Difference Between Fishing And Fish Farming
For many thousands of years, fish and shellfish have been an important part of the human diet. They are an excellent source of protein and an important source of vitamins A and D. In addition to their use as human food, fish and shellfish are ground up to produce fish meal, which is used as food for chickens and other domesticated animals. Fish oils are important, too. Large amounts are used to make margarine and to manufacture paint, lubricants, grease, soap, and other products.
More than 100 million tons (91 million metric tons) of fish are harvested annually. Almost two-thirds of this is edible fish. Most of the edible fish are marine species such as cod, haddock, halibut, herring, and salmon, but freshwater species such as trout, carp, eel, and catfish are also important to the harvest. As demand for fish has increased, people have begun to eat fish previously shunned, such as shark, monkfish, and sea robins.
Modern Fishing Technologies
Commercial fishermen use nets, lines, dredges, and traps to catch fish and shellfish. The method used is determined by the habitat of the fish sought and by the type of vessel used by the fishermen.
The most widely used nets are trawls, seines, and gill nets. A trawl net is a large, tapered net bag that is towed through the water. It has a wide mouth that is held open by floats attached to its upper edge. Fish are trapped after entering the mouth of the trawl. The mouth is then drawn closed, and the net is hauled aboard the ship. Trawl nets are primarily used to catch shrimp and fish such as cod and haddock that live on or near the bottom of the sea.
Raising fish and shellfish to maturity under controlled conditions is called fish farming. It is a branch of a broader field called aquaculture, which also includes raising seaweed and other marine and freshwater products.
Fish farming is believed to have begun in China several thousand years ago. It was also practiced in ancient Egypt; a bas-relief in an Egyptian tomb built about 2000 B.C.shows tilapia, an African food fish, being harvested in an artificial pond. Today, fish farming is widespread: worldwide, the industry produces more than 45 million tons (41 million metric tons) of fish annually. According to the United Nations Food and Agriculture Organization, the aquaculture industry’s contribution to the total world food-fish supply has more than quadrupled in the past 50 years. Nearly one half of all fish for food comes from this increasingly popular source.
Only certain species of fish are suitable for this method of farming. Fish that migrate or roam widely, such as salmon and herring, cannot be raised to maturity in small enclosures. In contrast, mussels, which spend their adult lives attached to a stationary object, can be easily cultivated.
Some species can be raised in captivity but will not breed under these conditions. Yellowtail, an important cultured fish in Japan, is such a species. The Japanese collect the young (fry) in the early spring, then raise the fish in ponds or floating net cages. Some hatcheries use hormone injections to induce egg laying in fish that do not spawn spontaneously in captivity. The fry are then transferred to special ponds.
Fish are often transferred from one pond to another as they grow. For instance, trout fry are kept in small tanks. Once they are able to swim and eat on their own, they are moved to rearing ponds. When they are bigger, they are transferred again.
Carp is the most widely cultivated fish species. Other important farmed fish include tilapia, catfish, mullet, milkfish, and trout. Important shellfish include mussels, oysters, and shrimp.
Some practices integrate fish farming and agriculture. This is economically practical, because the wastes from one process can be used as fertilizer for the other. For example, catfish farmers in the southern United States often alternate between a crop of catfish and a crop of soybeans. When the catfish have been harvested and the pond drained, the nutrient-rich wastes that collected on the bottom of the pond reduce or eliminate the need for chemical fertilizers for the soybeans. Soybean products form part of the feed for the catfish. In many Asian nations, fish farming and duck raising are similarly combined.
Fish farmers also have found that polyculture—raising several species with different food habits—tends to increase yields. In China, a farm pond may contain a variety of carp: common carp, which eat insects; grass carp, which eat grass; and silver carp, which eat phytoplankton.
Initially, fish farmers simply stocked fish in confined areas, then harvested the fish when they reached a suitable size. Increasingly, however, practices similar to those used in agriculture have been applied to fish farming. Fertilizers—either organic wastes or chemicals—are now added to ponds. Many fish farmers raise their fish on special nutritionally balanced diets rather than depending on the environment to provide sufficient food. For instance, most catfish farmers in the United States feed their stock a high-protein food made of soybeans, grain, and fish meal.
Salmon have an unusual life cycle. They spawn in freshwater streams, then migrate to the ocean, where they spend most of their lives, returning to the streams only to breed. Norway farms these fish by enclosing them in large natural bodies of seawater, such as fjords. Other countries, however, find it much more efficient to “ranch” salmon. They hatch and cultivate the fish until they are one or two years old. Then the tiny fish are released to make their way to the ocean. Two to five years later, when it is time to breed, the salmon return to their home stream. Some are harvested, but enough are allowed to breed to ensure a steady population.
The scientific study of fish farming is relatively young. Much still needs to be learned about the biology and farming of fish and shellfish. Among the major goals of researchers is the use of selective breeding and genetics to produce fish with more-desirable characteristics, such as resistance to disease or improved ability to spawn in captivity. Ecologists warn that aquaculture may have a dark side too. Pollution, disease, and habitat destruction are some of the dangers that research must address.