Herring communicate primarily by releasing tiny bubbles of gas from their bodies, producing rapid bursts of sound that other herring can detect. These sounds, which researchers named Fast Repetitive Ticks (FRTs), were first described in a 2003 study and represent a method of sound production never previously documented in any other animal.
How Herring Produce Sound
Herring have an unusual anatomical feature: their swim bladder, the gas-filled organ that helps them control buoyancy, is connected to both their stomach and their anal opening. When herring need to communicate, they push air from the swim bladder out through the anal duct, creating a stream of fine bubbles. These bubbles produce a distinctive rapid clicking noise as they escape.
Each burst consists of 7 to 65 individual pulses, with an average of about 32 pulses per burst. A single burst can last anywhere from 0.6 seconds to 7.6 seconds, with the average coming in at around 2.6 seconds. The sound covers a broad frequency range, from 1.7 kHz up to at least 22 kHz, meaning part of the signal extends into frequencies that are ultrasonic to the human ear.
Researchers initially considered whether the sounds might just be a byproduct of digestion or of herring gulping air at the surface. Both explanations were ruled out. The gas release appears to be deliberate, not a digestive accident, and it doesn’t depend on the fish having recently swallowed air.
Both Species Do It
Pacific herring were the first species observed producing FRT sounds in controlled lab conditions. When researchers then studied Atlantic herring, they found the same behavior. Video analysis confirmed that the sounds in Atlantic herring were also associated with bubble expulsion from the anal duct region. The fact that both major herring species share this mechanism suggests it serves an important biological function rather than being a quirk of one population.
Why Herring Make These Sounds
The leading hypothesis is that FRT sounds help herring maintain their massive schools, especially in low-light conditions. Herring are famous for forming enormous, tightly coordinated groups that can number in the hundreds of millions. During the day, they rely heavily on vision to track their neighbors. At night or in murky water, acoustic signals could fill that gap, giving each fish a way to sense how many others are nearby and where they are.
There’s also a connection between the sounds and stress. Gas release from the anal opening has been observed when herring are startled or scared, and it occurs during rapid changes in depth (ascending or descending in the water column). This raises the possibility that some bubble release serves a dual purpose: adjusting buoyancy while simultaneously signaling alarm to nearby fish. A sudden increase in clicking from multiple fish could function as a kind of group warning.
Schooling Density and Sound
Herring don’t always school with the same intensity. During overwintering, they pack together in extremely dense formations, while during spawning they spread out considerably. This variation in density likely affects how much acoustic communication matters. In a tightly packed school, each fish can feel its neighbors through pressure changes in the water. In a looser formation, sound may become more important for staying connected. Predators like killer whales appear to recognize these differences too, adjusting their own hunting calls depending on whether herring are schooling tightly or spread out.
A Stealthy Signal
One of the more interesting aspects of herring communication is the frequency range. The broadband pulses reaching up to 22 kHz and beyond sit in a range that many common marine predators are not well equipped to detect. Most predatory fish hear best at lower frequencies. This means herring may have evolved a signaling system that works as a kind of private channel, audible to other herring but largely invisible to the animals trying to eat them. It’s a elegant solution to a core problem in animal communication: how to coordinate with your group without advertising your location to predators.
Other Ways Herring Coordinate
Sound isn’t the only tool herring use. Like most schooling fish, they rely on their lateral line, a sensory organ running along each side of the body that detects vibrations and pressure changes in the surrounding water. This lets them sense the speed and direction of nearby fish and react almost instantly to changes in the school’s movement. Vision plays a major role during daylight hours, with each fish tracking the position and orientation of its nearest neighbors.
What makes the FRT discovery significant is that it added a third layer to this picture. Before 2003, nobody knew herring were actively producing sounds to communicate. The combination of vision for daytime coordination, lateral line sensing for split-second reactions, and acoustic signaling for maintaining contact in darkness gives herring one of the more sophisticated group communication systems among fish.