Do Fish Know When It Is Raining?

Stand beside a stream on a wet afternoon and watch the surface break into countless tiny rings, and a question almost asks itself. Rain is the one kind of weather that arrives right inside a fish’s world, rather than somewhere above it. From the bank it looks and sounds dramatic. But does the fish down below notice anything at all? To answer that, we need to know two things: what rain sounds like underwater, and what fish can actually sense.

Start with the rain. According to NASA’s Earth Observatory, a raindrop makes sound underwater in two ways: the splat of the drop hitting the surface, and the bubble that often forms beneath the water during the splash. For most raindrops, the sound of the bubble is the louder part. And the size of the drop matters enormously. Small drops, about 0.8 to 1.2 millimetres across, are described as remarkably loud, producing high, bell-like tones between 13 and 25 kilohertz.

Other drops behave very differently. NASA reports that medium-sized drops, around 1.2 to 2.0 millimetres, are relatively quiet, much quieter than small ones. Large and very large drops make energetic splashes that trap bubbles of different sizes, producing loud sound that includes relatively low frequencies of about 1 to 10 kilohertz. The pattern is so distinctive that scientists at the University of Washington built an acoustic rain gauge that measures rainfall over the ocean simply by listening to it.

Now the fish. The International Game Fish Association explains that fish have no outer or middle ears, but they do have inner ears containing ear stones called otoliths, which vibrate in response to sound and trigger sensory cells. It cites research suggesting that bass have a narrow range of hearing, with a peak around 100 cycles per second, weak hearing above 200, and no hearing above 600. Many fish also have a lateral line, a row of sensors along the body that interprets low-frequency vibrations and works with the inner ear.

A 2013 review in the Journal of Experimental Biology describes those lateral line sensors as detectors of water motion, most sensitive at low frequencies of roughly 100 to 200 hertz, and suggests that in shallow water fish probably detect sound by combining their ears and lateral line. Put that next to NASA’s figures and something interesting appears. Much of the underwater sound of rain sits in the thousands of hertz, far above the range of the bass described by the game fish association.

So do fish know when it is raining? These sources cannot say for certain, and fish are not all alike. But the physics suggests that a fish may not experience rain the way we imagine: the bright, bell-like chorus that fills the water may be largely out of its hearing range. Whatever a fish does make of a downpour, it is probably a very different thing from the drumming we hear from the bank, which makes the question even more intriguing than it first appeared.