Oxygen-depleted regions of tropical oceans are expanding, restricting habitats for fish and other marine life, an international team of scientists said

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

The largest decline in oxygen levels occurred in the northeast Atlantic,

"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