The growth of brown trout (Salmo trutta Linn.); the growth of two-year-old trout at a constant temperature of 11.5 degrees C.

Since fish form an important source of human food, a considerable amount of data is available concerning their distribution and size under natural conditions. This is especially true for the Salmonidae because of their popularity with sporting fishermen. With the development of commercial hatcheries, there has been a certain amount of experimental work concerning the effect of various factors on the growth and life history, particularly of 'trout'—Salmo trutta, S. irideus and SalveUnus fontinaUs. However, it is impossible to define the effect of any particular factor on trout growth, since several environmental factors varied at the same time, without being measured accurately. In some experiments (Surber, 1935; Pentelow, 1939) individual trout were isolated and their growth recorded, while in others (McCay & Tunison, 1935, 1937; Wingfield, 1940) the fish were kept in groups and their average sizes recorded at intervals. There is thus no record of the effect ofintraspecific relationships on the growth of individuals. The investigation now to be described was an attempt to study the growth of individual trout when the environmental factors were as completely controlled as possible. Observations on the early growth of trout fry under such conditions have already been reported (Brown, 1946), but these are very delicate and are difficult to identify as individuals in the early stages, so 2-year-old trout were used for a more detailed investigation, especially of the relation between food and growth in different environments. The present paper is an account of observations on trout living at a constant temperature of 11 -5° C, and experiments concerned with the effect of temperature on growth will be reported later. The following factors were controlled or recorded throughout: temperature, amount and intensity of illumination, rate of water flow, chemical composition and aeration of the water, quality and quantity of food, volume of living space and number of individuals in each group. The following were investigated: the growth of individuals in groups, the effect of crowding on growth, changes in growth rate with age, the relation between food and growth and the effect of duration of light on growth. The number of individuals studied was limited by the space and time available, but the observations are published since some of the results were unexpected.