“…At large complex obstacles, where fish are delayed and their migration thwarted, passage occurs primarily during daylight Gowans et al, 1999Gowans et al, , 2003Null & Niemela, 2011), whilst at less complex structures and natural by-pass channels passage generally occurs at night (Dunkley & Shearer, 1989;. Light intensity preference for passage requirements appears to be site specific and related to the visual orientation needs at each given obstacle (Banks, 1969;Thorstad et al, 2008). The timing of passage (in either day or night) may be an early indicator of passage difficulty for salmon, with daytime passage potentially indicating higher levels of passage difficulty.…”