People have a lot of ways to warn each other about danger.
We can yell, “Hey! Watch out!”
We can wave our arms quickly above our heads, or put our palms out to stop someone walking behind us.
Sirens and flashing lights also warn us about dangers like fire or a car wreck.
Fish don’t have arms or voices, so how do they warn each other?
Scientists at the University of Saskatchewan in Canada say that some fish communicate by smell. They have a fancier phrase for it, though: chemosensory cues. You might say that some fish can smell fear.
The researchers also call these fishy warnings a “disturbance cue.” When a fish gets scared by a predator it squirts out a bit of urea, which comes from the fish’s kidney and is high in the chemical nitrogen.
Fish gave the warning when they were chased, surprised or stressed by predators.
In the scientists’ tests the fish did not warn others all of the time. They did it most often when they were with fish they knew. When they were alone or close to strangers the fish were less likely to issue a warning. And the fish in the study didn’t get alarmed when the warning came from a fish they didn’t know.
When other fish sense the disturbance cue they responded in a few different ways. Sometimes they would freeze in place, other times they might swim quickly away. If they were with a group of fish they knew they might bunch up together. All of these are ways they might be able to avoid being eaten by another fish, reptile or bird.
Studies have shown that other animals use disturbance cues, including frogs, crayfish and sea urchins.
This all makes me wonder: When I catch a fish, does it try to warn others? That would help explain why I don't catch too many fish.
— Brett French, firstname.lastname@example.org