Dave Tickler and Philippe Bouchet from the University of Western Australia are the hard working “pelagic camera” team on this Pristine Seas expedition to Rapa and Marotiri in far southern French Polynesia. They deploy camera systems to float below the surface out in the open ocean, and record footage of the marine life far from the shallow reefs where the divers are making their surveys. They see a lot of sharks every day and yet, just as we report from our manned dives, they are puzzled by one big question: How can there be so many sharks and so little obvious prey? Here Dave and Philippe present their initial thoughts.
By Dave Tickler and Philippe Bouchet
Twenty-nine pelagic cameras have so far been successfully deployed around Rapa and Marotiri to capture life beneath the waves in remote sites beyond divers’ reach.
Suspended at a depth of 10 meters and armed with a canister full of smelly fish bait, the cameras are like “floating eyes” that work in pairs and record marine biodiversity for two hours at a time as they drift across underwater plateaus, reefs, slopes, seamounts, and the abyss. With only dumbbell weights, buoys, and a length of rope required to make them operational, they’re also are a great illustration of how cutting-edge ocean exploration does not have to be the stuff of rocket scientists—simple designs can yield intriguing and much needed data.
Our greatest challenge has been to operate, deploy, and retrieve the machines whilst also battling howling winds, confused seas, regular rain squalls, and heavy swells—the most trying conditions we’ve ever been in. However, all those long, cold, and wet hours spent in the Zodiac motorboat have been worthwhile, as our footage has revealed sharks turning up at almost every baited camera rig—up to 20 in one instance. Mostly we have been seeing Galapagos sharks, but the cameras we set in the more remote western area of Rapa also captured two young tiger sharks, evidence that these apex predator species also use this area.
The questions raised by our dive team in the shallow areas also resonate in the deep blue. The apparent lack of large quantities of fish life raises the question of what this seemingly abundant and healthy population of sharks feeds on; and, while the young, inquisitive sharks are a pleasure to watch as they investigate our instruments, we are also forced to wonder: where are the adults?
A couple of possible hypotheses are being discussed on board. Perhaps these sharks are feeding at night on nocturnal prey? Or perhaps they are feeding away from the coastal reefs in the fringes of, or even beyond, the edge of the Rapan plateau? The snippets of views of the sea floor gathered from by the automated drop cameras seem to confirm this and show an abundance of life on the deeper reefs.
An all-too-familiar explanation for the lack of fully grown sharks might be the on-going trade in shark fins, or bycatch in commercial fisheries. An intriguing, and more optimistic, alternative is that these remote outcrops of reef are acting as nursery grounds and crèche areas for sharks, with the adults living in deeper offshore waters or even migrating to far off reefs.
This second explanation would cast the Rapa and Marotiri systems as vital hubs in the life cycles of two important shark species, placing greater urgency on the proposed protection of this area. As always our scientific endeavors are creating as many questions as they answer: with tools such as satellite tracking to map the long-range movements and diving behavior of the sharks, and genetic analyses to find out how closely sharks at Rapa are related to other sites in this part of the Pacific Ocean, future teams visiting the Austral Islands (perhaps in the context of it as a marine protected area) will be able to answer them.
The Pristine Seas expedition to Rapa is sponsored by Blancpain and Davidoff Cool Water.