Herring School For Safer Sex Hidden below the deep, dark ocean, millions of herring come together in a nightly ritual migration in order to mate.

Herring School For Safer Sex

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From NPR News, this is ALL THINGS CONSIDERED. I'm Robert Siegel. Every now and then, scientists build a new gadget that shows them something they've never seen before. And that's what's happened with ocean scientists who tested an advanced form of sonar out on the North Atlantic.

It revealed something amazing beneath the surface, an enormous gathering of herring, as many as half a billion. The scientists describe their discovery this week in the journal Science. NPR's Christopher Joyce reports on what they're calling a city-state of fish.

CHRISTOPHER JOYCE: Nicholas Makris doesn't know much about fish. He's an engineer, but the device he invented, a powerful kind of sonar that can scan wide swaths of the ocean, got biologists pretty excited.

Makris and a team of those biologists took his device out into the North Atlantic, to the Georges Bank, a hot sport for marine life. It was, he says, a shot in the dark, but soon, one of his colleagues saw something unusual on the sonar screen and told everyone to keep an eye out.

Dr. NICHOLAS MAKRIS (Engineer, Massachusetts Institute of Technology): Hey, you better watch what's going to happen here and stick around here because I think you're going to see something. And she said just watch, just watch, just watch, and there it happened. It was really amazing.

JOYCE: At sunset, herring that were spread out near the bottom of the ocean started to drift closer to each other. When they reached a critical density, the fish suddenly started to group into clumps. As each fish synchronized its movements with nearby fish, the clumps rapidly coalesced into a vast school 25 miles across.

On the sonar screen, Makris says it looked like the kind of wave created by spectators in a sports stadium.

Dr. MAKRIS: Somebody stands up, and then the person next to them stands up, and then the next and the next, and before you know it, actually within seconds, you have a wave propagating from one end of the arena to the other.

JOYCE: As the fish aligned themselves with each other, these waves moved through the school 10 times as fast as any individual fish could swim, the same way a wave in a stadium crowd moves faster than people can.

Iain Couzin is a biologist at Princeton University who studies this kind of behavior in everything from fish to locusts to people. He says each fish has to copy the movement of the next fish so that the whole mass can most instantly as one.

Dr. IAIN COUZIN (Biologist, Princeton University): It seems that the fish are very sensitive to the behavior of those around them, and this can cause these sort of very rapidly propagating waves of change of behavior, and therefore it changes the population dynamics, you know, incredibly rapidly.

JOYCE: This is the first time scientists have observed such a chain reaction start to finish on such a large scale, according to Nicholas Makris. His new sonar, it's called Ocean Acoustic Waveguide Remote Sensing, made it possible.

Dr. MAKRIS: Below the surface, things were going on that nobody knew about, and it's because we just couldn't see it. With this new technology, we're essentially taking the darkness out of the ocean and just seeing things that we just couldn't see before, I mean, literally city-states of fish.

JOYCE: And this city-state moved. Overnight, the entire school swam south to warmer waters to mate. The next day, the fish returned to where they started and dispersed, and when the sun set the next evening, it all started again.

Iain Couzin at Princeton says mating exposes the fish to predators, and behavior like this reduces each fish's risk of getting eaten.

Dr. COUZIN: It's very important that they synchronize their behavior to mate at the same time and to move together into particularly dangerous parts of the ocean.

JOYCE: Nicholas Makris, a professor at MIT, compares the remote-sensing project to what astronomers do.

Dr. MAKRIS: When you put the Hubble telescope out there and look into the heavens, you don't know what you're going to see, and it's the same thing in the ocean. I mean it's dark, it's dark like space, and if you can't sense it with your eyes, and then you augment with these other tools, you find things that you just weren't expecting.

JOYCE: Makris's expedition was part of the Census of Marine Life, a 10-year-long effort to catalog life in the oceans. Christopher Joyce, NPR News.

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