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How Bleached Coral Reefs Permanently Change Local Fish Communities

A new study found that bleached reefs had fewer predators such as snappers and groupers and more plant-eating fish such as parrotfish and rabbitfish.
A school of fish pass over a coral reef at Hanauma Bay on January 15th, 2005, in Honolulu, Hawaii.

The bleaching of coral reefs could permanently change the composition of the fish communities that inhabit them, a new study has found.

The research, published online June 18th in the journal Global Change Biology, traced changes in the mix of species on reefs in Seychelles back to coral bleaching caused by spiking ocean temperatures in 1998.

For reasons that scientists are still puzzling out, higher water temperatures sometimes cause corals to evict their symbiotic algae tenants, draining the reef of its color and often leading to the widespread death of the corals themselves. In the study, bleached reefs had fewer predators such as snappers and groupers and more plant-eating fish such as parrotfish and rabbitfish, ecologist James Robinson and his colleagues found.

"Other reefs have reported recovery of fish communities within 10 years, so we really didn't expect Seychelles fish to get stuck in these new states," Robinson, the study's lead author and a senior research associate at Lancaster University in the United Kingdom, said in an email.

Reefs with fish communities that bounced back to their earlier compositions—like the Great Barrier Reef, for example—are often protected and thus don't have to deal with the compounding stress of human pressures like fishing, Robinson said.

The team's study drew on a series of species surveys on coral reefs in Seychelles between 1994 and 2017. Analysis of that data revealed that, when corals were able to mount a comeback, the diversity of fish changed markedly.

In places where the corals didn't come back, seaweed often engulfed the reef and brought with it a medley of species skewed toward herbivorous fish. Emperors and wrasses, which feed on reef-dwelling invertebrates, also moved in. On the other hand, tiny damselfish and other similarly diminutive species disappeared, along with the large predatory fish.

What's more, these altered communities remained in place for more than 15 years after the bleaching occurred, instead of drifting back toward what they had been before 1998. With the timespans between these bleaching "events" decreasing to less than a decade in many places around the world in the wake of global climate change, the authors suggest that communities aren't likely to regain their former arrangement.

"If habitat continues to change, such as loss of corals and more algal growth on recovering reefs (and Seychelles' reefs did bleach again in 2016)," Robinson said, "then we'd expect fish communities to change further."

Protecting reefs didn't appear to have much of a benefit for recovering communities, at least the way they're set up in Seychelles.

"This may be because small protected areas are less effective for larger predatory fish which move in and out of protected areas, and so marine reserves may only help bleaching when they are sufficiently large," Robinson said.

Morphing reef ecosystems also have implications for the people of Seychelles. The country comprises a chain of more than 100 small islands in the Indian Ocean off the coast of East Africa, and many communities there depend on the bounty of the sea.

"We need to find ways in which ecosystem management can help fish recovery from bleaching, while continuing to support important ecosystem services such as fisheries," Robinson said.

Fisheries aimed at harvesting the most numerous fish found on bleached reefs appear to be one promising possibility. Robinson and his colleagues published a study in late 2018 in the journal Nature Ecology & Evolution showing that people living in Seychelles shifted their focus to more numerous plant-eating fish species.

That sort of adaptation is likely to be increasingly necessary in the future, Nick Graham, a professor of marine ecology at Lancaster University and an author on both papers, said in a statement.

"The new normal for coral reefs will be reef fish communities which have fewer species and are dominated by herbivores and invertebrate feeding fish," Graham said. "This will alter the way coral reefs function, and the fishery opportunities for coastal communities adjacent to coral reefs."

This story originally appeared at the website of global conservation news service Get updates on their stories delivered to your inbox, or follow @Mongabay on Facebook, Instagram, or Twitter.