Oxygen-depleted regions of tropical oceans are expanding, restricting habitats for fish and other marine life, an international team of scientists said in a study published Thursday.

The researchers found that oxygen levels at 300 to 700 meters (985 to 2,300 feet) have declined significantly over the past five decades.

"The ecological impacts of this increase could have substantial biological and economical consequences," a summary of the study said.

The team of oceanographers said it remained unclear if the reduction in oxygen levels could be linked to climate change, according to the study published in the May 2 edition of Science.

"Whether or not these observed changes in oxygen can be attributed to global warming alone is still unresolved. The reduction in oxygen may also be caused by natural processes on shorter time scales," said co-author Lothar Stramma from the Leibniz Institute of Marine Sciences in Kiel, Germany.

The largest decline in oxygen levels occurred in the northeast Atlantic, with much less dramatic changes seen in the eastern Indian Ocean, the study said.

The oxygen-poor areas could possibly reach coastal zones through currents flowing from the mid-depth tropical oceans, said co-author Janet Sprintall of Scripps Institution of Oceanography at the University of California at San Diego.

"The width of the low-oxygen zone is expanding deeper but also shoaling toward the ocean surface," said Sprintall.

The study was carried out as part of a long-running program on climate variability and predictability led by the World Climate Research Programme, which looks at climate through the interaction of ocean and atmosphere.