Determining the factors that influence salmon predation in the Sacramento-San Joaquin Delta: Insights from sonic telemetry

Understanding the factors that influence predation of juvenile salmon is a priority for those that study and manage salmon. Many factors have the ability to directly and indirectly influence predation rates of salmon including: predator population size, consumption rates of predators, intra- and interspecific interactions of predators, habitat mosaics, and water quality. In the Sacramento-San Joaquin River Delta Chinook salmon Oncorhynchus tshawytscha smolts have had low survival emigrating out of the delta in recent years (2-5%). Predation is probably a contributing factor, but there is limited information on predation rates of salmon. To gain information on what factors influence predation of juvenile salmon we used multiple techniques including: examining predator diets using genetic methods, determining relative predation rates using predation event recorders, mapping habitat, determining predator population size estimates from multi-pass electrofishing, mark-recapture methods, and hydroacoustics sonar surveys, and examining predator movement using sonic telemetry between nine study reaches covering 25 river km of the San Joaquin River. This talk will highlight each of the aspects of this study and then focus on the use of sonic telemetry to examine the distribution and movement of four known salmon predators (largemouth bass Micropterus salmoides, striped bass Morone saxatilis, white catfish Ameiurus catus, and channel catfish Ictalurus punctatus) using VEMCO V-9 acoustic coded telemetry transmitters (180 kHz, V9-2H). We found that predators differed in number of movements between study reaches and time spent in each reach. By combining the distribution and movement results with the information gained from other aspects of this study (consumption rates, population size, and habitat relationships) we aim to determine which factors are most important for influencing predation of juvenile salmon within the Sacramento-San Joaquin River Delta