How the Continual Movement of Wildlife Regulates the Natural World

Each night, as the line that separates day from night sweeps across the face of the ocean, a vast wave of life rises from the ocean’s depths behind it. Made up of an astonishing diversity of animals—myriad species of minute zooplankton, jellyfish and krill, savage squid and a confusion of fish species ranging from lanternfish to viperfish and eels, as well as stranger creatures such as translucent larvaceans and snotlike salps—this world-spanning tide travels surfaceward to feed in the safety of the dark, before retreating to the depths again at dawn.

Known as the diel vertical migration, this nightly cycle is the single largest movement of life on Earth, with some estimates suggesting the biomass of the animals that make the journey may total 10 billion tons or more. So dense is this cloud of bodies, in fact, that in World War II, scientists working on early sonar were perplexed by readings showing a phantom sea floor that rose and fell at dusk and dawn. Now dubbed the deep scattering layer, this phenomenon unsettled many commanders, and later gave rise to research into the possibility submarines might be able to disguise themselves within the obscuring fog.

The animals that make this journey mostly reside in the twilight of the mesopelagic zone. Beginning approximately 200 meters below the surface and extending to approximately a kilometer down, the mesopelagic is the transition between the well-lit upper layer of the ocean and the stygian gloom below. Its upper boundary is defined as the point where a mere 1 per cent of the Sun’s light still penetrates; its lower limit lies at the point where all light is finally extinguished.

This lightlessness is both sanctuary and trap. Down in the gloom of the mesopelagic, zooplankton and other animals are safe from the predators that patrol the sunny waters above. But no sunlight also means no photosynthesis, and no food save that which drifts down from above in the slow rain of dead plankton, decaying fish and other organic matter known as marine snow. And so the residents of the mesopelagic wait, safe in the dark, until night falls above, and then rise to feast, some on the plankton, some on their fellow migrants from the deep.

As our understanding of the diel vertical migration has grown more sophisticated, more detail has emerged. In the Arctic, for instance, where the Sun barely rises for months at a time, the migrating animals time their movements to the appearance of the Moon instead; elsewhere, solar eclipses and even heavy clouds can trigger movement upwards. More significantly, this colossal movement of life does not simply provide food for the denizens of the mesopelagic, it plays a vital role in maintaining life on our planet.

The eddies created by this movement of trillions of bodies help move nutrients, oxygen and heat through the ocean, with some studies suggesting the diel migration may play a bigger part in ocean circulation than the wind. It even helps regulate the climate. For as the migrating animals consume the photosynthetic phytoplankton, they ingest the carbon the phytoplankton have absorbed from the water and transport it downwards, away from the surface and into the mesopelagic.

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Immense and profound as it is, the nightly dance of the diel vertical migration is only one of countless migratory journeys undertaken by animals above and below the waves. As the days and seasons cycle past, animals and other organisms shift with them, tracing patterns across the surface of the planet. Some of these journeys cover staggering distances: humpback whales, for instance, travel many thousands of kilometers every year, swimming from Antarctic waters to their breeding grounds in the tropics and back again. Sea turtles also undertake startlingly long voyages: in late 2017 a loggerhead turtle released from an aquarium in Cape Town travelled more than 37,000 kilometers back to her breeding ground on Western Australia’s Pilbara coast.

And Arctic terns—tiny black-headed birds that weigh a mere 100 grams—leave their breeding grounds in the Arctic as the northern summer ends and travel south, to the waters off Antarctica, where they spend the southern summer on the islands and the edge of the ice before heading north once more, a wandering path that covers an average of 70,000 to 90,000 kilometers a year.

These extraordinary journeys are only the most extreme examples of an ocean’s more widespread tendency. Like animals on land, creatures in the ocean are constantly in motion. In some cases their movement is hostage to natural forces, as they drift with the currents or the winds. In other cases it is deliberate, as they propel themselves through the ocean. Like the more passive wanderers, these journeys sometimes take advantage of currents. In the Pacific, mako sharks ride the enormous rotating currents known as gyres, moving with them in search of the prey that gathers on their edges.

Likewise, loggerhead turtles use the North Atlantic Gyre as a sort of conveyor belt, slipping into the Gulf Stream in the waters off Florida as hatchlings, then riding the gyre in a great loop across the Atlantic towards Portugal and North Africa before it bears them back to their breeding grounds. Still others move more freely, wandering great distances—such as albatrosses, some species of which will travel the distance to the Moon and back seven times over across the course of their fifty-year lifespan, or the seals that regularly swim back and forth across the icy waters between Antarctica and the Subantarctic islands, and sometimes as far as Australia and New Zealand.

This movement of life is not confined to surface waters and the skies. In 2020 researchers found evidence of what they dubbed “a great migration” deep beneath the South Atlantic. By analyzing seven and a half years of time-lapse photos produced by two observatories positioned 1.4 kilometers down on the continental slope off Angola, they found the numbers of fish fluctuated according to the season. More significantly, these fluctuations were synchronized with changes in the abundance of life in the upper layers of the water, suggesting the fish were following the slow fall of nutrients from the surface, their annual movements tied to the seasons just like those of animals far above.

Human cultures have also long relied upon these vast, interlocking cycles of migration. Coastal Indigenous cultures in Australia possess sophisticated understandings of the seasonal movements of fish and other marine animals. Up and down the east and west coast many greet the arrival of the humpbacks from Antarctica with songs and ceremonies, and incorporate the arrival of other species into the systems of knowledge that shape their relationship with Country. In some Queensland cultures the number of rainbow lorikeets helps predict the size of the mullet run, while the D’harawal People whose Country lies just south of Sydney knew that when the coast myall, or kai’arrewan, flowered, it meant the prawns were schooling in the estuaries.

On the opposite side of the Pacific, the Salish people and many other First Nations cultures regard the salmon that course through their coastal waters and rivers as gift-bearing relatives, and the annual migration that sustains not just human communities, but bears and whales and many others, as part of the complex interplay of human and non-human agencies that structure the world. The salmon do not just provide food, they are a reminder of the existence of more-than-human presences, and of the reciprocity that underpins First Nations understandings of the world.

For Europeans these seasonal fluctuations were also an integral part of the natural calendar. But despite this, European science largely rejected the idea of animal migration until well into the eighteenth century. Faced with the annual appearance and disappearance of migratory birds European scholars argued these species somehow hid themselves during the winter months, hibernating in holes, burying themselves, or—as Swedish priest Olaus Magnus suggested of swallows in the sixteenth century—submerging themselves in the mud at the bottom of lakes and streams.

Others followed Aristotle in believing some birds underwent a kind of metamorphosis, changing from one species to another as summer verged into winter, before changing back again with the arrival of the spring. Thus the robin and the redstart could never be seen in the same place, because, like avian versions of Clark Kent and Superman, they were actually the same creature. On those rare occasions when migration was considered a possibility the explanations were even more fanciful than the stories of lakes filled with slumbering swallows: in 1684 the physicist and philosopher Charles Morton argued that the storks that nest in Europe in the summer spend their winters on the Moon, declaring that the journey took a month each way, and the birds were aided in their journey by the lack of atmospheric pressure and gravity.

At least in part, these theories reflected the constraints of their historical context: without any means of following birds and other animals, or of easily sharing observations with people in other parts of the world, Europeans had little access to information that would challenge the assumptions of their predecessors. Yet there was another dimension to their resistance to the idea of migration. As European empires expanded outwards they took possession of new territories, violently subjugating their inhabitants and transforming their landscapes.

The horrifying cruelty and violence of this process demanded ways of imagining other peoples that placed them outside the sphere of moral concern. And so Europeans began to develop racialized hierarchies that treated Africans and Indigenous peoples as less than human. As these divisions hardened, the idea of a world in which racial categories mapped onto levels of social and cultural development—and therefore human worth—became naturalized, giving rise not just to the repulsive race science that would find its ultimate expression in the gas chambers of Auschwitz and Treblinka, but even the organization of the planet’s space: notions of the natural industry and advancement of those from cool climates and the degeneracy of those from the tropics were commonplace, and in 1858, when British zoologist and geographer Philip Sclater proposed the system of zoo-geographic realms that with minor alterations remains in use today, he drew explicitly upon these hierarchies.

The notion that birds and other animals might move from place to place conflicted with these assumptions: after all, if animals were not confined to one part of the world where did they fit in the hierarchy? Yet as the European powers continued to expand, it became harder and harder to deny the reality of animal migration. At first most of these examples concerned land birds: sailors occasionally reported great flocks of swallows and other birds skimming across the waves or nesting in the rigging, and as early as the 1770s, Gilbert White, English clergyman and author of the luminous A Natural History of Selborne, cited reports of the movement of swallows across Gibraltar and Andalusia as evidence the birds were traveling south in the winters, and noted the crowds of birds the changing seasons brought to parts of England: “more,” he thought, “than can be bred in any one district.”

But the first unambiguous evidence of long-distance migration did not arrive until 1822, when a hunter in the north of Germany shot a stork with something embedded in its neck. On examination it turned out to be a 76-centimeter-long spear of African origin. The bird was sent to the University of Rostock where it remains on view today, the spear angled grotesquely upwards through its long neck, its diamond-shaped head high above the stork’s.

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From the book Deep Water: The World in the Ocean by James Bradley. Copyright © 2024 by James Bradley. Published on July 2, 2024, by HarperOne, an imprint of HarperCollins Publishers. Excerpted by permission. Featured image: Anna Varona, used under Creative Commons BY 4.0