- Related Topics:
- aquaculture
- bycatch
- hunting
- fishing
The term shellfish is generally applied to all invertebrate marine organisms having visible shells. They may be broadly categorized as crustaceans and mollusks.
The crustaceans include lobsters, crabs, crayfish, and both shrimp and the closely related but larger prawns. The shells consist mainly of a hard, inedible substance called chitin. Crustaceans molt frequently during growth. Blue crabs are eaten when molting and soft-shelled. Marine lobsters are eaten when about five years old and have by then molted about 25 times.
With the development of satisfactory freezing techniques in the 1940s, shrimping expanded considerably, becoming a global operation. The United States is a major consumer, importing shrimp, mainly frozen, from more than 60 countries. South Africa and Australia have developed a worldwide market for rock lobster, and Japan and Russia dominate the world market for king crab.
The major mollusks consumed as food are oysters, mussels, clams, scallops, whelks, and snails. The best-known marine snail is the abalone, encountered in many warm waters. This group also includes the octopus, squid, and cuttlefish, popular seafoods in Mediterranean countries and the Far East.
Sea cucumbers (holothurians), or sea slugs, are usually marketed under the name of trepang or bêche-de-mer. Rich in protein, they are eaten in China, Southeast Asia, Australia, and Italy.
Mammals
Ocean mammals include such cetaceans as whales, porpoises, and dolphins, as well as seals and walruses. Whales are a source of meat, fats, and oils, hormones such as insulin, and chemicals. They exist at all levels of ocean food chains. The blue whale mainly devours small reddish shrimp called krill, while the formidable killer whale feeds on salmon, seals, and sharks. The number of species, although still large, has declined considerably. See the article whaling.
The hunting of porpoises and dolphins preceded whaling in history. Dolphins were eaten in ancient times around the Mediterranean, and Xenophon and his Greek army found sizable stores of salted dolphin meat in earthenware vessels on the Black Sea coast. Their use as food there continued until banned by the Soviet Union in the interest of preserving the animals for biologic research. Many tropical islanders still hunt dolphin on a large scale. Freshwater dolphins are caught in many of the world’s great rivers, including the Ganges, Indus, and Brahmaputra, the Amazon, and the Río de la Plata. The dolphins of Chinese rivers have been eradicated, but a number survive in the lake regions of the upper Yangtze.
Seaweeds and plankton
Marine plants may be divided into two groups: grasses and algae. There is only one subaquatic grass of any significance, namely eelgrass. Algae that grow in a fixed location, generally called seaweeds, may be categorized according to colour, into green, brown, red, or blue-green. Brown algae, sometimes called kelp, may grow to exceptional sizes; some specimens attain a length of 50 metres or more.
Seaweeds are heavily exploited in many parts of the globe for human as well as animal food. Several species are extensively cultivated on the coastal shelves of China, Japan, the Philippines, and elsewhere. Brown species in particular are harvested in Japan and made into a number of food products. Several are used as material for various thickening agents.
Cultivated red seaweeds belong to the genus Porphyra. Their sun-dried, blackish fronds are shaped into sheets and used in the Orient as a wrapping for rice. Harvested along the coasts of Ireland and Scotland, red seaweed is made into a powder and used as the main ingredient of a kind of bread called laver. Seaweeds contribute to the diet accessory nutrients such as vitamins B6 and B12.
Phytoplankton does not offer man a suitable food and can hardly be used even as feed for animals. Many species are toxic; the rest are scarcely digestible. In addition, most plankton finish their life cycle within a few days or weeks and are usually devoured by predators. Consequently, the amount of plankton in the water at any given moment is small, even though total plankton production over a year may, in a particular water, well exceed that of fish. Plankton harvesting is therefore very difficult, because of the volume of water that must be sieved, but several attempts to develop a feasible harvesting device have been undertaken. The Japanese, Burmese, and East Indians have managed to develop profitable fisheries for certain tiny shrimp that feed on plankton. The shrimp are dried or fermented into pastes. Elsewhere similar plankton-fed shrimp are sun dried and sold as a snack.
Unicellular green algae, such as Chlorella and Scenedesmus, have been artificially cultivated, yielding 75 tons per hectare (30 tons per acre) per year, compared with the standard wheat yield of 2.5 to 3.7 tons. However, the process is costly, since algae, in addition to harvesting, require decolorization and special processing to remove or break down the cell walls through drying and enzyme action in order to become digestible. It is far more efficient to use such plankton directly as fish feed in cultivation ponds or in the raising of cattle and poultry. Blue-green algae easily create waterblooms, slimy accumulations that may be dried in the sun and molded into small loaves with a nutlike flavour and high in protein. This food is extracted from Lake Chad in tropical Africa, and the Aztecs made a similar product. In China a scum called lan, collected from ponds and freshwater lakes, provides sustenance for large numbers of people. A related scum, keklap, found in Java, is used chiefly as fish feed. Another species is made into dried sheets in Japan and prepared for food by heating in water. Successful cultivation of some blue-green species has been carried through on a semicommercial scale.
Georg A. Borgstrom John C. SainsburyFish finding
Traditionally, sea fishermen have known the time and place to find their catch, but the history of fishing has demonstrated more than once that even old and rich fishing places can become exhausted quite suddenly. This is especially true with pelagic fish like herring, pilchards, or sardines. The herring yields of the Schonen fishery and later on of the Bohuslaine fishery (1744–1809) in the Baltic Sea fell so severely that the very existence of the Hanseatic League was compromised. This sudden change did not result from overfishing but was caused instead by natural fluctuations in the development of stocks. In modern times, sardine fishing collapsed off the California coast in 1952, followed by the Peruvian anchovy fishery 20 years later. Similar disasters have occurred in other parts of the world not only because of overfishing but also for natural reasons. When this happens new fishing places must be found. It is difficult to explain how good fishing places in great depths were found in ancient times, but fish in shallow waters, fjords, or small bays can easily be seen. On the high seas, fish can be located when they surface temporarily, and fish searching by direct observation from a vessel is important even today. Airplanes and helicopters are commonly used in purse seine fishing. An experienced air-spotting pilot can detect fish under the surface and identify species by observing the shoal’s form or colour or behaviour and sometimes by the presence of accompanying birds. During the night, fishes can be located through the phenomenon of bioluminescence; i.e., when their passage through the water causes tiny marine organisms to luminesce. Accompanying birds have played an important part in fish searching for centuries, because a concentration of birds can be seen from a distance. Very often the birds are not attracted by the fish sought but by smaller fishes and squid, which may have taken refuge from large species by swimming to the surface. Other animals may also indicate fish concentrations by their presence. Porpoises, for example, are known companions of tuna, and tuna purse seiners often set their nets where porpoises have been seen. To find fish in deeper waters by other means was difficult if not impossible in the past. Herring fishermen used signal lines to find their prey in deep waters. These were long wires dropped from a boat; the fisherman holding the line in his hand could feel the vibration caused by the fish touching the line, which was named the herring’s telephone. Other fish were also found by signal lines, often tied with fishing gear. In modern industrial fisheries, experiments have been made with direct listening for fish, but this method has been found impractical. Sea fishermen have also learned to judge where fish can be expected by observing environmental conditions. The colour of the water and the presence of current or of a borderline between different water bodies are some common fish indicators. One of the most important physical properties for fish finding is the temperature of the water. The use of thermometers was one of the first practices fishermen learned from oceanographers, not only for fish finding but also for forecasting availability of the desired species. Aerial and satellite surveys of these properties are becoming of increasing importance.
The first experiments using electrically generated sound pulses and their echoes to locate fish were undertaken in Britain during the 1930s, and by the 1950s fish-finding echo sounders had become an essential aid to catching. As these units worked vertically, they only showed fish immediately below the vessel, so that a logical development was the application of sonar in order to search horizontally around the boat. For many years the machines provided only a “black on white” paper display of the resulting echoes, and interpretation of the displays was dependent upon the skill of the skipper. Gradually, improvements were made in the quality and quantity of information displayed, enabling monochrome signals to be displayed on a television-type screen. A big breakthrough came with the advent of microprocessor technology, which made it possible for fish-finding sonars and sounders to rapidly analyze the signals that their high performance transducers picked up from the sea. Information regarding size, abundance, and movement of the fish is now displayed in many colours, provides the skipper with a wide range of scales, and enables him to focus on and expand information at a particular depth or location.
Other instruments have become vital to fishing operations, especially radio- and satellite-transmitted position-fixing equipment such as Decca Navigator, Loran, and Satnav. These enable a skipper to return to the precise position where fish are spotted or to a particular location such as a coral reef or where gear has been set. Microprocessor technology allows information from various instruments such as sonar, radar, Satnav, and Loran to be fed into a single television screen that provides the skipper with information processed to suit his needs. The vessel’s movements, shown on the screen, can be integrated with navigation and fishing charts fed into the display from computer memory banks. By linking these instruments to the control of winches, engines, and rudder, fully computerized fishing operations are possible.
