A Jewel Among the Rubble of a Dead Coral City
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 seventh in a series of special reports from the field that will continue throughout his team’s three-week expedition.
Our reef dive on Aitutaki, Cook Islands looked like it was going to be wonderful: a vast expanse of complex coral like cement on the bottom, disappearing into the distance in the classic ridges and channels of a growing reef. But as we descended a different picture emerged…this reef was not alive.
The coral skeletons were still there, a complex, interdigitated mosaic of branching, plate-like and mounding species. But 99 percent of the coral colonies were dead, entombed by a layer of crustose coralline algae that covered all the colorful native coral and left a ghostlike, chalky pall. The reef was still inhabited by fish – but not the normal kinds. There were no damselfish or planktivorous chromis, trigger fish, hawkfish or butterfly fish. But large numbers of algae-eaters were there: tiny surgeonfish and small parrot fish. Living like vagabond children in an abandoned city, they wandered through the silent towers of coral, picking at any fleshy algae that might grow on the mausoleum-like surfaces. They were stalked by fearsome predators – huge moray eels lurked in passageways, ready to pounce on the unwary. Groupers stalked the channels and hid in plain site waiting for prey as the coral city steadily decayed around them.
A giant clam - a living jewel among the rubble of the dead reef.
One bright jewel gleamed just ahead, and as we swam toward it, we could see it was a fair-sized giant clam, nestled between coral skeletons on a ledge 95 feet down. Clams share with corals their reliance on an extra food supply from internal symbiotic algae. So clams at this depth were perhaps a sign that the reef was still capable of supporting life. I turned to see our dive guide Dawn pull out his knife and slice the clam off the reef. Alarmed and uncertain, I couldn’t ask by sign language what I wanted to know. Was he moving it, maybe to a shallower site where its internal algae could gather more light?
Giant clams are a delicacy in the Cook Islands and Dawn was a 100% native Cook Islander. Was he planning to eat it? I couldn’t be sure, and kept an eye on the clam…and on the reef.
What had happened to the coral? A reef this extensive spoke of a vibrant coral community within the past 5 - 10 years. Now all that remained were Crown-of-Thorns starfish plastered across the few remaining coral colonies, consuming the thin layer of living tissue, and leaving behind a stark white, dead skeleton.
Crown-of-Thorns have plagued Cook Island reefs for years, ripping through coral communities, reducing them to pavement. Outbreaks have come to many of the high islands of the Pacific, and to some of the low atolls. Strong freshwater runoff from land, especially from poorly used agricultural plots or commercial developments, fertilizes the near shore seas, and bolsters the populations of baby starfish. At first the young starfish remain hidden in a healthy reef, but when the 3-inch multi-armed predators emerge, the corals have no defense.
Crown-of-Thorns, predator of coral. Photo by Joi Ito via Flickr, Creative Commons License.
Near the end of our dive, Dawn dropped the clam at the boat anchor and used his knife to dislodge dozens of starfish while we explored the shallows. But time is the only antidote to this problem…eventually the starfish to die off naturally and the corals grow back. If the outbreaks come faster and faster, then there is not enough time for recovery between outbreaks. This is why pigs are the best friends of starfish…pig farms wash so much waste into the water that the coastal ocean chemistry changes, and starfish babies are favored. Other poor land use practices contribute to the mess, easily seen in the brown, sediment laden water in a tropical stream after a heavy rain.
There was little enough to see on this dive that my mind turned to this problem. But I wondered if I should hide Dawn’s Giant Clam so he couldn’t ascend to the boat with it. For the reef, the only hope lay in the corals recovering from healthy corals elsewhere - and the kind of baby coral re-growth we had seen in the Coral Kindergarten on Rarotonga. Keeping enough coral alive in the back reef, where starfish seldom venture, to parent the next generation, might be a way forward. Our experiments here might help with this.
Our dive guide, Dawn, at Aitutaki reef in the Cook Islands.
As the signal flashed among us to head up to the boat, I warily watched Dawn re-collect the clam. But he ambled off a few yards and found a place to nestle it into the reef, angling it up to face the sunshine it needed to grow into a behemoth clam. Back on board our dive boat, he was surprised we had been worried…of course he’d preserve such a jewel.
We chose a more hopeful scene for our surface interval…the giant clam farm and coral gardens of the nearby back reef. There maybe we could find the parents of the next generation of corals that the dead reef behind us so sorely needs.