How Sonar Finds Schools of Fish — Thanks to Their Swim Bladders

A fish's swim bladder was not made to be found by sonar, but it is. This gas-filled organ helps a fish stay at the right depth without wasting energy — and it also reflects sound so well that scientists can use it to locate whole schools of fish.

Sonar data on fish, combined into a montage. Credit: NOAA.

Why fish have swim bladders

If fish had to swim constantly just to hold their depth, they would burn a lot of energy. Many fish instead rely on a swim bladder, a gas-filled organ along the back of the body, to control their stability and buoyancy in the water. The same organ works as a resonating chamber that can produce and receive sound.

How fisheries sonar works

Scientific divers can observe the underwater world directly, but they cannot see everything. Sonar fills the gap: it surveys a large area quickly, without destroying anything and with very little disturbance.

Sonar sends out pulses of sound and measures how strongly they come back. Anything with a density different from the water around it — fish, plankton, air bubbles, the seafloor — can return a signal. In a fish, the resonant swim bladder contrasts most with the water, and the bigger the fish, the stronger the echo.

Rebuilding the reef in three dimensions

The real value of fisheries sonar is that it captures a "virtual reality" of how fish use coral reef ecosystems. Combined with GIS and other visual tools, it lets scientists reconstruct the ecosystem. NOAA scientists build three-dimensional maps from sonar data to learn where fish spend their time, what kinds of terrain they prefer, and possibly where they spawn.

Why it matters

Tropical island communities depend on healthy populations of reef fish for recreational and commercial fishing and for tourism. Knowing where fish gather, and why, helps managers protect coral reefs. Together with seafloor mapping, fisheries sonar reveals the conditions fish prefer at their "hot spots."

Sources

Based on "How do scientists locate schools of fish?" from NOAA's National Ocean Service, a work of the United States government in the public domain.