A new study has cast doubt on one theory of how marine species could survive the current decline of coral reefs stemming from climate change and human activity, according to Science News.

About half of the world’s coral reefs have disappeared in the past three decades, and scientists are predicting a total 90 percent loss of the world’s coral reefs by 2050. Corals are especially sensitive to fluctuations in water temperature, and oceans have been getting hotter in recent years. Pollution, over-fishing, development, and agricultural runoff have also fueled the problem, according to The Independent.

Coral reefs produce oxygen and despite accounting for only a tiny amount of the world’s ocean area, provide habitats for one in four of all ocean species. They also help to protect shorelines from storms, and fuel both tourism and fishing based economies. Medical researchers have even looked into coral reefs for treatments for everything from cancer to arthritis to bacterial infections.

As scientists have begun to grasp the full scope of this decline, one theory, called the “deep reef refugia” hypothesis, has suggested that species whose habitat has been destroyed could simply move to deeper reefs, called mesophotic reefs, below the level where sunlight can penetrate. Many species that live in shallow waters have also been found at these lower depths, between 30 and 150 meters below the surface.

However, the new study, published in the journal Science on Friday, suggests that these species may have more difficulty than expected surviving in the deeper environment. One of the study’s coauthors, California Academy of Sciences zoologist Luiz Rocha, points out that even if species can be found in both habitats, it doesn’t mean they could adapt to living their exclusively.

“When you start looking into details, a lot of these species don’t actually live in these depths,” according to Rocha.

The team examined 687 coral species and 1,761 species of fish, diving up to 150 meters in areas near the Philippines, Curaçao, Bermuda, and Micronesia. They found that roughly half of the species were found in both shallow and deeper reefs, with the other half only found only in one or the other.

They also found that the deeper reefs were themselves facing threats like coral bleaching as a result of rising temperatures. The researchers found waste like broken glass and fishing line, even in the deeper reefs.

The research doesn’t state conclusively that many species could not adapt to deeper reefs, but it does raise further questions as to whether such migration can be counted on to save a majority of marine species dependent on declining coral reefs.

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