There are two basic types of oceanic fish farming. Coastal farming involves shallow-water farms located on the coast, along rivers, or in lakes. Open ocean farming involves deep-water farms that are not tied to a specific coastal location. There are important differences in these two approaches to oceanic aquaculture.
Coastal fish farming is the predominant form of oceanic aquaculture today. In a coastal aquaculture facility, pens or fish cages are deployed along the coastline, often in a protected bay or inlet. Most crustacean and mollusk farming is done inshore, using racks on which the food animals are grown. Depending on the species being farmed, the nutrients for the farmed food animals might come from the water itself, from the provision of forage fish, or from the addition of nutrients into the water in soluble form.
There are advantages to farming fish along the coast. Because the facility is located near the shore, storms are attenuated by the proximity of the landmass, and both workers and shipping infrastructure are close at hand. Operators can bring in supplies and export product from the convenience of local railheads and ports. Because the water is generally shallow at the coastline, it is also possible for operators to use ranch-style techniques, building habitats on the sea bottom for desirable fish such as abalone, and then simply catching the fish in the normal fashion without needing to closely manage the population.
Coastal farms have a number of significant disadvantages as well. Coastal areas are subject to intense competition from other uses such as recreational activities, fisheries, ports, and renewable energy development. Fixed-site aquaculture also has fairly severe environmental impacts on the delicate coastal ecosystem. Fish farms produce large quantities of waste and excess nutrients, which settle onto a fixed location on the sea bottom. This can utterly disrupt or even extinguish the local benthic ecosystem, causing repercussions to fisheries and tourism. Disease among the fish stocks is also a major problem, and because coastal fish farms can be close to one another, a disease which decimates one population can spread to other operations, even crossing species barriers.
Offshore fish farming, or deep-ocean aquaculture, cuts the ties to the shore, although not to the sea bottom. Generally, an offshore facility is tethered to the bottom and anchored to buoys, so that cages can move up and down in the water column but are still at a fixed location in the ocean. An offshore aquaculture can be sited almost anywhere, and does not have to compete with pleasure boating or fishing fleets. In addition, the much greater area (and stronger currents) available for dispersal of waste products and excess nutrients means that the environmental impact is significantly reduced or eliminated. Disease transmission is less problematic, and experimental offshore aquaculture operations have found that parasitic infestations are much more easily managed in the relative isolation of an offshore facility. Offshore locations are also more able to coexist with other uses of the same area of the ocean; since the cages can be moved deeper into the water than is possible in a coastal area, other uses such as boating can be accommodated.
Offshore aquaculture has its own set of problems, however. The expense of building cages that can withstand the storms and currents of the open ocean is considerable. Because the environmental conditions are more rigorous in the open sea, fish escapes are more likely to occur, which has ramifications both in terms of cost and in the potential for the introduction of invasive species to ecologically vulnerable areas of ocean. In addition, the laws and regulatory environments that apply close to shore (within the three-mile limit, for example) become more complex further out to sea; for this reason, off-shore aquaculture in the United States has primarily developed only in areas of the ocean that are unambiguously under Federal jurisdiction so that operators can have a predictable legal environment in which to do business.
Despite the potential issues and problems that arise in both coastal and offshore aquaculture, it is a certainty that these areas of economic activity will continue to grow and expand. Fish are a vital part of the global food infrastructure and aquaculture is rapidly becoming the dominant way in which this sector produces its output.