During the breeding season, the sight of a red belly on any other stickleback triggers aggressive behavior in a male stickleback. this is called:

This is called innate behavior.

Innate behavior is a natural gene controlling behavior that occurs in an organism without the need for preliminary learning or experience. This type of behavior is demonstrated by all members of a species in a very similar way whenever they are exposed to a certain stimulus. From the question given, the aggressive behavior being triggered in a male stickleback when it sees a red belly on any other stickleback during the breeding season is called innate behavior.

The aggressive behavior in male stickleback in called a fixed action pattern. Fixed action pattern is a sequence or series of actions that occur as an innate behavior in animals. The actions are triggered by external stimulus called the sign stimulus which involve sight of something. They trigger an unchangeable pattern of behavior. In the case of the stickleback, once the male stickleback sees the red belly of the female stickleback, a signal is sent to the brain that it is mating season and they start being aggressive.