Dr. Richard Pyle
Cocos Island is perhaps best known for an abundance of large marine animals, sharks in particular. Most iconic of these are the vast …

This field report first appeared on the NY Times Green blog December 28, 2011.

Hammerhead sharks have been gradually declining at Cocos Island.  There are still large schools in many places here, but those schools are found at fewer sites. Photo: Avi Klapfer.

Cocos Island is perhaps best known for an abundance of large marine animals, sharks in particular. Most iconic of these are the vast schools of scalloped hammerhead sharks, which can be found by the hundreds at various sites in the waters around the island.

Equally striking is the abundance of white-tip reef sharks. At any given moment during a typical dive here, one can look around and count at least half a dozen, and at certain times — usually at dusk or at night when they forage for food — you can see hundreds. Silky sharks can also be found in large numbers here, often among the hammerheads, or gathered just below the surface around an anchored ship at night when the ship’s lights are on.

But it doesn’t end there — other sharks regularly seen at Cocos include blacktips, Galapagos sharks, silvertip sharks, tiger sharks and the occasional whale shark.

This morning, when our plans for a deep rebreather dive were canceled because of strong currents, the filmmaker Howard Hall and I made a dive off Isla Manuelita, at the northern end of Cocos Island. As we swam along the eastern side of Isla Manuelita, we were surrounded by the usual abundance of reef fishes large and small (and, of course, the omnipresent whitetip reef sharks).

Suddenly we encountered a plume of cold, clear water accompanied by a lot of turbulence. We continued further until we rounded a corner in the reef to find a most unusual sight: thousands of fishes up in the water column tumbling wildly in a raging current.

A huge Manta Ray hangs in the current, allowing its planktonic food to come to it.  Photo: Howard Hall.

After waiting as inconspicuously as we could near the edge of the channel, we spotted a pair of hammerhead sharks. They were swimming hard, but the current was so strong that they weren’t moving forward at all. Eventually, they turned and disappeared into the gloom in a down current.

A few minutes later, a large shadow appeared in the distance. As it drew closer to us, we could see that it was a huge manta ray. It, too, was swimming hard into the current but remained almost perfectly stationary, hovering a few feet above our heads. Mantas feed on plankton filtered through their cavernous mouths, and this one was holding position in the current with its mouth wide open while its food was delivered straight inside.

Then things got a bit more exciting. A pair of large Galapagos sharks came in to check us out. A short while later, two more arrived. Howard and I stayed near the rocks, sheltered from the current, while this procession of large animals passed before us — more hammerheads, then the manta ray again, then an eagle ray, then more Galapagos sharks. From my perspective, it was everything that diving at Cocos Island should be.

And yet … it wasn’t. Even though we saw about half a dozen, Howard explained that there used to be many more hammerhead sharks in this area.

Today the waters surrounding Cocos Island are protected by the Costa Rican government, and unlike many such protected areas around the world, the enforcement here is effective despite persistent attempts at poaching. But the protected area only extends 12 miles around the island, whereas sharks can migrate hundreds of miles.

Cocos nonetheless serves as an important case study in the impact of marine protected areas on ocean health and biodiversity, a prime area of focus for the One World One Ocean conservation campaign.

Thousands of sharks are slaughtered around the world every year for their fins, which are used in soup and other products In many cases, only the fins are taken and the carcasses are dumped into the sea. Recently, at nearby Malpelo Island, hundreds of dead, finless hammerhead sharks were found discarded on the sea floor. Without question, this is among the most deplorable fishing practices on earth.

One of several Galapagos sharks that stayed near us in the channel during our dive. Photo: Howard Hall.

Yet not all of the shifts in shark populations are necessarily caused by humans. For example, in 1998, when Howard and Michelle Hall were filming “Island of the Sharks” here at Cocos, they arrived during a strong El Niño period to find warm, crystal-clear waters and absolutely no hammerhead sharks. Months later, when the sea had cooled to more normal temperatures, the sharks began to return.

When the famous undersea explorer Hans Hass was here in the 1950’s, tiger sharks were abundant. Yet this species was extremely rare at Cocos throughout the 1990’s. And then, a few years ago, they mysteriously began to return.

Still, there is no question that shark fishing is a primary culprit. Silky sharks, which used to be very common here even a few short years ago, are almost nonexistent. No known natural process can account for their absence; we do know that this species is heavily targeted by the shark-fishing industry.

Cocos Island would serve as a perfect laboratory for studying many aspects of shark biology, from the nocturnal pack-feeding behavior of white-tip reef sharks to the ephemeral presence of tiger sharks to the dwindling populations of hammerheads and silky sharks, and many, many other topics. The research and filming opportunities here are almost limitless.