Influence of Physical Factors on Density of Stocked Brown Trout (Salmo trutta fario L.) in a Danish Lowland Stream

Physical factors and brown trout densities were studied in a small Danish lowland stream. The densities of brown trout larger than 15 cm were significantly correlated with gradient, mean depth, coefficient of variation in current velocity 7 cm above the bottom, the ratio between wetted perimeter and width, amount of overhanging banks and degree of macrophyte cover. Coefficient of variation in current velocity 7 cm above the bottom was the most important factor for brown trout density (rs= 0.8364, 24 df, (P < 0.001)), which supports the idea of this value as a measure of stream complexity. A rather small relation between trout density and amount of overhanging bank cover (rs= 0.4179, 24 df, (P < 0.050)), contrary to the closer relationships found in previous studies, is discussed as an effect of the self-shading capacity of this rather narrow and deep stream.