The major portion of the total fish harvest consists of few fish species, which are divided into two primary groups. Pelagic species – those which live in the near-surface layers of the oceans, this include several species of herring, tuna, salmon, anchovies, pilchard, sardines, menhaden, and mackerel. Demersal species – fish that live in the near-bottom layers of the ocean, this includes cod, sole, halibut, haddock, hake, and flounder. Large catches are also made of a group of fish classed commercially as SHELLFISH – shrimp, lobster, scallops, oysters, clams, crabs, mussels, and squid. WHALING was once a major part of the fishing industry. Overfishing has endangered many whale numbers, however, and the field has lessened in importance.
Almost all large pelagic and demersal fish catches are made over or near the continental shelf, the underwater plateau around the continents and large islands. In these waters temperatures, water depths, and the currents that influence the amounts of available food create an environment that is highly favourable to the existence of large schools of fish.
The animals living in and on the bottom of the continental shelf serve as additional food sources for demersal fish. Also, most species spawn on continental shelves, and the main nursery grounds of many species are also in coastal regions. The main fishing grounds are located on the wider continental shelves of the mid and high latitudes. The single most important area is the North Pacific, where as much as one-quarter of the world’s fish catch is taken.