The Secret Language of the Sea: Why Do Schools of Atlantic Herring Use High-Frequency Bursts of Underwater Flatulence to Communicate in the Dark?

In the vast, pitch-black depths of the Atlantic Ocean, maintaining a cohesive social structure is a matter of life and death. For the Atlantic herring (Clupea harengus), staying close to the school provides safety from predators, but when the sun sets, visual cues vanish. To solve this problem, these fish have evolved one of the most unusual communication methods in the animal kingdom. Scientists discovered that these schools emit high-frequency sounds produced by expelling air from their anal ducts—a phenomenon colloquially known as underwater flatulence. But why do schools of Atlantic herring use high-frequency bursts of underwater flatulence to communicate in the dark? Far from being a digestive accident, this behavior is a sophisticated tool for social signaling and predator evasion.

Understanding the Mechanics: What is an "FRT"?

To understand this behavior, we must first look at the biology of the herring. Researchers from the University of British Columbia and the Scottish Association for Marine Science identified these sounds as "Fast Repetitive Ticks," or FRTs. Unlike human flatulence, which is a byproduct of digestion, herring FRTs are produced by air stored in the swim bladder.

• The Source: Herring swallow air at the surface and store it in their swim bladder.
• The Action: To communicate, they pulse this air through a narrow opening called the "ductus pneumaticus" near the anus.
• The Sound: This results in a burst of high-frequency clicks that can last up to several seconds.

Because these sounds are linked to air storage rather than food intake, they are entirely voluntary and can be modulated based on the social context of the school.

Maintaining Social Cohesion in Total Darkness

The primary answer to why do schools of Atlantic herring use high-frequency bursts of underwater flatulence to communicate in the dark lies in social organization. Herring are obligate schoolers, meaning they rely on the group for protection. During the day, they use their eyes to maintain their position relative to their neighbors. However, at night, visual contact is impossible.

According to research published in the journal Biology Letters, the frequency of these FRT sounds increases significantly as the density of the school increases. This suggests that the sounds act as a "roll call" or a spatial anchor. By emitting these high-frequency bursts, individual fish can sense the proximity of their peers through their specialized hearing, ensuring the school does not drift apart in the shadows.

The "Private Channel" Advantage

One of the most fascinating aspects of this behavior is the frequency of the sound itself. Most predatory fish, such as cod and halibut, have a hearing range limited to lower frequencies. In contrast, herring have evolved specialized "prootic bullae"—gas-filled structures connected to the inner ear—that allow them to detect much higher frequencies.

By using high-frequency bursts, herring create what scientists call a "private communication channel." This offers several evolutionary advantages:

• Stealth: They can signal to one another without alerting nearby predators that lack high-frequency hearing.
• Clarity: High-frequency sounds dissipate quickly in water, meaning the signal stays within the immediate school rather than traveling long distances and attracting distant threats.
• Precision: These short, sharp bursts provide clear "pings" that help the fish triangulate their position within a moving mass of thousands of others.

The Discovery and Its Scientific Impact

The discovery of FRTs was so unique that the research team, led by Ben Wilson, was awarded the Ig Nobel Prize in 2004—a prestigious award for research that first makes people laugh, then makes them think. Their study proved that fish communication is far more complex than previously thought. Before this research, it was widely assumed that most fish sounds were mechanical or accidental. The realization that herring use "gas-mediated" sounds to navigate the darkness transformed our understanding of marine acoustics and social evolution.

Conclusion

The question of why do schools of Atlantic herring use high-frequency bursts of underwater flatulence to communicate in the dark reveals the incredible ingenuity of evolution. What might seem like a humorous biological quirk is actually a vital survival strategy. By utilizing high-frequency pulses of air from their swim bladders, herring can maintain school density and stay safe from predators in conditions where vision is useless. This "private channel" of communication ensures that even in the deepest, darkest waters, the school remains a single, protected unit. As we continue to explore the mysteries of the ocean, the humble herring serves as a reminder that nature often finds the most unexpected solutions to the challenges of survival.