At the south end of Fakarava Atoll, a 35-mile-long rectangle of coral in French Polynesia, a narrow channel cuts through the barrier reef. Every June thousands of camouflage groupers congregate in that channel, in an area the size of two to three football fields, to spawn the next generation. Violent tidal currents funnel through every six hours, filling and emptying the lagoon. The groupers, fat and about two feet long, are not alone: Hundreds of gray reef sharks assemble as well, to stalk them. The female groupers, like other reef fish, spend at most a few days in the spawning grounds. Yet for some reason the males, which lead solitary lives most of the year, spend weeks crowded into this treacherous place—until finally the whole mass of fish spawn at once, releasing clouds of eggs and sperm into the water. The locals told us it happens at full moon.
My team and I have spent the past four years trying to document and understand this amazing, mysterious spectacle. For a total of 21 weeks, we’ve dived day and night—about 3,000 diver hours in all—into the 115-foot-deep channel. In our first year, 2014, marine biologists Johann Mourier and Antonin Guilbert made the first accurate counts: There were some 17,000 groupers and 700 gray reef sharks in the channel. (The fish are protected here by a biosphere reserve.) That year I completed a continuous 24-hour dive—a technical feat that required the support of the whole team. The point wasn’t to set a record. The point was to observe the fish the way a biologist would animals on land, uninterrupted for a long time.
At dusk on that first night, I watched crustaceans and mollusks emerge from the bowels of the reef—and then retreat at the flash of my light. I watched the camouflage groupers darken their skin and withdraw into crevices to sleep. And I watched the sharks come alive, as if they’d awaited this moment. By day they swim languidly—wakeful groupers are too quick for them. Now, after nightfall, the sharks swarmed along the seabed by the hundreds. The water was electric with them, and I realized I’d underestimated their speed. Their agitation was disturbing: Owing to the gas mixture I was breathing for this 24-hour dive, I couldn’t ascend to safety whenever I wanted. I had to remain at depth with the sharks.
