"Our studies found that one species of fish could not even survive in water just 3 degrees Celsius [5 degrees F] warmer than what it lives in now," said lead study author Jodie Rummer from the ARC Center of Excellence for Coral Reef Studies at James Cook University.
Climate change places equatorial marine species most at risk as oceans are projected to warm by 2 to 3 degrees Celsius by the end of this century, threatening many species that only experience a very narrow range of temperatures over their entire lives, she said.
"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."
Warmer temperatures would limit activities crucial to a species' survival, such as evading predators, finding food and generating sufficient energy to breed, the researchers said.
That could lead to declines in fish populations as species may move away from the equator to find refuge in areas with more forgiving temperatures, Rummer said.
"This will have a substantial impact on the human societies that depend on these fish," she said, noting that concentration of developing countries lies in the equatorial zone where fish are crucial to the livelihoods and survival of millions of people.