Climate change and warming waters have driven thousands of ocean species poleward from the equator, which is threatening marine ecosystems and the livelihoods of people who depend on them, researchers reported Monday.

The Proceedings of the National Academy of Sciences of the United States of America Journal (PNAS) compared data on roughly 50,000 marine species over three sets of 20-year periods up until 2015, revealing that the exodus from tropical waters is speeding up.

The tropics have long harboured an outsized proportion of marine life, but could see that diversity disappear if climate change is not brought to heel, PNAS journal authors warned.

Senior Author and Professor of Marine Biology at the University of Auckland, Mark Costello, told reporters, "Global warming has been changing life in the ocean for at least 60 years," adding that his findings showed "a drop of about 1,500 species inhabiting areas around the equator.

"This will continue throughout the century, but the pace will depend on how we reduce -- or not -- greenhouse gas emissions."

Notably, poleward migration, which is migration directed or going towards either the North Pole or South Pole, was more evident north of the equator, where oceans witnessed more warming than south of the equator.

In addition, it was also more prevalent among open water fish than so-called benthic species, which are species of marine life that live on ocean floors or sea beds.

"Benthic species can only move during their floating life-stage, and thus their shift [poleward] is between generations," Costello explained.

On the other hand, species living in high seas can move with the water masses over the course of their lifetime.

Marine life in tropical waters declines when annual average sea temperature increases over 20 to 25 degrees Celsius, depending on the species, the study found.

- 40 percent drop by mid-century -

"The 'missing' tropical species [migrating poleward] are likely following their thermal habitat as subtropical waters warm," said a professor of ecology at Nelson Mandela University and co-author, David Schoeman.

Notably, fossil records indicated that the same phenomenon happened 140,000 years ago, the last time global surface temperatures increased, becoming as hot as they are now.

Based on data from the Ocean Biodiversity Information System, the study does not analyze how each species of marine life will adapt to new environments.

Generally speaking, open-water species are likely to fare better, previous research found.
Additionally, the impact on commercial fish stocks in the tropics is not addressed either, although it is clear which parts of the world will be hit the hardest.

"Indonesia and other nations near the equator, such as in West Africa, have the most to lose because their stocks can only decrease," as no new species will replace those leaving, Costello noted.

On a global basis, about 1.3 billion individuals live in coastal tropical areas, many of which rely on fisheries and commercial fish for sustenance.

In the 'Nature' journal, a recent review estimated the maximum catch potential of tropical fish stocks in so-called exclusive economic zones -- 370 kilometers from the coast -- would decline 40 percent by mid-century if global warming continues unabated.

Many Pacific island nations, combined catch of skipjack and yellowfin tuna -- the two most exported fish -- are expected to drop up to 40 percent under the same scenario, while coral reef fish consumed locally could plunge even further.

The study in PNAS began with the University of Auckland doctoral dissertation of Chhaya Chaudhary.