Although the general distribution of the most important commercial species has been known for many years from the observations of fishermen, detailed investigations sometimes reveal that the population is not biologically homogeneous, but comprises several more or less separate breeding groups. For the rational exploitation of a stock it is necessary to know of the existence of such subdivisions, the limits of the distribution of each, the extent to which interchanges may occur between them and the contribution that each makes to the fishery in any area.

In some cases, measurements of the sizes of fish may point to a heterogeneity of the population. Normally, the length/frequency curve for fish of equal age from the same stock is unimodal. A curve having an obviously bimodal or polymodal form for fish of the same age group suggests that a mixed population is being sampled. Sometimes anatomical or physiological differences indicate subdivisions of the population. Precise studies of the anatomy of fish from different areas may reveal slight differences in structure; for example in number of vertebrae, the number of fin-rays or the pattern of rings in scales or otoliths. There may also be physiological differences between different parts of the population; for example, in salinity or temperature tolerances, fecundity, breeding season or blood cell counts. Attention has also been given to biochemical and serological studies as evidence of genetic differences between subdivisions of fish stocks (de Ligny, 1969). Fishery research attempts to discover if such differences are characteristic racial features, or are simply variations due to environmental causes.

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