The increasing use of fish farming is essential if stocks are to be main-tained in the face of increasing pressures from anglers, to say nothing of pollution, water abstraction, and commercial netting. These occur on both sides of the Atlantic, and as stocks are regenerated from selectively bred fish-farm introduction, it is possible that the local variants will gradually disappear.
In Britain the continual existence of the rainbow is heavily dependent upon the fish farmer. In the vast majority of British waters rainbows become spawn-bound and fail to breed naturally. They can only be maintained by stocking.
The exceptions are the Derbyshire Wye, the Chess and the Misbourne, where rainbows have established breeding colonies. They also breed in Blagdon Reservoir in Somerset co-existing with the brown trout.
Rainbows are very suitable for introduction to reservoirs because they live only for four or five seasons, and can be stocked in a range of sizes. In most farm conditions a one-year-old fish may be between 4 and 8in, attaining between 6 and 12in in its second year. A third year fish may be between 9 and 16in, and a fourth year fish between 14 and 20in. These are average figures and can be exceeded with suitable feeding and water conditions. A few years ago a 10lb rainbow was exceptional. Recent introductions at Avington fisheries, where 20lb fish are being produced on high-protein diets, indicate that selective breeding has improved future catches.