Wild fish stocks have declined and the commercial harvest of fish from oceans has reached a plateau, so the seafood industry has turned to aquaculture (see page 230) to meet increasing demand for seafood (Figure 8.17). Over the past three decades, aquaculture production has increased by about 9% annually, a growth rate exceeding that of all other forms of food production. By 2011 this resulted in more than 40% of total fisheries production (Figure 8.18).

Aquaculture occurs in marine, brackish, or freshwater environments all over the world. Farmers raise fish in ponds or in mesh cages suspended in water, while shrimp and crabs are generally grown in ponds or tanks. In intensive aquaculture, farmers feed fish and crustaceans specially formulated diets for optimum growth. By contrast, filter-feeding shellfish such as oysters and mussels can be suspended in the water column on racks or attached to lines, where they feed by filtering plankton and other organic matter from the surrounding water. Properly managed aquaculture systems can provide an environmentally friendly source of dietary protein. However, just like agriculture, aquaculture has the potential to threaten the biodiversity and health of ecosystems in several different ways.

Critics of aquaculture point out that in all but the most secure systems, some of the organisms being cultured will escape into the wild. For example, cage-grown fish can escape if the netting confining the fish is torn. Pond-raised fish or crustaceans may jump or crawl into nearby waterways. Escapees may become invasive species (see Chapter 3), such as the Asian carp, which escaped from aquaculture ponds to colonize the Mississippi River system and which now threaten the Great Lakes. Even native species pose threats: When domesticated individuals mate with wild, locally adapted individuals, it generally reduces the fitness of wild, locally adapted populations. Domesticated fish have genetic characteristics that have been selected for confined rearing, whereas wild fish have genetic characteristics optimized for their native environments. Therefore, when aquaculture fish breed with wild fish, they introduce genes that decrease adaptation of wild populations to their environment.

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Currently, there are no genetically modified varieties of fish used in aquaculture. One company, AquaBounty Technologies based in Massachusetts, is seeking approval from the U.S. Food and Drug Administration (FDA) for its faster-growing, genetically modified (GM) Atlantic salmon. Despite protective measures the company has taken, some scientists and conservationists believe that these fish will threaten wild fish populations if they escape.

Aquaculture can also be a significant source of water pollution. As in agriculture on land, runoff from fish and shellfish feeding operations contains nutrients, especially nitrogen and phosphorus, which can produce noxious algal blooms that impair water quality and can kill wild fish. Aquaculture waste may also contain enough organic matter to deplete oxygen supplies in waters receiving the waste, again potentially resulting in fish kills.

Shrimp farms are increasing pressure on mangrove forests, one of the most valuable and endangered tropical ecosystems on Earth. Mangrove forests grow in coastal waters, where they protect coastal areas from storm surge and may protect against tsunami damage. The roots of mangroves also provide protection from predation for young fish, so they additionally act as nurseries that enhance the productivity of coastal tropical fisheries. Mangroves also happen to be desirable locations for shrimp farms because mangroves grow along the shores of warm tropical oceans in soft sediments ideal for pond building.

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As the pond culture of shrimp has increased from 72,000 metric tons (83,000 tons) in 1980 to 2.5 million metric tons (2.8 million tons) in 2009, so has the clearing of mangrove forests (Figure 8.19). Shrimp farms currently account for about 10% of total mangrove loss, along with urbanization, agriculture, and removal for fuel and construction. Globally, approximately one-third of mangrove forests have been cleared due to deforestation and development.

One serious issue with aquaculture is that fish farmed at high densities are highly susceptible to parasites and pathogens, which can then be transmitted to wild stocks. For instance, areas with cage-grown salmon are associated with a decline of neighboring local populations of wild salmon and trout (Figure 8.20). Most of these declines appear to be the result of infestations by sea lice, Lepeophtheirus salmonis, a small parasite that grazes on the external mucus layer of salmon and its close relatives.

While aquaculture may appear to be a low-impact alternative to fishing, it does not necessarily reduce the stress on marine fish populations. Raising carnivorous species, such as Atlantic salmon and some species of shrimp, still requires harvesting wild fish for fish meal and fish oil to provide the captive fish and shrimp with sufficient protein and fats. Both the fish meal and fish oil are derived from small forage fish, such as anchovies, herring, and mackerel, which are important species in oceanic food webs because they transfer trophic energy from small primary producers and zooplankton to higher trophic levels.

Some 20 to 30 million metric tons (22 to 33 million tons) of forage fish are harvested each year for aquaculture feeds. This number represents between one-fourth and one-third of the global fish catch. By 2009 aquaculture was consuming 68% of the global production of fish meal. Removing forage fish from the ecosystem for fish meal production reduces the availability of that food to fish higher in the marine food web. Because of energy loss on trophic transfer, it can take between 2 and 5 kilograms of wild caught fish to produce a single kilogram of fish in aquaculture.