Rummer added that many species in that region only experience a very narrow range of temperatures over their entire lives, and so are likely adapted to perform best at those temperatures.
This means climate change places equatorial marine species most at risk, as oceans are projected to warm by two to three degrees Celsius by the end of the 21st century, researchers said in a press release.
"Such an increase in warming leads to a loss of performance," Rummer said. "Already, we found four species of fish are living at or above the temperatures at which they function best."
For the study, an international team of researchers studied six common species of fish living on coral reefs near the equator. They measured the rates at which fish use oxygen, the fuel for metabolism, across different temperatures -- at rest and during maximal performance.
Based on their results, at warmer temperatures fish lose scope for performance. In the wild, this loss would limit activities crucial to survival, such as evading predators and finding food. It would also affect their ability to breed.
Researchers said that because "many of Earth's equatorial populations are now living close to their thermal limits," there are dire consequences ahead if these fish cannot adapt to the pace at which oceans are warming.
Rummer predicts that there will be declines in fish populations as species may move away from the equator to find refuge in areas with more forgiving temperatures.
"This will have a substantial impact on the human societies that depend on these fish," she said.
Papua New Guinea and Indonesia are a few of the developing countries that lie in the equatorial zone, where fish are crucial to the livelihoods and survival of millions of people, researchers said.
Researchers said understanding the link between an organism and its environment is crucial to "developing management strategies for the conservation of marine biodiversity and the sustainable use of marine fisheries."
"This is particularly urgent when considering food security for human communities," Rummer added.
