Earth's Largest Migration Happens Every Night

Scientists report the mass vertical ascent of marine life - at night - is unprecedented in the wild world.

When anyone mentions great migrations, most people immediately think of The Great Migration when vast numbers of wildebeest and zebra travel from Kenya's Maasai Mara to Tanzania's Serengeti National Park and then back again.


Others may think of the Caribou in the Arctic but, let's face it, almost nobody would think of the incredible 3,000 mile flight of Monarch butterflies on their journey to and from Mexico through the western United States and Canada.


But, actually, the greatest maigration - by total numbers - happens every night and every morning, in the oceans.


At dusk, every night, countless millions of marine animals such as fish, squid, and crustaceans swim toward the surface to feed, reversing direction and returning to deeper water at dawn. This is the finding of new research by scientists with the Monterey Bay Aquarium Research Institute, reports OutdoorLife.


The researchers say the cover of night affords greater protection from larger predators, allowing sea life to swim more safely toward the surface, where they feed in energy-rich waters. These findings were made possible by innovative audio technology that recorded when animals migrated from the depths to the surface, and back again.


Their research documents sunset and sunrise movements of animals between the surface and the twilight zone, a layer of water that stretches from 660 to 3,300 feet deep. They explain that deep-sea creatures stay in these deeper waters during the day to avoid tuna, salmon, seabirds, and other predators that rely on sight to hunt their prey.


“Their movements are not just pre-programmed to go up or go down,” one of the researchers, Benoit-Bird, explained to Phys.org. “There’s a lot more nuance and decisions to be made night to night and even minute to minute, based on how hungry they are and how much of a risk there is.”


Data from the new audio technology also revealed that larger, more visible marine life may delay their dinnertime migration up to 80 minutes after sunset. Meanwhile, smaller animals began their migration to shallower waters sooner than larger animals - just 20 minutes after sundown - and faster swimmers migrated earlier than weaker ones.


Their findings shed new light on why deep-sea animals embark on their massive migration each night. Studying this massive migration is critical to understanding ocean health, says Benoit-Bird, calling the migration a “biological conveyor belt” that moves energy and carbon around in the oceans.

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