The Effect of Certain Environmental Factors on the Growth of Brown Trout (Salmo Trutta L.)

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Abstract
1. The factors responsible for seasonal variations in the growth of brown trout in any one environment are examined. 2. The probable basic importance of the water temperature is established. A critical temperature of 6°C. below which no growth takes place is deduced. 3. It is shown that the seasonal variations in growth observed do not result in corresponding variations in the width of the scale rings: such variations appear to be the direct result of fluctuations in available food supply. 4. Under natural conditions both water temperature and available food supply apparently act as limiting factors during the winter: during the summer, provided that the water temperature does not rise to a lethal level, only the available food supply is limiting. 5. The possible importance of light intensity as a limiting factor is indicated. 6. Differences in the growth rate of trout from different environments are also considered. 7. The role of dissolved calcium is investigated. It appears unlikely that the amount of dissolved calcium is responsible for the differences in trout growth in "hard" and "soft" waters. 8. It is suggested that such differences may be effected not by differences in the concentration of any one specific ion, but by departures from the optimum ionic balance brought about by variations in the relative concentration of any of the ions present.