Deep scattering perplexed sonar technicians during World War 2, as their systems appeared to be telling them that the depth of the seafloor kept changing from day to night. It was later revealed that a peculiar phenomenon that we now know as the deep scattering layer was screwing with their instruments, and it’s an example of accidental teamwork among marine animals.
The anomaly is connected to how sound travels and can be reflected, and hinges a lot on the bizarre swim bladders of animals that live suspended in the water column. These buoyancy bags help them to regulate depth by filling with and passing gas (some fish effectively fart to go deeper), but they also serve as a springboard for sonar signals.
What is the deep scattering layer?
The deep scattering layer is made up of lots of marine animals like fish, squid, and jellyfish, to name a few, many of which are bioluminescent. They can be found throughout the oceans but are typically seen at depths of around 300-500 meters (984-1,640 feet), though this changes through the day. Deep scattering layers tend to be deeper during the day and shallower at night when marine animals travel closer to the surface to feed (this is known as diel vertical migration).
Most marine animals are concentrated in deep scattering layers across the oceans, and they provide vital nutrients to the seabed in the form of “marine snow”, a deep-sea dietary staple. The deep scattering layer is therefore a mobile buffet for predators who will target them when hunting, which is one of the reasons why these congregations move around throughout the day.
A handful of small species doesn’t tend to do much to sonar readings, though midsize gatherings are sometimes confused for megalodon. However, when small marine organisms gather in densities thick enough to constitute a deep scattering layer, they can create a “false bottom”.
What does the deep scattering layer do to sonar?
The “false bottom” effect occurs when a dense layer of marine animals interrupts sonar signal to such an extent that it reads as a physical barrier. It’s an interesting observation, but one that becomes disorientating when you’re relying on that same signal to asses where the seafloor is. According to the National Oceanic and Atmospheric Administration (NOAA), diel vertical migration meant early technicians during World War 2 were taking one ocean depth measurement during the day, only to see the seafloor bounce back to just a few hundred meters’ depth at night.
Fish swim bladders are partly to blame for this rebounding effect. The gas-filled organ helps them to maintain the right buoyancy to stay at their desired depth in the water column without wasting a lot of energy swimming to stay there.
Fish will let air in and out of the bladder to move up and down in the water column, but the organ is also very good at reflecting sound waves. Since sonar works by emitting acoustic pulses into water to detect reflections, you can see why things get so complicated when gassy fish swim into the picture.
And on that subject…
The herring farts that almost launched nuclear war
It's perfectly feasible that in the 1980s a major diplomatic incident between nuclear superpowers could have been triggered by fish farts. After a Russian submarine ran aground on the south coast of Sweden, they took it as evidence that the Soviet Union was infiltrating their waters.
The submarine was returned to international waters, but the Swedish government remained alert, convinced that Russian subs could still be operating near their territory. Suspicion grew when they started picking up elusive underwater signals, though they could never find anything.
It wasn’t until a team including Magnus Wahlberg, a professor at the University of Southern Denmark, got involved that the source of the sound was pinned down. Herring are unique in that their fish bladders connect directly to the anal opening, but being very small they can only let out a little air at a time so the sound is more of a fizzing than a parp. As it happens, that fizzing was exactly what the naval personnel had been hearing.
The ocean is still home to many mysteries, but as history has shown us, a few of them could turn out to be nothing more than fish gas.