Assessing Juvenile Chinook Salmon Behavior and Entrainment Risk near Unscreened Water Diversions: Large Flume Simulations

Juvenile Chinook Salmon Oncorhynchus tshawytscha from California's Central Valley must pass thousands of unscreened water diversion pipes during their out-migration to the Pacific Ocean. The number of fish that become entrained into (drawn through) these diversions at different hydraulic and environmental conditions is currently unknown. We tested the ability of juvenile Chinook Salmon to avoid entrainment into a 0.46-m-diameter unscreened water diversion pipe while swimming in a large-river-simulation flume. Fish swimming experiments were conducted at 0.15, 0.38, and 0.61 m/s sweeping velocities (simulating the river current) with 0.42 and 0.57 m ³/s water diversion rates during the day and at 0.15 and 0.61 m/s with a diversion rate of 0.57 m ³/s in turbid water and during the night. The number of fish entrained during day experiments ranged from 0.8% (SE, 0.3) to 8.5% (SE, 0.3). The percentage of pipe passage events resulting in fish entrainment nearly doubled at the 0.57 m ³/s water diversion rate (1.7%) compared with that at 0.42 m ³/s (0.9%). In clear water conditions during the day, more fish became entrained at the higher water diversion rate (0.57 m ³/s) and slower sweeping velocity (0.15 m/s), with fish entrainments starting 38.6 cm (SE, 1.6) from the center of the pipe inlet, where fish experienced an increased velocity gradient and a mean resultant velocity of 0.74 m/s. Fish entrainment was strongly influenced by the number of pipe passages per experiment, rather than by swimming orientation or time spent in the flume. More fish were entrained at the faster sweeping velocity (0.61 m/s) in turbid water during the day and at night, indicating that juvenile Chinook Salmon may use nonvisual guidance (e.g., lateral line system) to avoid water diversions in slower currents. These results help to provide a scientific basis for protecting out-migrating juvenile Chinook Salmon exposed to unscreened water diversions.