The environmental impact of fishing includes issues such as the availability of fish, overfishing, fisheries, and fisheries management; as well as the impact of fishing on other elements of the environment, such as by-catch. These issues are part of marine conservation, and are addressed in fisheries science programs. There is a growing gap between the supply of fish and demand, due in part to world population growth. Similar to other environmental issues, there can be conflict between the fishermen who depend on fishing for their income, and fishery scientists whose studies indicate that if future fish populations are to be sustainable then some fisheries must reduce or even close.