In almost all Harvest Moon games there have been fish ponds, duck ponds and the like; however, they do not always allow the breeding of fish. Harvest Moon usually makes the breeding as simple as throwing one or two fish in a pond, which increases their size (Small -> Medium -> Large) and growing their quantity if there are one or two Large Fishes inside. The player may need to feed them every day, though this varies between games.
In games that revolve around fish breeding or taking care of a virtual aquarium, fish tend to have stats that can be raised, and can pass on these stats to offspring. Offspring may receive stat boosts depending on their parentage, and might even emerge as a new species if certain requirements are met.
Most often the ultimate goal of fish breeding is either getting the rarest species, raising a fish with the highest possible stats, winning fish races, or simply generating better ingredients for cooking.