Each fish, swimming alone in an aquarium, interacted with a 12cm semi-buoyant thermometer. They would nudge and bump the object with their noses in a manner distinctive to each fish. Collisions between thermometer and aquarium glass were loud enough to be heard in the adjoining room. During 12 observation sessions, there were over 1,400 interactions between fish and thermometer. 

Were the fish playing? That’s what the research team concluded based on several criteria for animal play: it was voluntary behavior; it didn’t involve a clear survival purpose such as feeding, mating, or fighting; it was repetitive but not neurotic; and it was only performed when the fish were not stressed or scared. I’ve witnessed play-like behaviour in fish on two occasions during visits to a reclaimed wetland in south Florida: small minnows leaped over floating twigs and reed stems—playful behaviour that has been anecdotally reported by many people.