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> Especially if among fish in their species:
> 
> https://www.sciencedaily.com/releases/2019/04/190418141739.htm
> 
> "The USask researchers discovered that wild fish release chemicals called 'disturbance cues' to signal to other fish about nearby dangers, such as predators.
> 
> ***
> 
> "Fish signaled most when in the presence of familiar fish, but signaled far less or not at all when in the presence of strangers, or when on their own.
> The signals provoked a 'fright response' in fish they knew, including freezing, dashing about and then shoaling tightly together. Fish use this behavior to defend themselves against predators.
> 
> "'When minnows were present alongside familiar minnows, they were much more likely to produce signals that initiated close grouping of nearby fish, a strategy used to avoid being eaten by predators," said Bairos-Novak, who is now at James Cook University, Australia.
> Disturbance cues are voluntarily released by prey after being chased, startled or stressed by predators.
> 
> "One of the main constituents of the signal is urea, found in fish urine.
> 
> "Fathead minnows, caught at a lake, were placed in groups with familiar fish, unfamiliar fish or as isolated individuals. The research team then simulated a predator chase. The fish responded by shoaling, freezing and dashing when they received a signal from a group they knew. But they did not take significant defensive action when receiving cues from unfamiliar fish or isolated minnows.
> 
> "Disturbance cues are voluntarily released by prey after being chased, startled or stressed by predators."
> 
> Comment:  Plants warn of danger by  releasing gases. It is not surprising that evolution has provided a similar mechanism in fish, certainly an example of convergence.