The Fish That Farts

In 1981, a Soviet submarine ran aground in Swedish waters. The incident immediately drew the attention of the Swedish Navy, which began deploying a variety of monitoring systems to track activity in the surrounding sea. Soon afterward, naval operators started detecting sounds that resembled submarine propellers. Sweden launched extensive searches for the suspected intruding submarines, yet every attempt ended the same way: nothing was ever found.

Even after the Cold War ended and the Soviet Union dissolved, the mysterious “submarine noises” continued to appear in Swedish monitoring systems. The situation grew tense enough that in 1994 the Swedish Prime Minister formally wrote to Russian President Boris Yeltsin, asking why Russia kept sending submarines into Swedish waters and demanding that such activities stop. Yeltsin's reply was perplexing. He stated that he had no idea what the Swedish government was referring to.

After fifteen years of pursuing what seemed like phantom submarines, the Swedish Navy eventually turned to acoustic specialists and marine biologists for help identifying the true source of the sounds.

When the experts listened to the recordings, they proposed a possibility that surprised the navy: the sounds might not come from submarines at all, but from living organisms. To investigate this idea, the team interviewed local fishermen. The fishermen provided an intriguing observation. When schools of fish were trapped in nets, the water surface above them sometimes appeared to boil.

Clupea harengus。圖源：Gervais et Boulart - Les poissons Gervais，H.

Researchers then conducted experiments with live fish in aquariums. They discovered that Atlantic herring (Clupea harengus) could indeed produce audible sounds. However, the noise from a single fish did not resemble the distinctive “frying bacon” sound that the navy had been detecting. The key realization came next: herring in the ocean often form enormous schools. When many individuals release sound simultaneously, the combined noise closely resembles the sizzling sound previously interpreted as submarine activity.

The mechanism behind these sounds is surprisingly simple. Atlantic herring release gas through the anus—essentially, they fart. Scientists refer to this phenomenon as Fast Repetitive Ticks (FRT). When large numbers of herring release gas at the same time, bubbles rise to the surface. As the bubbles burst, they generate the crackling noises that Swedish sonar operators once mistook for submarine propellers. In other words, the Swedish Navy had been listening to massive schools of fish farting.

Why do Atlantic herring fart at all?

Researchers found that the herring's swim bladder is connected to the digestive tract and ultimately to the anus. When gas accumulates in the swim bladder, it can be expelled as bubbles through the anal opening. Further studies revealed another intriguing pattern: herring tend to release these gas bubbles primarily at night. Because herring possess highly sensitive hearing capable of detecting extremely faint underwater sounds, scientists proposed that these emissions may function as a form of communication between individuals within a school.

This communication system, however, carries certain risks. Most predatory fish cannot detect the sounds produced by herring FRTs, but dolphins and whales can. Marine mammals are capable of using these sounds to locate herring schools. At the same time, herring themselves are able to hear the vocalizations of cetaceans, allowing them to detect approaching predators.

For years, Sweden believed it was being stalked by Soviet submarines. In reality, the mysterious underwater signals were produced by vast schools of Atlantic herring quietly communicating in the dark.

Author: Bai Leng

Reference:

1. María, Braeuner . (2022). Herring Farts Almost Provoked a War (That Biology Prevented). BraeuNERD.

2. Wahlberg, M. & H. Westerberg. 2003. Sounds produced by herring (Clupea harengus) bubble release. Aquatic Living Resources.