Substrate Preferences of Brown Trout Fry (Salmo trutta) m Artificial Stream Channels

Brown trout fry (Salmo trutta) of two sizes (43 and 32 mm mean total length) preferred the coarsest gravel when allowed to choose between three sizes (8–16, 20–30, and 50–70 mm) at two temperatures (12.9–18.1 and 6.0–6.9 °C) in artificial stream channels. Fry hid more in the substrate in daytime than at night and in daytime also hid more at low temperatures than at high temperatures. Increasing the average water velocity from 4 to 14 cm∙s−1 displaced the 43-mm fry slightly downstream at night. The 32-mm fry showed a weaker preference for the coarse substrate and, in general, had less pronounced diurnal substrate preferences.