Grazing parrotfish help protect threatened Caribbean coral reefs

By on March 11, 2015

Prior to the 1980s, sea urchins were abundant in the Caribbean. They were important to the region’s ecosystem and played a vital role in helping to protect the region’s coral reefs. When their populations began to slip, ocean acidification, pollution and disease left reefs there a lot to contend with.

Following the drop in sea urchins, the number of parrotfish in the Caribbean began to go up, to the point that they are now the dominant species around reefs there. And it seems that the parrotfish might be a good replacement for urchins, according to research led by scientists at Florida International University.

The popular fish is reeled in by fishermen across the Caribbean but can also serve as a control for algae growth on reefs, researchers find. It does this by eating the growth away, but also in the neat way that it moves around.

“We have done a lot of behavioral work looking at different species and determining exactly what they’re eating and how they’re feeding,” said Tom Adam, a postdoctoral scholar at FIU during the investigation who is now an assistant research biologist at the Marine Science Institute at the University of California, Santa Barbara. “The thing that parrotfish do that other fish don’t do — the thing that sea urchins do — is they scrape the substrate as they feed.”

Midnight and rainbow parrotfishes swim alongside a Caribbean coral reef. (Credit: Florida International University)

Midnight and rainbow parrotfishes swim alongside a Caribbean coral reef. (Credit: Florida International University)

This behavior helps to clear space on coral reefs that is taken up by algae growth and helps fend off coral shading. And it’s special because most other fish just “crop algae like lawnmowers,” says Adam. That doesn’t open up new space on corals.

For the investigation, scientists looked at about a dozen species of parrotfish and three species of surgeonfish, tropical fish known for their scalpel-like fins. Of these, the parrotfish species are the most diverse, says Adam. Some are small, while others grow up to 20 centimeters in length. And many of them have different types of algae that they prefer to eat.

“Some parrotfish are really associated with higher leaf reefs, while others are associated with low leaf ones,” said Adam. “To find out where the fish are grazing, we used behavioral observation surveys to figure those out.”

On most reefs in the Caribbean, parrotfish are dominant by biomass, says Adam. And it’s not really surprising to find that they have an outsized role in helping coral reefs in the area. Still, their behavior isn’t something that can be predicted accurately 100 percent of the time.

“Something that isn’t well known: When parrotfish are grazing, they can actually eat baby corals or eat larger corals,” said Adam. “Some feed inadvertently and intentionally on corals themselves.”

So the idea of using them to help protect, or restore coral reefs in the Caribbean or even globally, is less straightforward than many might think. But it’s still a pretty good one.

“The findings are incredibly applicable when we think about ocean acidification, global warming and large mortality events of corals,” said Adam. “If we can better manage herbivore populations, we can help reefs to be more resilient, as well as other things that can’t be managed locally.”

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