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At Night, Fish Communicate With Special Calls, Whistles and Grunts

Dropping a hydrophone into an underwater cave helps researchers make sense of the din

This "Stars and Stripes" toadfish living in Okinawa Churaumi Aquarium in Japan is one member of a very noisy kind of fish (YURIKO NAKAO/Reuters/Corbis)
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We tend to associate noise with man-made sounds: the blare of car horns, clamor of passing trains and the shouts of construction workers. Yet nature itself has a lot to say, too, though noise pollution is making it harder to hear. And increasingly we are learning that even underwater creatures aren’t safe from human cacophony. (Whales are notoriously sensitive to ship noise.)

The soundscapes under the sea can be rich, surprising and vital—for example, baby oysters use sounds to find a place to settle down. Now, a new study shows that fish that swim during the day babble differently than fish at night do. 

Already, scientists knew that many sea-dwelling critters make sound. Emily Anthes writes for the New Yorker how some of this submarine symphony is played:

Clownfish chirp and pop by gnashing their teeth together. Oyster toadfish hum and blare like foghorns by quickly contracting muscles attached to their swim bladders. Croaking gourami make their signature noise by snapping the tendons of their pectoral fins. Altogether, more than eight hundred fish species are known to hoot, moan, grunt, groan, thump, bark, or otherwise vocalize. Carol Johnston, an ecologist at Auburn University, is partial to the sounds made by lollipop darters, small fish native to Alabama and Tennessee. “They sound like whales,” she told me.

It’s not the soaring song of, say, bowhead whales, but it is a surprisingly deep, resonant "woawm-woawm-woawmfor such a cutely named, small fish. All the songs and chirps and groans and more are mainly used during courtship, competition between males and in defense of the nest or other territory. 

But with all those varied noises, scientists wonder how each species picks their signals out of the din. In the new study, researchers deployed an underwater sound detector called a hydrophone in a cave nearly 400 feet below the surface, off the coast of South Africa. 

They recorded 2,793 distinct sounds, which they divided into 17 groups based on shared characteristics. The descriptions for those groups include "two or three clear pulses separated by 45-50 ms (milliseconds)," "isolated boom," "coarse pulses in a regular series" and "one or two pulses grouped following by a train of around eight pulses. Sometimes the first group of pulses is absent." The researchers couldn’t conclusively identify all the calling fish because of low light levels, but they did have some guesses. Some were toadfish, others members of the family Holocentridae (variously known as soldierfish or squirrelfish), and still others different kinds of grouper.

Here's a recording from a different group of a toadfish humming and growling:

The researchers found, though, that when night fell, the sounds changed dramatically. The calls associated with daytime fish often overlapped in terms of their frequencies and pulse characteristics. In contrast, nighttime calls were more distinct from each other. 

Since fish swimming during the day can use visual cues as well as auditory ones, they can afford this kind of communication jumble, the researchers write. Night-swimming fish, on the other hand, are only able to tell other species apart by the noises they make, so they have to be unique. At night, the sounds fish make are more important. The researchers published their findings in Proceedings of the National Academy of Sciences

We already know that on land, animals have different strategies for distinguishing their noises from that of other species, including humans. Frogs of the same genus will call at different frequencies depending on their species. Birds sing longer, higher songs to compensate for city noise. It seems that fish too have strategies for making themselves heard. It’s a good thing, because human noises are not the only non-animal sounds washing though the seascape. Glaciers melting create some of noisiest places in the ocean

This new information is critical for figuring out the effect that human noises might have on fish populations, the researchers note. More hydrophone recordings can tell us whether we need to quiet down and could perhaps reveal some of the originators of the many odd underwater sounds already detected by the U.S. National Oceanic and Atmospheric Administration. After all, it’s the ocean and there be monsters.

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