Rainbow trout Oncorhynchus mykiss are also masters of aquatic espionage, according to new research from Sweden. Because fights can result in serious injury or even cost them their lives, they find out what they’re letting themselves in for before wading in. Jörgen Johnsson and Anders Åkerman at the University of Göteborg watched what happened when juvenile trout who had observed other pairs fighting were matched against either the winner they had just watched or a fish from an unseen contest. When a trout had inside information as to its chances of winning, it became aggressive more quickly in fights it eventually won and surrendered earlier in fights that it lost.
(Animal Behaviour, vol. 56, pp771-6).