Warmer oceans will shift habitats of marine animals (June 8 is World Ocean Day)

Warmer water temperature and decreased levels of oxygen in the ocean will act together to push marine animals away from the equator to places in the ocean where the oxygen supply can meet their future needs, says a study.

The warmer water temperature will speed up the animals' metabolic need for oxygen, as also happens during exercise, but the warmer water will hold less of the oxygen needed to fuel their bodies, similar to what happens at high altitudes, the researchers said.

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 your metabolism goes up, you need more food and you need more oxygen," said lead author Curtis Deutsch, associate professor of oceanography at University of Washington.

"This means that aquatic animals could become oxygen-starved in the warmer future, even if oxygen does not change. We know that oxygen levels in the ocean are going down now and will decrease more with climate warming," Deutsch said.

The study centred on four Atlantic Ocean species: Atlantic cod that live in the open ocean, Atlantic rock crab that live in coastal waters, sharp snout seabream that live in the subtropical Atlantic and Mediterranean, and common eelpout, a bottom-dwelling fish that lives in shallow waters in high northern latitudes.

Deutsch used climate models to see how the projected temperature and oxygen levels by 2100 due to climate change would affect these four species' ability to meet their future energy needs.

If current emissions continue, the near-surface ocean is projected to become 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.

For all four species, the equator-ward part of the range would become uninhabitable because peak oxygen demand would become greater than the supply.

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

The authors believe the results are relevant for all marine species that rely on aquatic oxygen for an energy source.

The study was published in the journal Science.