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69. In contrast to mammals and birds, individual fish may show a very wide range of growth rates, matched to correspondingly wide differences in food intake. Within a natural group of salmon or trout in a stream, individual fish may show variations in growth rate both as a result of their own feeding preferences and variable feeding opportunities. Salmon which smolt in the wild, at about 125mm body length, may be between 1 and 4 years old. It is not uncommon for salmon and trout in the wild to survive long periods of food deprivation, for example over winter. During the return migration in freshwater, adult salmon may cease feeding altogether for periods of up to one year.

70. Farmed salmon and trout may also show wide variations in feeding and growth rates but, in general, they are fed regularly, at least daily, and grow at rather faster rates than fish in the wild. Food provision may be reduced at times to regulate growth or to reduce the fat content of the tissues and may be withheld altogether prior to transport or slaughter, or as a treatment for disease.