April 15, 2019 — Not long agoI went snorkeling in the Pacific Ocean, a half mile off the southwest coast of Oahu. The flanks of the Hawaiian island are steep there, and the bottom quickly disappeared beneath us as we motored out to the site. Looking back, I could see the green slopes of the Waianae Range rising to 4,000 feet behind the beach. Normally the mountains shield the water here from the trade winds. But on that day a breeze created a light chop that nearly obscured what I had come to see: a thin, oily slick of surface water, rich in organic particles, in which newborn fish were feeding and struggling to survive their first precarious weeks.
Plunging my face into the sheen, I found myself looking inside a fish nursery: The water was dotted with life you would ordinarily never notice. Fish eggs drifted like tiny lanterns, their yolk sacs glowing in the sunlight. Fish larvae small as ladybugs darted about. A sergeant major damselfish the size of a dime appeared huge by comparison as it fluttered past. Below us, a school of 12-inch, bigeye scad—like mackerel but with enormous eyes—fed on everything that had the misfortune of being small.
My guides that day, oceanographer Jamison Gove and fish biologist Jonathan Whitney of the National Oceanic and Atmospheric Administration in Honolulu, are nearly three years into a research project that aims to make sense of this chaotic scene. The larval stage is the “black box” of fisheries science: Fertilized eggs go in, and young fish come out—but what happens inside remains sketchy. The larval fish are so small and fragile they’re exceedingly difficult to study. The overwhelming majority will never become adults. Yet fish populations around the world, and the animals that eat them, depend on just how many larval fish make it, and in what condition.