The Largest Migration on Earth Is Vertical

Most of the animals in the nightly vertical migration are small copepods. But trillions of krill, shrimp, squid and jellyfish, such as these, also participate.

Every night on Earth, a great migration takes place. It’s bigger than the ones of caribou, wildebeest or zebras on land or Arctic terns in the air. But while this stupendous, nightly migration overshadows all the others, you’ve probably never heard of it. And even more surprising is the fact that it’s vertical.

This upright, mass movement rises from below: from the depths of the sea to the surface of the ocean. And while many of the animals on this journey are so tiny that they are invisible to the naked eye, they are as energetic as any. They swim upward as far as 1,500 feet each evening and then return the same distance in the morning, traveling tens of thousands of body lengths every day.

Why do they do it? Like most animals on migration, they do it to eat. But adding to the wondrous scope of this natural phenomenon—the human equivalent of walking 25 miles each way to get to and from breakfast—is that along their way, these animals are helping to sequester carbon dioxide, thus reversing some of the damaging CO2 emissions perpetrated by humans.

Photosynthesis takes place in microscopic plants that grow in the sunlight on the water’s surface.

Carbon sequestering carried out by sea life

Each evening as the sun sets, an estimated five billion metric tons of sea life move from the bottom of our oceans to feast on microscopic plants that grow in the sunlight on the water surface. They ascend only in the darker hours to avoid predators that hunt by sight. Before dawn, these animals—roughly weighing as much as 17 million 747 airplanes—reverse course, sinking or swimming down to spend another day in darkness.

Most of these creatures are small, translucent crustaceans called copepods. But trillions of krill, jellyfish, shrimp, squid and other ocean residents join the voyage. Just one of the rising animals—the bioluminescent lantern fish, only six inches long—is abundant enough to outweigh the entire planet’s annual fisheries catch.

By eating the products of photosynthesis in the surface waters at night and swimming downward each day, the migrating animals potentially move a tremendous amount of carbon from the surface waters to the deep.

Bioluminescent organisms live throughout the water column, from the surface to the seafloor, and from near the coast to the open ocean.

As a zooplankton consumes nutrients at the top and heads back down, it excretes a fecal pellet, which another individual slightly lower down consumes and excretes, and so on. The collective effect can move nutrients down from the surface as much as 53 percent faster than would happen by gravity alone, according to one recent study.

Beyond shuffling nutrients, the migration pumps carbon down, making it a critical player in carbon sequestration—and thus a boon to the climate. Similar to trees on land, microscopic plants at the ocean surface convert atmospheric carbon dioxide into organic matter. When migrating zooplankton consume this plant matter and carry it down, they sequester carbon in the depths, where it may remain for hundreds or thousands of years.

Remarkably, the vertical migration takes place even in the darkness of winter at the North Pole under several feet of ice that is covered with snow. In a 2016 study, researchers used acoustic devices moored to the sea bottom across the Arctic and found that zooplankton flee to the dark depths to escape the faint light of the rising moon. The researchers were so doubtful that such minimal light could drive the migration that they implanted electrodes in the optic nerves of krill to measure the amount of light needed to elicit a response. They found that only a few photons were enough.

As populations of larger fish decrease, commercial fishing moves down the food chain. Already, krill paste and lantern fish protein concentrate are sold as commercial products.

So, although most of the individual migrators are minuscule, their staggering numbers mean that the amount of plant material they eat each night is enormous and makes up an important part in the global carbon cycle.

Ladders of longitude created by clouds of creatures

This natural marvel began to be recognized just 76 years ago.

In 1942, a U.S. Navy research vessel, the USS Jasper, was testing new sonar technology off the coast of California when it reported sound waves being deflected from a mysterious, dense layer more than 1,000 feet below the surface. It stretched for more than 300 miles, leading researchers to think that it might be the seafloor itself. Other sonar pioneers soon found similar layers all across the Atlantic, Pacific and even in lakes worldwide. Yet exactly what the cloudlike layers were remained an enigma—and a peril for the Navy, which feared they could hide enemy submarines.

Three years later, a researcher from the Scripps Institution of Oceanography, a department of the University of California, San Diego, used crude plankton nets to conduct nighttime surveys of marine life at various depths and became the first to report that the thick fogs were actually masses of living creatures.

Fishing the vertical migration takes food away from salmon, which could starve them out, leaving less available food for bears.

Now termed the “deep scattering layer,” the DSL can be hundreds of feet thick and extend for hundreds of miles at various depths across the world’s oceans. En masse, these creatures resemble an almost endless cloud of drifting snow, yet they are spectacularly varied. Different species may prefer to hang out at different depths by day and night, or at different temperatures and salinity gradients. For some smaller creatures, such as copepods, seawater can seem viscous, making their migration feel like slogging through molasses. In contrast, many bony fish species inflate their swim bladders for quick ballooning to the surface and then deflate them for a speedy descent. Some animals may travel only a few dozen feet on their expeditions, while others traverse several thousand feet. The result, researchers say, is less like a coordinated mass movement from the depths to the surface and back again and more like overlapping ladders of migration.

An unviewed vertical migration kept from vanishing

Unfortunately, the vertical migration is in jeopardy. If the Arctic becomes ice-free or if ice melt causes the ocean to become more stratified, it could alter the patterns of nutrient flow and carbon cycles, with unforeseen consequences. Plant life at the ocean surface, for instance, produces about 20 percent of the Earth’s oxygen—one in every five breaths we breathe. In our other oceans, as commercial fishing decimates populations of larger fish, the tendency is to move down the food chain. Current commercial products that take advantage of the migration already include krill paste and lantern fish protein concentrate, mainly as feed for fish farms. Targeting the migration also takes food away from salmon, tuna and whales, possibly contributing to their starvation.

For now, this out-of-sight pageant goes on every night, following its ancient rhythms and continuing to shape the diversity and productivity of our oceans. But like so many other natural wonders on Earth, the vertical migration could disappear if we’re not vigilant.

Here’s to finding your true places and natural habitats,

Candy