Imagine you spend your whole career chasing after something you cannot see, but only listen to its “typical sound”, and it is a matter of national security or maybe even a World War III trigger. You have the sound, your sonars show there is something out there, you are not going crazy. But you cannot seem to find it… Fifteen years later it turns out it was only a small fish. A big school of small fish…farting. This happened to the Swedish Navy not so long ago, built up diplomatic tensions between Sweden and Russia, but in the end, a little biology brought back peace to the Baltic Sea.
Sweden’s famous “neutrality” did not leave them out of the way of conflict during past wars. Swedish territory was put to the test several times, their location was a good reason to be prepared, and we now know that they were not that neutral after all. But Sweden was not directly involved during the Cold War (1947 – 1989/91). However, their neighbors from across the Baltic Sea were the main actors in this war: the (back then) Soviet Union.
Naturally, it was a cause of great concern when in 1981 a lost Russian submarine got stranded on the Swedish coasts in the Baltic Sea. Not only was it in Swedish territory unexpectedly but also a restricted zone near a Swedish naval base. Coincidence? We still don’t know. The Russians claimed a malfunction of their navigation systems and were eventually escorted back East. During the rest of the Cold War, the swedes remained vigilant and increased surveillance including several unsuccessful submarine hunts within Swedish waters. Their military hydrophones and sonars were often detecting a lot of submarine activity – or so they thought.
But the Cold War came to an end between 1989 and 1991, so why was the Swedish Navy still detecting a lot of submarine activity for several years after that? Tensions built up to the point that in 1994 Sweden’s Prime Minister Carl Bildt wrote a letter to Boris Yeltsin, first president of the Russian Federation, asking what was going on and to put an end to this. Yeltsin replied: “dude, I have no idea what you’re talking about” (or, you know, a diplomatic version of that).
It was until 1996 that the Swedish Navy decided to show all this acoustic evidence to the first civilians that ever listened to this “typical sound”: bioacoustics experts (bioacousticians?) and fish biologists Magnus Wahlberg and Håkan Westerberg. Keep in mind that by this point the Navy had been listening and chasing these sounds for fifteen years. FIFTEEN!!!
In his TEDtalk, Wahlberg describes what the Navy was calling “typical sounds” as the sound of frying bacon. Wahlberg and Westerberg suspected a biological origin of the “typical sound”. They talked to local fishermen who said sometimes their fishnets “boiled”. They first studied Baltic herring fish in aquariums and noticed that they do make some sounds – but it was not like frying bacon sounds. They then realized they needed to scale up because these fish travel in big schools (huge schools – Atlantic herring can travel in schools of up to four cubic kilometers!!! Although I haven’t found a reliable estimate for the Baltic sub-species). It turned out that a lot (a lot!) of these fish together expelling little bubbles of air from their anuses make the fishermen’s nets look like they are “boiling” when the bubbles reach the surface and they make the frying-bacon sound. The “typical sound” the military had been after for over fifteen years was nothing more than farting fish.
But how does a fish fart?
When we go swimming, we need to invest a lot of energy to stay afloat, and it can be exhausting (or a good cardio workout). But then, are fish constantly exhausted? That wouldn’t make a lot of evolutionary sense.
Many bony fish species control their buoyancy with a special organ called the swim bladder. The swim bladder helps fish stay afloat passively and save that “muscle energy” for when they need to escape from predators, fishermen, or biologists. You can picture the swim bladder like a balloon inside a fish’s body. They can inflate or deflate it to control where they are in the water column.
There are different types of swim bladders regarding how they are connected to other organs and how they function. Some species have the swimming bladder connected to their mouth and can refill it at the water surface; others have lost these connections and exchange gases solely through their circulatory system or have a gas gland that fills it up with carbon dioxide produced through biochemical reactions in other organs. Others, like the Baltic herring (Clupea harengus membras), have their swim bladder connected to both their stomach (thus indirectly to their mouths) and their anus.
Other fish, like cartilaginous fish (sharks and rays), do not have swim bladders. Instead, they store a lot of oil in their liver to help them with their buoyancy. As you might recall from childhood experiments: oil is less dense than water.
But let’s get back to the Herring.
Fish physics is really interesting, and we can go on for a while, but for today let’s focus on the acoustics of the Baltic herring’s anal breeze. For a while, it was thought that herring use their mouth to fill the swim bladder through the “pneumatic duct” (a little channel that travels through the stomach) when they’re at the surface of the water. When they need to release gas (to avoid exploding when ascending or when stressed by fishermen’s nets, for example) they do not burp it out: they fart it out. But many questions remain regarding how herring actually fill their swim bladders since a study on Atlantic herrings reported that most of the farts were produced at night and not necessarily related to when they gulped air from the surface. So how do they store or “re-fill”? We don’t know yet; we just know they fart (sure, some might argue they are not “farts” because they are not produced in the intestinal tract, but we are using a liberal definition of fart as “any emission of gas from the anus”).
Why all the nighttime farting, you ask?
Interestingly, the gas-filled channel in the herring also connects their swim bladder to the otolith organs (related to direction and “hearing”) and part of their “detection system” (or lateral sensory cells) in their heads. This means that herring are good listeners!
By now several studies have shown that clupeid fish (the herring family) can sense hydro-acoustic differences in their surroundings and communicate through their farts, using their swim bladders as a resonating chamber. This is particularly useful at night to stay close to the schools when their visibility is impaired. Wahlberg and Westerberg had also argued that the herring probably release gas for a social reason and not so much for physics.
Funnily enough, the paper on Atlantic herring named the farts “Fast Repetitive Ticks”, or FRTs. For a while, scientists thought only members of the same species could hear their own high-pitched FRTs to share their “secret messages” and successfully swim away from predators, but now we know some predators – some whales – are also capable of hearing the herring FRT conversations – as de Swedish Navy did too.
So now you know, consult with scientists before you spend over fifteen years being terrified by your staple food!
Fun Fact: During World War II sonar operators were often confused by the resonance of a lot of aquatic fauna swim bladders and other “inner bubbles” of some planktonic animals. A weird phenomenon of an ocean “false bottom” (now known as the “deep scattering layer”) gave the impression that the ocean floor was deeper during the day than at night, when in fact this was just “a layer” of wildlife.
If you learned something new here and would like to support this blog, maybe [kofi] ?
References
Wahlberg, M. & H. Westerberg. 2003. Sounds produced by herring (Clupea harengus) bubble release. Aquatic Living Resources. 3(16) 271-275.
Wilson, B.; R.S. Batty & L.M. Dill. 2004. Pacific and Atlantic herring produce burst pulse sounds. The Royal Society Biology Letters. 271 S95-S97.
Sharpe & Dill. 1997. The behavior of Pacific herring schools in response to artificial humpback whale bubbles. Canadian Journal of Zoology 75(5), 725 – 730.
Doksætera, L.; O.R. Godø, & N.O. Handegard. 2009. Behavioral responses of herring (Clupea harengus) to 1-2 and 6-7 kHz sonar signals and killer whale feeding sounds. The Journal of the Acoustical Society of America. 125, 554.
Whaa?? They be FARTING??!?