When fish would experience 'mountaineering effect'

Global warming would result in warmer water temperatures that would in turn speed up the aquatic animals' metabolic need for oxygen.

But the warmer water will hold less of the oxygen needed to fuel their bodies, similar to what happens at high altitudes, warns a new University of Washington (UW) study.

It's the combination of physical exertion and lack of oxygen at high altitudes that creates one of the biggest challenges for mountaineers.

Researchers said the same principle will apply to marine species under global warming.

"If your metabolism goes up, you need more food and you need more oxygen. This means that aquatic animals could become oxygen-starved in the warmer future, even if oxygen doesn't change," said lead study author Curtis Deutsch, a UW associate professor of oceanography.

"We know that oxygen levels in the ocean are going down now and will decrease more with climate warming," Deutsch added.

The study found that these changes will act together to push marine animals away from the equator.

About two thirds of the respiratory stress due to climate change is caused by warmer temperatures, while the rest is because warmer water holds less dissolved gases.

If current emissions continue, the near-surface ocean is projected to warm by several degrees Celsius by the end of this century. Seawater at that temperature would hold 5-10 percent less oxygen than it does now.

Viable habitats would shift away from the equator, displacing from 14 percent to 26 percent of the current ranges.

"The Atlantic Ocean is relatively well oxygenated. If there's oxygen restriction in the Atlantic Ocean marine habitat, then it should be everywhere," Deutsch said.

Climate models predict that the northern Pacific Ocean's relatively low oxygen levels will decline even further, making it the most vulnerable part of the ocean to habitat loss.

"For aquatic animals that are breathing water, warming temperatures create a real problem of limited oxygen supply versus elevated demand," said co-author Raymond Huey, a UW professor of biology.

"This simple metabolic index seems to correlate with the current distributions of marine organisms, and that means that it gives you the power to predict how range limits are going to shift with warming," he said.

The findings were published in the journal Science.