Evaluation of strobe lights for vertically displacing juvenile salmon near a filling culvert intake at the Hiram M. Chittenden Locks, Seattle, Washington

In May 1998, we evaluated the use of strobe lights for vertically redistributing salmon smolts from in front of a culvert used to fill a 15.2 m deep navigation lock chamber in Seattle, Washington. Flows into the culvert during low tides were up to 62.3 cm/s, with maximum instantaneous water velocities of 1.7 m/s. We installed ten strobe light flash heads around the perimeter and in front of the north-filling culvert. Fish densities near the culvert entrance were monitored hydroacoustically. Strobe lights were evaluated using paired on and off treatments during daytime (N = 32). In control treatments before fills, most juvenile salmon were within 5 m of the intake floor. With strobe lights turned on, the fish distribution shifted upward, resulting in a 7-m displacement of the peak densities towards the surface. Mean fish densities at the depth of the culvert (8-13 m) decreased by more than 96% during strobe light-on treatments as compared with control treatments. Based on paired t-tests, mean fish densities in 1-m strata below a depth of 9 m were all significantly lower (x = 0.05) during on treatments than during off treatments. In control treatments during fills, the distribution of juvenile salmon was relatively uniform, with slightly greater mean densities 8-10 m from the surface. During strobe light,on treatments, juvenile salmon were distributed higher in the water column, resulting in a 3-m displacement of the peak densities towards the surface. Mean fish densities at the depth of the culvert decreased by 87% during strobe light-on treatments as compared with control treatments. Based on paired t-tests, mean fish densities in all 1-m strata below a depth of 8 m were significantly lower (a = 0.05) during on treatments than during off treatments. Nighttime tests were also conducted, but nighttime densities were too low for meaningful strobe light evaluation.