For a quarter-inch fish larva, making a one-way vertical trip of 1,000 feet is the equivalent of a human swimming more than 50 miles-in just an hour or so. Some of them ascend from more than 3,000 feet below. Current estimates indicate some 10 billion tons of animals make these excursions every day. Humans are mostly unaware of this daily aquatic movement, known as diel vertical migration, but it's the largest routine migration of life on Earth. As sunset and sunrise slide from east to west every 24 hours-across the Pacific Ocean, then the Indian, the Southern and the Atlantic-swarm after swarm make the same upward journey, retreating as daylight returns. The multitudes remain near the surface at night, but just as the first beams of morning light begin to cascade across the sea, they are already turning back down to the deep. And yet, just before the sun disappears, the swarms begin to rise on a clandestine journey to the surface.Īs they climb, clusters of other zooplankton join in: copepods, salps, krill and fish larvae. Scientists long considered these tiny animals to be drifters, passive specks suspended in the ocean, moved by the whims of tides and currents. Every evening around the world trillions of zooplankton, many smaller than a grain of rice, hover hundreds of feet below the surface of the sea, waiting for their signal.
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