December 21, 2017 — Each year between February and June, the fish gather to spawn in Mexico’s Colorado River Delta. The fish, a type of croaker called the Gulf corvina, meet in water as cloudy as chocolate milk. It’s a reunion for the entire species, all members of which reproduce within a dozen-mile stretch of the delta.
When the time is right, a few days before the new or full moons, the male fish begin to sing. To humans, the sound is machine guns going off just below the waterline. To female fish, the rapid burr-burr-burr is a Bing Crosby croon. Make that Bing cranked up to 11.
Marine biologists who recorded the sound describe the animals as the “loudest fish ever documented,” said Timothy J. Rowell, at the Scripps Institution of Oceanography in California. Rowell and Brad E. Erisman, a University of Texas at Austin fisheries scientist, spent four days in 2014 snooping on the fish with sonar and underwater microphones. The land surrounding the delta is desolate, Rowell said. Fresh water that once fed wild greenery has been diverted to faucets and hoses.
But the delta is alive with the sound of fish. “When you arrive at the channels of the delta, you can hear it in the air even while the engine is running on the boat,” Rowell said. Sound is measured differently in water than in air; still, the fish are as loud as lawn mowers, Rowell said.
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