* Predation: One of the most common reasons why fish chase other fish is to eat them. Fish that are predators are typically larger and more powerful than their prey, and they use their speed and agility to catch their food.
* Territory: Fish that are territorial will often chase other fish away from their territory. This is done in order to protect the fish's food and breeding grounds.
* Mating: Some fish species chase each other as a way of courting. The male fish will typically chase the female fish, displaying his fins and colors in an attempt to attract her.
* Play: Fish may also chase each other as a form of play. This is especially common among young fish, who may chase each other for hours at a time.