Ocean farming

Although oceans cover about 70% of the Earth's surface, most food is produced on land areas through agriculture and animal husbandry. This is because 90% of the ocean is unproductive. The productive areas are near shore, such as continental shelfs, upwellings , coral reefs, mangrove swamps (also known as mangal communities), and estuaries. The primary reason why these near shore areas are more productive than the open ocean is because of the supply of nutrients which are needed for plant (primarily phytoplankton ) productivity, which fuels the food chain/web .

A worldwide plateau in harvesting of natural fish catch was reached in about 1989, so commercial and sport fisheries in the oceans cannot be increased unless something is changed. One possibility is to enhance production through fertilization of the water. Another, more practical, possibility is through aquaculture . As wild harvests decrease and per capita seafood consumption increases, aquaculture should have an important role. For example, the U.S. imports 60% of its seafood, which contributes to its trade imbalance. Aquaculture can help by providing jobs as well as food products for domestic consumption and for export.

Aquaculture done in seawater is known as mariculture . Fish and shellfish are grown in improved conditions to produce more and better food in environments with lower predation and disease. There are several forms of aquaculture: intensive, extensive, and open ocean. In extensive aquaculture, relatively little control is exerted by the mariculturist. These typically occur near shore where organisms are grown in several different ways: floating cages or pens; cordonedoff bodies of water which can be fertilized for enhanced production; or racks and other structures (to grow shellfish). For example, oyster farmers in the Chesapeake Bay place clean substrates in selected areas of the bottom mud each summer to collect young oyster larvae (called "spat") as they settle down from the plankton . These young oysters attach and grow on the substrates, which are periodically transferred to new locations to prevent them from being covered with mud and silt . After several years, the oysters are large enough to be marketed. Other culture methods for bivalves such as oysters and clams utilize the stake method, which involves the use of bamboo or other poles driven into the sediment and placed approximately 6 ft (2 m) apart. Nylon ropes hanging from floats can also be used to collect larvae. Following growout, the shellfish are collected and sold on the market. Farmers in the Philippines are able to produce nearly 1.5 tons per acre (0.60 tons per ha) of mussels and 1.7 tons per acre (0.70 tons per ha) of oysters annually. These techniques all have relatively low cost and maintenance. Sometimes there are problems with pollution from domestic and industrial waste, and periodic blooms of toxic dinoflagellates (a type of phytoplankton), which create red tides. Red tides can be unhealthy and even lethal to humans.

Intensive mariculture systems involve the production of large quantities of marine animals in relatively small areas. They are relatively expensive to build and maintain, so typically only highly-valued species are used, such as lobsters, shrimp, halibut, and certain other fish. Intensive systems can sometimes be located in tanks on land, using either pumped salt water from the nearby ocean or recirculated artificial seawater if the system is located farther away from the ocean. Typically, however, nearshore or offshore floating tanks or pens are used. The organisms are raised from eggs to market size in these controlled environments. Salmon have been raised in sea pens (netted cages) for over 20 years along the Pacific coast of the United States and Canada and along the Atlantic coasts of France, Scotland, and the Scandinavian countries, especially Norway. This process typically involves the growth of salmon from fertilized eggs through the fry stage in indoor fresh water hatcheries. This is followed by the transfer of parr (the stage after the fry in which the fish develop vertical lines along their sides) to outdoor tanks or cages in salt water. The fish are fed either commercial pellets or "trash" fish such as herring, capelin, menhaden, and anchovies. The feeding process can be quite labor-intensive, and thus translates into one of the highest expenses for the salmon farmer. After one to two years, the salmon go through a process called smoltification, which involves a physiological change that allows them to live in salt water. The smolt are then transferred to salt water cages for another one to two years until they reach a market size of about 4-12 lb (2-5 kg). Rainbow and sea trouts are handled in a similar way, although they are usually larger when moved into sea pens. As with the extensive method, problems can arise through pollution and red tides.

Most mariculture industries use both nearshore and offshore cages for raising the fish. The nearshore cages are necessary for raising young fish, but the offshore cages are better for raising fish to maturity because of enhanced growing conditions (cleaner water and lower mortality ). Improvements are still needed, such as methods to remove dead fish, make grading (sorting) of fish possible, make harvesting easier and feeding possible (particularly during adverse weather), and make fouling removal easier. It will also be helpful to construct 24-hour living accommodations on some of the larger offshore units.

A third form of mariculture is known as ocean ranching, or "enhancement aquaculture." This can be compared to cattle ranching on land. For example, salmon are raised in hatcheries until they reach the smolt stage, and are then released at a particular point along the shore where they swim away until they reach reproductive maturity. Like other anadromous fish, they return to that same area several years later, where they are recaptured, processed, and sold on the market. Salmon are ideal fish for this because they are "selfherding." However, normal returns range from only 1-20%, depending upon the water quality of the return environment and the release size of the fish. For example, Coho salmon fry that were around 0.5 oz (14 g) showed a 1-2% return while those twice this size had a 7-8% return rate. Recent research has shown that released salmon can be imprinted to return to salt water sites, which has the added advantages of lower cost (due to not needing more expensive property located along streams that enter the ocean), and better meat quality (which declines when salmon swim from salt to fresh water). The Sea Run, Inc. salmon company of Kennebunkport, Maine, rears Pacific pink and chum salmon from eggs to fingerlings in a hatchery located near the discharge of an oil-fired power plant. The water has a characteristic temperature and smell which attracts the salmon back to the same area each year. They have also experimented with the release of morpholine and phenyethyl alcohol (synthetic organic compounds) to the water to aid in imprinting.

Similar ocean ranching techniques have been used in other countries. For example, in Japan the natural catch of Red Sea bream has declined for the last 24 years (more than a 51% decrease) due to over-exploitation, so these fish have been raised in tanks and cages since 1962. Other species commonly raised in Japan include salmon, flounder, mackerel, bluefin tuna, shrimp, scallops, and abalone. Many of these species have a high consumer demand and thus are highly priced. Abalone can also be grown to a small size of approximately 1 in (3 cm) and used to re-seed areas where the natural population has declined. They can then be harvested as adults about three to five years later. However, recapture rates are typically low (only 0.5–10%).

Ocean farming or ranching is becoming popular worldwide. In Equador, Mazatlan Yellowtail fish are grown to commercial size (2.7 lb; 1.2 kg) in six months with a 90% survival rate. Other species raised in this country include flounder, snook, red drum, and Pacific pompano. In Australia , oysters are commonly raised via sea ranching. Scallops and salmon are now raised in Canada. Mussels, oysters, clams, bream, turbot, and salmon are common mariculture species in Spain, where the warm water allows these species to be grown to market size in a shorter time.

Recent work has been done on algal turf mariculture around some Caribbean islands in natural, unfertilized waters. Test plots have shown that they can raise up to 25 million tons (23 million metric tonnes) of dry algae per year without harming the natural ecosystem . This product can then be used for direct human ingestion or as food to raise marine invertebrates (such as West Indian spider crabs, conchs, and whelks) as well as some herbivorous fish. This would be particularly helpful because conch are now an endangered species through over-harvesting in this area.

Ocean ranching, along with other forms of aquaculture, is an exciting field that should be in greater demand in the future. Our progeny may depend on it more for global food production to feed the increasing human population.

[John Korstad ]

RESOURCES

BOOKS

Landau, M. Introduction to Aquaculture. New York: Wiley, 1992.

PERIODICALS

Adey, W. H. "Food Production in Low-Nutrient Seas." BioScience 37 (1987): 340–8.

Shelty, H. P. C., and G. P. Satyanarayana Rao. "Aquaculture in India." World Aquaculture 27 (1996): 20–24.

OTHER

Reinertsen, H., L.A. Dahle, L. Jørgensen, and K. Tvinnereim. Fish Farming Technology. Proceedings of the First International Conference on Fish Farming Technology, Trondheim, Norway, August 9-12, 1993. Brookfield, VT: Balkema Publishers, 1993.