Coral reefs are dying off around the world. To further his work on the issue, Dr. Stephen Palumbi, director of Stanford’s Hopkins Marine Station, Pew Fellow in Marine Conservation, and OWOO Science Advisor is in American Samoa and the Cook Islands in search of the corals most likely to survive. This is the fourth in a series of special reports from the field that will continue throughout his team’s three-week expedition.
By Stephen Palumbi, PhD.
We made an exciting discovery on our first dive in Rarotonga, one that made us smile through our regulators, and extend the dive to make sure we were right about we were seeing. We found a coral kindergarten.
From 35 to 75 feet deep there were young corals growing everywhere. They were up to the size of a saucer growing on old dead coral boulders and stacks. Every square yard or so there was a young colony growing, laying down a good base of attachment to the surface and sending up tentative branches from the center. And there were a variety of coral species in the nursery.
These little guys are the culprit: crown of thorns star fish. Photo via Jon Hanson, Flickr creative commons.
People do not flock to Rarotonga for the fabulous reefs; it’s a beach crowd that wanders along the shore. The dive shops are struggling and the corals here are well known to be mostly dead. The cause was an unfortunate triple whammy. The first disaster was an eruption of crown-of-thorns starfish in the early 1970s. Millions of starfish appeared on Rarotongan reefs and ate the coral (an unusual choice for any starfish but the Crown of Thorns, which finds tiny polyps encased in slime-coated rock appealing). The reefs recovered from this in about ten years according to local coral expert Dr. Teina Rongo. The second whammy was another crown-of-thorns outbreak, but this one was followed by series of powerful typhoons that churned up the corals into a pile of broken crockery. These disasters left a scarred dead zone of coral rock from which the reefs have yet to recover.
Our dives confirmed this history. The bottom is littered by broken and overturned coral heads and plates, with wide swaths of bare limestone rock where corals used to live. Live colonies are restricted to rare lumbering Porites mounds, a type of coral that even the crown-of-thorns disdains as dinner.
Coral recruits from Rarotonga – the next generation grows up. Photos by Steve Palumbi.
But in an unlikely place just east of the biggest town in the Cook Islands, among the coral stacks and sandy canyons a reef is trying to grow back from baby corals. Two things stand out as major reasons why this place might be re-growing. The water is clear – even on a bad day for diving. We dove the morning after a torrential rain, and most other spots along the coast were murky with runoff. But this site retained the clear blue water and glimpses of sandy bottom that divers love to see. Corals love it too, and thrive in clear water that lets in lots of sunlight.
The second important factor is that the old coral, though dead, was not covered in a fur of algae. It was scraped clean by parrot fish, surgeon fish, sea urchins and other algae eaters. Coral babies cannot settle down and grow on algae-covered rock. They need a clean nursery – and the fish are the nurses of this neonatal shore.
Good water and fish equals new coral. At least on one place in Rarotonga.
Next: why are there still fish some places in Rarotonga? Poison is the answer.
Aerial view of Rarotonga, Cook Islands courtesy NASA.