A Choreographic Feeding Frenzy
This field report first appeared on the NY Times Green blog December 30, 2011 as part five in a six-part series.
|During the day, Whitetip Reef sharks, Triaenodon obesus, are always around, but never hunting or feeding. Photo: Richard Pyle.|
Whenever I see a reef with an enormous abundance of large predators, I always wonder, where does the food come from?
In a typical community, the plants and other “primary producers” represent the largest biomass, followed by the herbivores, then various tiers of predators, each level in the hierarchy having successively lower total biomass in the system. In a few cases, however, this “pyramid” of decreasing biomass can seem to be inverted.
For example, there are reefs in the remote Northwestern Hawaiian Islands where huge aggregations of top predators like Galapagos sharks (Carcharhinus galapagensis) and Giant Trevally (Caranx ignobilis) appear to represent far greater biomass than the smaller reef fishes and invertebrates these predators feed upon. In theory, there is no way that these so-called “inverted pyramids” in feeding guilds can be sustainable, so when I see these kinds of ecosystems, I cannot help but wonder how they work.
With all of the sharks and large predatory fishes, the reefs around Cocos Island appear to support this sort of inverted pyramid. In the case of the hammerhead shark (Sphyrna lewini) and other large predatory sharks, it is easy enough to imagine that they might migrate to different areas to feed. They only appear to occur with impossible abundance at Cocos because they form aggregations in certain specific areas when they are not feeding.
|When the hunting sharks find their prey, a brief frenzy breaks out as they drive their prey from hiding. Photo: Richard Pyle.|
If you spread these same sharks evenly out across the entire area, they would not seem to outnumber their prey in terms of biomass. It is not so easy, however, to explain the abundance of whitetip reef shark (Triaenodon obesus). It seems like no matter where you dive at Cocos, you can look around and at any given moment during a dive and see a dozen or more of these sharks.
In other words, the whitetip reef sharks are already spread out evenly around Cocos, and huge numbers of them are still around. You would think that with so many sharks competing for the same food source, they would need to be constantly hunting all day long. But after hours of diving here, I had not yet seen a single whitetip reef shark hunting for a meal.
And then we did our first night dive.
Edwar Herreño and Felipe Chacon (the divemasters aboard the Argo) gave us a briefing after dinner about what to expect. We were told to stay six feet off the bottom and to shine our lights where they shined theirs.
When we reached the bottom, it was immediately obvious that the whitetip reef sharks were behaving very differently from what we had seen earlier the same day at the exact same spot. Instead of swimming slowly over the reef or resting on the bottom, the sharks were now swimming rapidly and somewhat aggressively, poking their heads into holes along the reef.
Every few minutes the sharks would suddenly converge on a single rock, twisting and turning their bodies in a frantic effort to find a reef fish to eat. Suddenly, a Pacific creolefish (Paranthias colonus) or scrawled filefish (Aluterus scriptus) would dart out from the reef, and within seconds it would be devoured by one of the sharks. As we continued our night dive, we saw this same pattern over and over again.
|The biomass of predatory fishes at Cocos is incredible. Photo: Richard Pyle.|
Howard Hall has filmed the mob-feeding behavior of these sharks at Cocos before. He believes there is an advantage to all of the sharks in hunting in this fashion, because when one shark chases down a meal, in doing so other small fishes are driven from hiding and eaten by other sharks. It certainly is a topic that should be examined as part of a future research project.
Edwar and Felipe had instructed us to point all of our lights in the same direction so they could keep the group together and focus our attention in one spot. But many times throughout the dive, I noticed these momentary feeding frenzies happening just outside the range of our lights. At other times, we would be following a pack of sharks as they were hunting, and then suddenly the sharks would swim off into the darkness. When we followed them, we would invariably find a feeding frenzy that was just finishing up.
These observations are important because there has been some concern that the divers’ lights were disrupting the natural shark feeding activities in some potentially harmful way. But it was very clear to me, at least, that we were passive observers.
Our lights neither assisted nor impeded their feeding behavior. Sharks have evolved to use other senses (smell, detection of pressure waves caused by their prey and small electrical sensing organs called Ampulae of Lorenzini); they do not use vision to hunt for food. This drama has been performed every night for thousands of years.
If we can manage to protect these animals from our own encroachment, which the One World One Ocean campaign aims to do, it will continue for thousands more.