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Rainbow parrotfish may be behind coral bleaching in part of the Florida Keys

A pesky fish may be the culprit behind bleached tropical coral off the coast of the Florida Keys, according to research from the University of Georgia.

The rainbow parrotfish is a beautiful teal green and yellow fish commonly found in the Atlantic from Florida all the way to Argentina.

The fish's beak-like mouth may be the source of annual coral bleaching in Buttonwood Sound during April and May, a time of year that isn't known for the hot temperatures typically blamed for bleaching.

The study found more than one in three corals in areas frequented by parrotfish were bleached, with about a quarter of the bleached corals showing bite marks. In areas without the fish, only 3.2% of the corals were bleached. None of those corals had bite marks.

"What does this mean for the coral? Well, if I were a coral, I would dread seeing these parrotfish," said William Fitt, lead author of the study and a professor in UGA's Odum School of Ecology. "I don't think there's any advantage to having parrotfish around for this coral."

Low algae, high number of bite marks leave corals vulnerable

The study, in the Journal of Marine Science and Engineering, relied on NOAA data that measured water temperatures every hour from 2010 to 2017 each April and May. Only three days in May 2017 posted water temperatures high enough to potentially bleach the coral.

The bleached and healthy corals didn't show any development, growth or maturation differences either.

Much of the bleached coral did, however, show numerous bite marks, leaving it with less tissue and reducing the algae that live within the coral. That stress may be the source of the coral bleaching, according to the researchers.

Further examination of the corals also found levels of the beneficial algae had decreased during the spring, when they would normally rise, suggesting that something was stressing the coral.

That something, Fitt said, is likely the parrotfish. The fish migrates into the area each year in the early spring and leaves in the summer, coinciding with the increased coral bleaching.

Coral bleaching may lead to disease, starvation, death

Coral bleaching happens when coral is stressed by high temperatures or other factors.

The typically golden brown coral pushes out beneficial algae that live in their tissues, turning a pale pinkish color. In a healthy coral, that algae feeds the coral and gives the coral its unique color.

Bleaching can be a death sentence for coral. Even when it's not fatal, it leaves coral vulnerable to disease and starvation. Multiple episodes of bleaching, like the annual occurrence in the sound, can kill the coral.

Fitt likened the parrotfish predation to mosquitoes.

"It could be a micropredator that's just an annoyance," he said. "But it's more than just annoying if there's a million mosquitoes on you. It depends on how many bites you get."

Most corals have stinging cells that deter potential predators from nibbling on them. This particular type of coral likely has fewer of those cells or less potent ones, Fitt said, explaining why the parrotfish so easily make a meal out of it.

It's unclear how prevalent this phenomenon is outside the Keys. But it's extremely common where Fitt studies.

While it's unknown exactly what effect this predation will have on the coral in the area in the long run, it likely isn't good.

"The most important thing is that parrotfish are stressing this coral out so much that it's producing a lot of this pink color, and that could be a major stress for this coral," Fitt said. "If you see a pink coral, that coral is having problems."