The Atlantic herring is one of the most abundant fish on Earth. The silvery, foot-long fish live in huge schools of thousands, to hundreds of thousands of members.
Fishermen have long known that herring schools can release large clouds of bubbles that are visible from the surface. The bubbles are released through the anal opening, connected to the fish's swim bladder, a special gas-filled organ that helps regulate buoyancy.
Herring gulp air at the surface and store it in their swim bladders. It's also long been known that herring make chirp-like sounds, especially when startled or in danger, but biologists didn't know how these chirps were produced.
A pair of Danish biologists recorded herring housed in special pressurized tanks that replicated conditions at the ocean depths where herring are found. They discovered that the chirping sounds were actually produced by air release from the swim bladder during the bubble production.
As it turns out, this was the first report of sound production by gas release in fishes.
This intrigued the scientists. Since the bubbles are released in response to stress, and occur whether the fish are ascending or descending, they don't think that it's simply part of controlling buoyancy.
Some scientists speculate that the sound and bubbles might work as a visual and acoustic screen to confuse predators, but more studies are needed to determine if the chirping bubbles have a behavioral function or not.