Algae-eating fish are critical to health of Coral Reefs

Coral reefs are jewels of the ocean–magnificent structures built over eons by corals that, in turn, support a wealth of other animals. Unfortunately, corals are dying on many reefs across the globe. Some reef inhabitants, like parrotfish, actually bite and scrape the reef while eating the algae, literally grinding down the reef! Surely being eaten must be bad for the reef?

Our Research team found exactly the opposite of what many people would expect–namely that reefs need parrot fish to grow healthy and strong. We found the reef grows fastest when there are lots of parrot fish around eating it!  Our team joined up with Aaron O’Dea from the Smithsonian Tropical Research Institute, to collect reef sediment cores in  Panama to understand what coral reefs looked like thousands of years ago before humans began fishing them.

The inside of the reef sediment cores look a bit like concrete–a mixture of coral and skeletons of animals that lived with them–including the teeth of parrotfish. To determine the rate of growth of the reef we obtained the ages of the coral skeletons along the length of our cores with high-precision Uranium-Thorium methods. Then we spent hundreds of hours at our microscopes, sorting parrotfish teeth out of the reef sediment.

So, what did we find? The more parrotfish teeth there are, the faster the reef grows!  We think this is because the fish eat algae that compete with corals:  Too much algae and the corals die.  Since only corals make reef rock, coral death slows down the growth of the whole reef. So, the parrotfish keep the algae in check and that makes for an abundance of corals and a fast-growing, healthy reef.

We also checked to see if other reef eaters had the same effect.  Many scientists have suggested that sea urchins, which eat algae off corals, might also help the reef grow.  But, when we counted up the number of urchin skeleton bits in our reef cores we could not find any link between the abundance of urchins and reef growth.  It seems that not all algae-eaters are equally important when it comes to making a healthy, fast-growing reef.

So what do we conclude from all this? If we want to protect reefs and all their amazing animals, we need to protect parrotfish from overfishing.  Several Caribbean nations have started to do this, with laws against catching them.  But those laws need teeth–parrotfish teeth! Our next step is to talk to local conservation and management groups so they have the best information to use in protecting their reefs.

To learn more about how we are using our reef cores as a time machine to track how Caribbean reefs have changed from the time of Columbus to the present, have a look at our paper online….