Alarm as marine species move home

City talk | Sarah Marsh 13 May 2021

During some summers, as the Caribbean water temperatures climb, the luminous coral colonies of gold, green and blue that ring the island nation of Cuba give way to patches of skeletal white.

The technicolor streaks of darting tropical fish flash less frequently. The rasping sounds of lobsters go quiet.

While Cuba's marine life has suffered from overfishing and pollution, there is mounting evidence that the warming of waters due to climate change may be taking a large toll as well - both off the island's coast and globally.

New research show the total number of open-water species declined by about half in the 40 years up to 2010 in tropical marine zones worldwide. During that time sea surface temperatures in the tropics rose nearly 0.2 degree Celsius.

"Climate change is already impacting marine species diversity distribution" with changes more dramatic in the Northern Hemisphere where waters have warmed faster, says study coauthor Chhaya Chaudhary, a bio-geographer at Goethe University.

While numerous factors like overfishing have hurt tropical species, the study published in the Proceedings of the National Academy of Sciences found a strong correlation between species decline and rising temperatures.

Fish diversity tended to either plateau or decline at or above 20 degrees Celsius, the researchers found.

While past studies have shown ocean warming is driving some species to migrate to cooler waters, the new study attempts to gauge that impact more broadly - analyzing data on 48,661 marine species including fish, mollusks, birds and corals since 1955.

The number of species attached to the seafloor - including corals and sponges - remained somewhat stable in the tropics between the 1970s and 2010, according to the study.

Some were also found beyond the tropics, suggesting they had expanded their ranges.

In other words, scientists say, species that can move are moving.

"In geological history this has occurred in the blink of an eye," says Sebastian Ferse, an ecologist at the Leibniz Centre for Tropical Marine Research. "To see such changes occurring so rapidly is something quite alarming."

For fixed species like corals, moving is not an option.

Ferse says" "One of the big questions is whether coral reefs as ecosystems and corals as species be able to move north or south fast enough to adjust to a changing climate?"

Having fleets of fish and other swimmers shift rapidly to more temperate waters could devastate the coral ecosystems they leave behind - along with any fishing and tourism industries that rely on them.

Such change"can have a really huge impact on some of the most vulnerable human communities around the planet," says Stuart Pimm, a conservation scientist at Duke University.

For Cuba, such an impact could unravel the island nation's efforts to manage its underwater gardens, although its corals have been less stressed by coastal development and pollution than corals elsewhere. They are considered more resilient to ocean warming.

"It's impressive to return to an area that experienced significant bleaching the year before but looks perfectly healthy a year later," says Daniel Whittle, who heads the Caribbean program at the Environmental Defense Fund.

Cuba opened its first coral reef nursery four years ago to research what species coped best with warming and eventually to repopulate depleted reefs. The country is also restoring coastal mangroves, which serve as fish nurseries and shelters.

Chaudhary and her colleagues plan to look next at the tropical species in decline or were migrating.


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