Guiding Migrant Salmon by Light Repulsion and Attraction in Fast and Turbid Water
attraction, canal, entrance, guidance, illumination, overfall, repulsion, salmon, trout, University of Washington
When simple strands of above and underwater lamps were placed along both banks of a canal 84 feet wide, statistically significant deflection was obtained in both clear and turbid water flowing at 6 feet per second average velocity. The effectiveness decreased as turbidity was increased. In relatively slow moving water, when the spectrophotometric transmission dropped to 55% in 1 cm of water, guidance by high intensity light (repulsion) became ineffective. In this turbid water guidance by attraction with extremely low levels of illumination was effective. Approximately 80% of the downstream migrant salmon and trout regardless of species or size, used the entrance compartment in which a .015 foot candle light was burning. Dim lights placed below and downstream from the overfall pulled fish out of a stilling basin, while bright lights blocked them. Under dim illumination the downstream migrants swam over the falls with less hesitation than was displayed in the dark and the population pressure did not build up. Intensity, not color, seemed to be the important variable. Electrical and auditory stimuli were not effective guiding agents when used for repulsion or attraction.