The Growth of Brown Trout (Salmo Trutta Linn.)

Studies of scale development in fishes which live in temperate zones have suggested that variations in the rate of growth of the fish may lead to the development of annuli and circuli on the scales. These variations in growth rate might result from : seasonal variations of the physical environment, especially of temperature and illumination;variations in the quality and quantity of the food supply; andphysiological changes within the fish, such as annual cycles of maturation of the gonads and spawning. The second and third factors might well be related ultimately to the first.