|Salmon fishery in Alaska. Note the man-made ladder of the salmon run.|
Ninety percent of the fish consumed by people are salt water fish. Fisheries can be the introduction of small fish into an ecosystem. In the fishery pictured in Alaska, they take the eggs and sperm of caught salmon and stir them together to produce small fry to reintroduce. This is called fry farming. At one time the fry would be lost in the salmon harvest.
Aquaculture is a very old practice. Oysters have been cultivated as far back as one century before the common era in the Roman Empire. Jack London wrote a short story about Oyster Pirates. East Coast Oysters were cultivated off the coast of California. Pirates would sneak in and steal the oysters to sell at much lower prices in the marketplaces.
Fish are grown and harvested in ponds, tanks and indoors in some locations. The technology needed to provide nutrients and recycle water are problems solved in these operations. India has a system of ponds in which six species of fish are grown. Some feed off algae and others feed off other components in the pond. Some fisheries have large cages submerged in the water to protect the fish from predation and for easy harvest when the fish are an appropriate size.