Fish adapts to hotter water but would rather move

Coral reef fish, which struggle to adapt to warmer ocean temperatures brought about by global climate change, may instead opt to relocate to cooler parts of the ocean.

“When fish have to adapt to increased temperature, there are physical consequences. They may not be able to handles stress, or reproduce, or even grow,” says marine scientist Jacob Johansen of the University of Texas at Austin. “But, when they seek out temperatures that they’ve evolved to be in over thousands of years, they can mitigate the impact of increasing temperatures and not sacrifice critical physiological processes.”

In experiments using a common coral reef fish, the blue-green damselfish (Chromis viridis) was acclimated to 2-4 degrees Celsius above their normal summer temperatures over a 27-week period.

Fish that were acclimated to the highest temperatures lost 30 percent of their body weight and some of them died, according to lead author Adam Habary of the University of Copenhagen.

“But we found that, when given the slightest chance, fish can seek out temperatures that they’ve evolved to be in over thousands of years, to mitigate the impact of increasing temperatures and not sacrifice critical physiological processes,” says Johansen.

Marine fish are faced with a tough decision. The ocean is warming at a faster rate than occurs on land, because oceans operate as sinks—trapping in the heat. Many ocean fish will need to adapt or move to avoid death.

This fish mimics the parents of its prey

Most prior research has focused on the capacity for animals to adapt to increasing temperatures, given that animals have adapted to changes in temperature in the past. However, previous adaptations happened at evolutionary timescales, roughly one degree Celsius temperature increase per million years. Current predictions for rising temperature are much greater, with sea surface temperature predicted to increase by 2-4 degrees Celsius by the end of the 21st century.

Instead of looking into how fish can adapt, the new study in Global Change Biology took a different approach by asking, what if fish moved? In fact, what if entire ecosystems were capable of moving to the cooler temperatures, towards the poles or in deeper water?

There is already evidence that many coral reef fish and pelagic fish, like tuna, have moved in response to warmer ocean waters. The researchers found evidence that this also might occur with blue-green damselfish, and they stressed the need to investigate more fish species, including commercial fish species that economies rely upon.

“This study shows, that there is a mechanistic explanation for why fish may move, faced with a choice, and now we have a way of testing it,” Johansen says.

Ripple effect as fish opt for cooler water?

Picking up and moving may not be the silver bullet for some species, particularly coral reef fish which are dependent on reefs relocating, too. Blue-green damselfish have a very small range, unable to go more than 60 feet in radius from their coral reef home. Corals cannot move pole-ward as fast as the temperature increases are predicted; if fish do not adapt, reducing critical processes like reproduction, they will have to move to deeper waters where living conditions are less than ideal.

In addition, ocean warming does not occur as a steady slide upward on the thermometer. It often occurs with more severe and frequent heating events. This has already been shown in the Great Barrier Reef, when an El Niño caused the temperature to rise above the thermal tolerance of the animals, causing a massive fish kill and widespread coral bleaching.

“It’s these transient periods that are causing the most damage,” Johansen warns.

Funding came from the Generalmajor J.F. Classen Foundation, Frøken Ellen Backe & Margaret Munn Tovborg Jensens foundations, as well as the Familien Muller-Geiels foundation to A.H. and an ARC Super Science Fellowship and infrastructure and research allocation from the ARC Centre of Excellence for Coral Reef Studies at JCU.

Researchers from the University of Copenhagen and James Cook University collaborated on the study.

Source: University of Texas at Austin

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Source: Futurity