Deep-sea fish sound off during nightly feedings
When people first started using sonar to navigate the oceans in World War II, they noticed that the sea floor seemed to rise during the night. During the day, sonar would report depths between 660 feet to 3,300 feet, but at night the so-called twilight zone would crawl towards the surface. In actuality, the topography wasn’t moving, but the fish and plankton were, and the sonar was reflecting off their millions of swim bladders, the organs fish use to hold or release gas to control their depth. Now a new dimension has been added to this nightly occurrence, but instead of listening to sonar, researchers are listening to the fish themselves.
Hearing a hum
For a number of years, scientists have been listening to the oceans with hydrophones. Sound carries very well in the water, which facilitates the long-distance auditory communication between many large sea mammals, like dolphins or whales. However, more continuous monitoring also revealed a strange humming throughout the ocean, which would then build towards nighttime. At 300 hertz or higher, the pitch of the buzzing noise was a bit high for a whale, and was usually sustained for too long to make sense as mammal noises.
After picking up these background hums for a long time, researchers started investigating more directly. The hydrophones found that the evening hum matched the timing of a nightly migration in deep-sea fish and crustaceans. As with the sonar, it was part of the swelling of activity from creatures native to the mesopelagic, or twilight, zone of the ocean. During the day, these animals stay at depths where very little light reaches, which helps them stay out of sight from predators. Once the sun is down, that cover of darkness is extended to higher depths, and billions of creatures rise up for a feast, in what is likely the largest migration of vertebrates on Earth.
Source of the sound
While a multitude of fish are once again the culprits behind a strange, nocturnal phenomenon, the exact mechanism behind this sound is still unproven. There’s a chance that it is purposeful, with fish making noise to communicate with each other, like a massive, underwater dinner-bell. There’s also a possibility that it’s something a bit more mechanical, and is just the sound of billions of swim bladders in action at once. Essentially, the mysterious hum of the ocean would then be the sound of fish farts as they changed depths to go eat. I probably don’t need to tell you which hypothesis my kids favor, but let’s just say they spent some time approximating these scenarios with their mouths after learning about this.
Source: Mysterious Ocean Buzz Traced To Daily Fish Migration by Christopher Joyce, the Two-way
