A hatchery is a place where fish are bred and raised. In general, salmon hatcheries work by capturing salmon when they return to fresh water to spawn. The hatcheries collect eggs and sperm (called milt), and mix them together. The fertilized eggs are then incubated, and hatched fish are raised in holding ponds. When they are considered mature enough to fend for themselves, the fish are released into a creek or river where they may reside for about a year or swim out to sea. In some situations, the juvenile fish are held for a period of time at a site away from the hatchery to allow them time to imprint to the natural waters. These "acclimation ponds" are intended to increase the number of fish returning to spawn in natural habitats rather than returning to the hatchery. Salmon raised in hatcheries often differ from wild salmon in their genetics and behavior. The negative effects of hatcheries on wild populations are being debated: for example, hatchery fish that originated from a distant river may intebreed with local wild salmon, or compete with them for food and habitat. Various hatchery reforms are underway including the development of hatchery and genetic management plans to guide hatchery operations and prevent negative impacts to ESA listed salmon species.