Predation and Predatory Fish Behavior in the Vicinity of a Floating Fish Guidance Structure in the Sacramento-San Joaquin Delta, California

California’s Department of Water Resources is mandated to investigate engineering solutions to reduce juvenile salmonid migration into the low-survival interior Sacramento-San Joaquin Delta. To this end, a 107-meter-long, 1.5-meter-deep floating fish guidance structure (FFGS) was tested at an important channel divergence of the mainstem Sacramento River in spring 2014, with On/Off position switching every tidal cycle. The present study addressed the concern that placement of the FFGS, a large in-water structure, could provide predatory fish habitat and therefore influence predation. Preliminary analyses suggest that the probability of predation of acoustically tagged juvenile Chinook Salmon was not significantly different between FFGS On and Off, but was inversely related to turbidity. Several aspects of predatory fish behavior also were not significantly different between FFGS On/Off positions: Standardized angling catch rate; residence time; and occupation of habitat zones near the FFGS. DIDSON monitoring results suggested that the density of predatory fish was greater nearer to the FFGS, and density near the downstream end of the FFGS was significantly greater with FFGS On. The preliminary findings suggest potential effects of the FFGS were limited to the area near the FFGS.