Abstract
Behavioural studies of fish towards the slope are conducted in relation to a set-net. Ten individuals of crucian carp Carassrus auratus are tested in an experimentai tank which consists of a flat and a sloped area in Experiment I and the sides of a trap box attached to the slope in Ex-perimentII.
In equal conditions of illumination in Exp. I, the distribution of fish tends to be increased in the flat area, which indicates that fish keep away from the ascending sloped area of the larger gradient. The repellent effect is typically observed in the case of gradient over 14 degrees. When the tank is partly shaded with black roofing board, fish are attracted into the darkened area. In the experimental combinations of sloped gradient and lower light intensity the repellant reactions towards the sloped area is diminished in the smaller gradient under 14 degrees. These reactions are attributed to the relative differences of light intensity between the slope and flat areas in the tank and also to the degrees of sloped gradients.
In Exp. II three divided areas, i.e., flat, slope and trap, are also examined in numbers of fish distribution in combination of lighting canditions. Numbers of fish tend to increase in the dazkened trap, while it varies with complex reactions of fish in the case of darkened slope and lighted trap.
