Response of Bull Trout Fry to Four Types of Water Diversion Screens

Abstract

Bull trout Salvelinus confluentus are currently listed as a threatened species in the western United States. Entrainment (the passage through screens of water diversion structures) has been identified as one cause of bull trout population decline. The objective of this study was to evaluate whether existing fish screen criteria developed for the Pacific Northwest are adequate to prevent bull trout fry from being impinged (having extended whole-body contact with a screen) or entrained by screened water intakes. Bull trout were exposed to four types of screens in an artificial stream. Recently emerged bull trout (median total length = 25.0 mm) were tested in groups of 25 at 6–7°C for approximately 16 h. Only one bull trout was entrained during all the experiments. Bull trout were regularly impinged on the screens but, in most cases, escaped impingement and survived for at least 24 h. This implies that the currently specified screen regulations for salmonid fry might not need to be modified for bull trout fry.