What is culling when fishing?

Culling, is a Word meaning to remove certain undesirable animals from an ecosystem in order to acquire a more healthy population. This can be practiced while managing a pond or personal body of water. However, culling is most often used in fishing when referring to tournament fishing. In a tournament where you can only weigh five fish, you would keep your first five keepers, than "cull" (remove them from your livewell) the smaller ones as you catch bigger ones to replace them.