Chromis viridis, from the Great Barrier Reef that is sensitive to temperature. Researchers acclimated the fish to a range of temperatures from 73 to 91 degrees Fahrenheit for more than six weeks. They then moved the fish into a two-chambered tank where the they could swim between chambers to increase or decrease the temperature. The experiment found the fish maintained the tank’s chamber at about 84 degrees, regardless of the temperature they had been acclimated to.

“We found that, when given the chance, fish sought out temperatures that they’ve evolved to be in over thousands of years to mitigate the impact of increasing temperatures,” Habary said. “Our study thus provides a mechanistic explanation for why we see fish relocate in this era of global warming.”

According to Johansen, previous research found that fish may be able to partially adapt to warmer temperatures, but certain important functions, such as growth, reproduction and metabolism, don’t work as well. Jodie Rummer, a senior research fellow at James Cook University in Australia and one of the co-authors of the study, said the fish tended to choose the temperature at which these physiological functions work best.

“They’re choosing a temperature that’s maximizing metabolic functions, where being alive is the cheapest and most effective,” Rummer said. “All of the basic maintenance costs of being alive become much more expensive under high temperatures.”

However, changing habitats brings its own challenges, according to Johansen. For example, fish that are dependent on specific species of coral have more difficulty migrating since the coral is unable to move to cooler areas as fast as temperatures are rising. Any new area that these fish move into will probably not be devoid of competition, either.

“If you’re a fish, sitting in your habitat where you have evolved, you have your own little niche and you’re doing well, and you want to move into a new habitat, obviously … there’s going to be lots of species already living there,” Johansen said.

Rummer said the ideal temperature for the fish is 84 degrees — the average summer temperature in the Great Barrier Reef. Climate change toward the end of the century could raise this average to 91 degrees, and the reef already briefly experienced these higher temperatures this summer.

“Those upper temperatures are kind of worst-case scenarios for these fish,” Rummer said. “It really tells us a lot about some of the limitations that these populations of fish might have, not only now but into the future.”

Johansen said oceans are warming faster now than at any time in recorded human history, and that the challenges caused by fish moving into already inhabited ecosystems raises questions about competition between native and invasive species relocating due to ocean warming.

“It’s a very complex question and it’s definitely not necessarily good news that a species can survive by moving, obviously for [those] other than that species.” Johansen said. “It’s a cause for concern at the same time as being a little glimmer of hope, but it’s something that needs to be studied much, much more before we have a full understanding of what’s going on.”

This story has been updated since its initial publication to clarify how tropical fish adapt. Jacob Johansen supervised the study, not assisted.