Observations were carried out in February–March and May–June periods during which water temperature ranges were 0.0–0.5 °C and 8.5–12.5 °C, respectively. Responses of fish to structures representing simplified environmental features, e.g. visual reference points, support against which fish could brace themselves in current, areas of slow water, overhead cover and areas of shade, were studied at three different water velocities. Winter and spring responses to illuminated and dark cover and to areas of rocky bottom were compared. Fish associated progressively more with experimental structures as more environmental features were incorporated into them. Degree of association with such structures and with other types of cover was much greater during winter. Degree of association increased with water velocity during both spring and winter. Increasing water velocities modified aggressive behavior. Fish displayed relatively more at low water velocities and nipped more at high velocities. It is possible that the association with and defence of specific locations on the bottom of a stream has selective value that alternates seasonally, providing the species with protection from predation and from displacement by current in winter, and with an efficient mode of food utilization in spring and summer.
