Wendy Science: The "Black Piranha" of Ascension Island

With his half-filled collection bag in tow, is high-tailing it out of the water. Although there’s more sampling to be done and he still has plenty of air left in his tank, the scientist, a curator of ichthyology at the , can’t afford to lose any more specimens. It’s not sharks that Rocha’s worried about. The that have followed our blood trail on earlier collecting dives are nowhere to be seen. This threat, though, is unrelenting, and the pursuers outnumber us by a wide margin. After 45 minutes underwater, Rocha has had enough.

“Sons o’ bitches,” he says with equal parts admiration and frustration as he fights his way through the surf and onto the narrow beach fringing this corner of Comfortless Cove. In his wake, a school of 30 or more () mill around like a group of nervous, but hopeful, dogs at a barbecue as the last few snacks are packed away. Fifteen minutes later, they haven’t given up. Even though the rest of the dive team carries nothing more than cameras and measuring tapes, the fish trail them, too, as they make their way back toward the beach.

In the following video, a specimen intended for Rocha’s collection bag quickly becomes the focus of a massive feeding mob of black triggerfish (video by Steven Bedard):

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Black triggerfish, or simply “blackfish” as the Ascension locals call them, are found in nearly every imaginable tropical reef habitat around the world. In most places, their numbers tend to be relatively modest. Here, and around a few other isolated oceanic islands, they literally swarm. Although the scientists aren’t entirely sure why the species has been so successful around these rocky outcrops, they think it’s probably a combination of the relatively low numbers of predators and competitors here, as well as the fish’s uncanny blend of omnivory and opportunism when it comes to food choice. You can’t possibly go in or even near the waters of Ascension without seeing black triggerfish cruising the shallows in large, loosely organized groups waiting for a meal, , to drift, swim or drop into their paths. And it’s just as common to see them grazing along the algae-covered bottom like herds of hungry goats.

Black triggerfish await the research team as they prepare for a dive at Ascension's Boatswain Bird Island. (Photo by Luiz Rocha)

Having read about and seen dozens of pictures of blackfish before I came to Ascension, I didn’t expect to like or appreciate them as much as I do. Photos of their dense, dark schools made them look more like pests than attractions, but they’re really quite beautiful and fascinating. Despite the name, they’re not actually black. While they often appear dark as pitch in low light, better illumination reveals a deep indigo interwoven with a complex pattern of olive green, light blue and gold as ornate as any Persian rug.

Sky-blue pinstripes delineate the fish’s ribbon-like fins, which propel them through the water with rhythmic flutters—a motion that brings to mind the flight pattern of a butterfly more than the aggressive tail swipes of a predator. Often the fish seem downright passive. On any given dive, it’s common to see individuals laying on their sides, molding their 12-inch-long, oval-shaped bodies into shallow depressions in the volcanic rock, an apparent effort to blend into the background until the danger has passed.

The black triggerfish’s name belies its brilliantly ornate coloration. (Photo by Luiz Rocha)

That timidity goes out the window when an energy-rich food source comes into view. Not only do the triggerfish brazenly snatch bites of flesh through the holes in Rocha’s mesh collection bag, they compete hard for nearly every potential specimen that ends up at the tip of his spear. Most of the time, he wins the battle, but not always. Regardless, whenever there’s an opportunity for a meal, a seemingly aimless school of triggerfish quickly becomes an unruly mob.

Mob feeding is actually the technical term biologists use to describe this sort of behavior—this overwhelming display of numbers and force. Naturally, the tactic is not restricted to harassing scientific divers, or to dispensing with the unwanted bits of flesh thrown back into the sea by local fishermen—as they do each afternoon at the pier in the heart of Georgetown, Ascension’s main settlement. These are the human-influenced, and probably exaggerated, versions of what is a fairly common feeding strategy among a number of reef fish species. In fact, most of the mob feeding we’ve observed in the waters here is herbivorous in nature.

As the marauding group of sergeant majors in the following video proves, triggerfish aren't the only Ascension residents that use mob-feeding to overwhelm other species guarding a territory (video by Steven Bedard):

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There are probably a couple of advantages to this sort of frenzied approach to feeding, even for herbivores. First, traveling in large numbers affords the members of a shoal of fish some level of protection from predators. But more relevant to feeding—and grazing in particular—a group of fish moving quickly and aggressively in large numbers stands a much better chance of overwhelming territorial species standing guard over a patch of habitat. In Ascension, the primary turf defenders that seem to make mob feeding a necessity are the diminutive () and (). Although they’re each around two inches long—just over one-tenth the size of a blackfish—these fish are no pushovers and spend the better part of each day driving off would-be grazers.