Fish farming has increased enormously in recent years, stimulated by the shortfall in traditional fishing catches due to declining population of fish caused by overfishing. By far the majority of fish are farmed for food, although some are grown to restock rivers and also to supply fish for ornamental ponds. Some shellfish such as oysters and mussels are also farmed. Work on a fish farm entails feeding, sorting the fish into different tanks as they grow, watching out for and treating diseases if they occur and general maintenance and preservation of the water quality. Salmon farmers have to breed the young fish in fresh water and then transfer them to large pens in the sea.