What is the biggest migration on Earth, and why does it happen every night?
Every dusk, millions of tonnes of animals rise hundreds of metres to feed, and every dawn they sink back into the dark. Sonar operators in World War II mistook them for the seabed.
▶ Start the storyEvery evening, a huge share of the animals living in the ocean's twilight zone swim up toward the surface to feed, and every morning they sink back into the dark. In terms of biomass, it is the largest synchronous migration in the world. Crustaceans, squid and fish all do it.
The twilight zone is the layer of ocean where light fades out: it begins where only 1% of surface light remains and ends where there is none, roughly 200 to 1,000 m down. It occupies about 60% of the planet's surface and about 20% of the ocean's volume, so this is not a corner of the sea. It is a large part of the biosphere.
The migrants were found by accident. During World War II, sonar operators were puzzled by what looked like a false sea floor 300 to 500 m down by day and shallower at night. It turned out to be millions of animals, above all small fish whose swim bladders reflected the sonar. The first people to see these readings even reported sunken islands.
Day
- Hiding at roughly 800 to 1,000 m
- Dark, safe from predators that hunt by sight
- Sonar sees a false sea floor
Night
- Feeding in the top 100 m
- Surface food, with less risk
- The false floor rises
Why go up and down? The leading explanation is predators. The ocean's surface provides an abundance of food, but it may be safest for many species to visit it at night, when animals that hunt by sight cannot see them. By day, they hide in the dark. The theory fits a detail: small, hard-to-see animals may start rising up to 20 minutes before sunset, while large, conspicuous fish may wait up to 80 minutes after dark.
It is also a clock, not just a reaction to light. Copepods taken from the wild kept their daily rhythm of migration in the laboratory even in constant darkness.
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Recap
Animals rise at dusk to feed where it is safe in the dark and return at dawn, guided by light and an internal clock.
💡 A trick to remember it · Up at dusk to eat in the dark, down at dawn before the light gives you away.
Surprising fact · Sonar operators in World War II mistook the migrating animals for a false sea floor, and the first readings were reported as sunken islands.
Sources (3)
No source, no claim. Every fact in this lesson (18 claims) cites at least one of these.