Fisheries

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Fisheries are regions of water where the levels of particular aquatic species are suitable for commercial harvesting. In addition to croplands and rangelands, fisheries represent one of the world's three major systems of food supply, as they provide food for approximately one billion people worldwide. The following is a list of some of the most commercially valuable species of fish and shellfish:

• Herring, sardines, and anchovies: these small fish feed on plankton near the surface of coastal waters and often form large schools.
• Salmon: these surface-dwelling (pelagic) fish live in the northern water of the Atlantic and Pacific Ocean. They are prized for their size and taste, and while they do not form schools, they are easily captured when they migrate in groups to freshwater streams for breeding.
• Cod: these bottom-dwelling (demersal) fish form large schools and feed mostly on crabs and mollusks.
• Mackerels and tunas: these pelagic fish are top carnivores that hunt in schools for smaller fish in the surface waters of the open sea
• Flatfish: species such as flounder, sole, and halibut live on the ocean bottom of the continental shelf.
• Invertebrate mollusks and crustaceans: species such as shrimp, crab, and lobsters actually command the highest market prices.

Fishing Methods

The commercial fishing industry has developed a variety of methods in order to catch marine species. Trawlers are fishing boats that drag large nets across the ocean floor. These nets catch the desired fish, but they also damage ocean habitats and capture huge quantities of bycatch, unwanted species that have little commercial value. Bycatch includes species such as dolphins, seals, and sea turtles, and it amounts to almost one-third of the total world catch each year. Purse-seine fishing is another method used by commercial fishermen, and it is used mainly to catch large pelagic species such as tuna. The purse-seine net surrounds the fish and closes like a drawstring in order to trap them. In the Pacific Ocean, these nets have killed large numbers of dolphins, which often swim directly above the schools of tuna. A third method is long-lining, in which ships send out fishing lines up to eighty miles long. These lines are hung with thousands of bated hooks, and they sometimes catch dolphins, pilot whales, sea turtles, and sea birds by accident. Finally, drift-net fishing uses nets made of invisible filament mesh in order to catch and hold fish by their gills. These nets catch almost anything that comes in contact with them, resulting in the unwanted capture and sometimes death of marine mammals, turtles, and seabirds. In 1993, the United Nations ordered a moratorium, or suspension, of large-scale drift-netting. This moratorium has significantly reduced the practice of drift-net fishing, but smaller drift-nets are still being used frequently in coastal waters.

Overfishing

The sustainable yield of a population of aquatic organisms is the largest amount of organisms that can be continually harvested without causing the population to crash. This yield often occurs when a population is maintained at half of its carrying capacity, or half of the maximum population that its ecosystem can support. Overfishing results when the harvest of a species exceeds that species' sustainable yield. In other words, so many fish are harvested that too few remain to breed and replenish the population, and as a result the population experiences a significant decline in numbers. When allowed to continue, overfishing often leads to the commercial extinction of a species, or the point at which it is no longer profitable for fishers to harvest them. If fishing fleets leave the region, the species may eventually recover. However, some weaker species may move closer to biological extinction, the complete disappearance of a species from the planet. This may be the case with the orange roughy in the waters of the South Pacific. Catches of this fish have dropped seventy percent over the past six years, and because they take twenty-five years to mature, the roughy may soon become extinct. Overall, over one hundred different types of fish are fighting for survival in waters in and around the United States. Many more are threatened worldwide, and unless we make significant changes to our fishing techniques and regulations, populations of fish will continue to disappear at alarming rates.

Aquaculture

Aquaculture is the commercial raising of fish and shellfish for food. As overfishing becomes an increasingly serious problem, the practice of aquaculture is growing at a rate of about six percent per year, and it currently supplies one-forth of the world's commercial fish harvest. There are two major types of aquaculture: fish farming and fish ranching. Fish farming uses a controlled environment, usually a pond or tank, to breed and raise fish until they reach the desired size for harvesting. Fish ranching, on the other hand, holds fish in captivity for the first few years of their lives, releases them, and then harvests the adults when they return to breed. This practice is used most commonly with salmon, as these fish almost always return to the exact location of their birth for breeding. Overall, the most common species of fish and shellfish that are harvesting through aquaculture include mussels, oysters, shrimp, clams, carp, tilapia, and salmon.

Aquaculture can be used to effectively remove pressure from overharvested fisheries, and in doing so it can help sustain marine biodiversity. Some other advantages of aquaculture include a high harvesting efficiency with no bycatch, a high yield of fish in a small volume of water, and the use of crossbreeding and genetic engineering in order to further increase yields. Even so, aquaculture does produce a number of harmful effects on aquatic environments. For one, large inputs of land, feed and water are required to make aquaculture profitable. In addition, aquaculture produces large and concentrated amounts of waste, which often leak into and contaminate surrounding areas. The huge quantity of water fertilizers used during aquaculture poses a particular problem, because they contain nutrients that cause eutrophication in nearby ecosystems. Since aquaculture uses very dense populations of organisms, these species are more susceptible to diseases, and once infected, they often transmit diseases to neighboring wild organisms.

Thinkquest Team "Fish," March 2005, Disclaimer and copyright information