The first requirement for any analysis of the composition of a stock of fish is a reliable method of obtaining representative samples. Where there are no market or mesh restrictions leading to the elimination of small fish from the commercial landings, these may include the full range of sizes and provide good samples of the fishable stock. If the commercial catch is not representative of the stock, it may still supply useful data if the sampling error is known. In most cases, however, special fishing gear operated by fishery research vessels is needed to obtain good samples of all age groups. The fishing grounds are explored using trawls with cod ends covered with fine-mesh net capable of retaining the smallest fish. For pelagic fish, drift nets may be used which include a range of mesh sizes suited to the different sizes of fish in the shoals.
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