California’s Central Valley salmon populations are in decline, and it is believed that one of the major contributors to these declines is low survival during residence in the Sacramento-San Joaquin River Delta. The mechanism of their mortality is unclear, but it is believed that a significant contributor is predation by the large populations of predators present there. However, it is currently not clear what proportion of juvenile salmonid mortality can be directly attributed to fish predation, largely because empirical data on predation has only been collected at limited spatial scales. In 2017, we quantified predation mortality rates, predator abundance, and relevant environmental covariates in 21 randomly selected study sites in the Delta, using a randomized selection protocol. Predation mortality rates were quantified using Predation Event Recorders (standardized predation monitoring devices), and predator densities were quantified using Dual-Identification Sonar cameras. This site selection protocol allowed for the inference of relationships between the environment and predation across a broader spatial scale than previous studies. Using these statistical relationships, we then developed the capability to produce high-resolution spatially and temporally-explicit predation risk estimates. We then put these predation risk estimates in the context of their impacts on migrating juvenile salmon, allowing us to assess the potential success of different potential survival-enhancing management actions.
Cyril J. Michel, University of California, Santa Cruz/NOAA-NMFS; Mark J. Henderson – USGS/Humboldt State University; Christopher M. Loomis – Humboldt State University; Joseph M. Smith, NOAA-NMFS-NWFSC, Seattle; Nicholas J. Demetras, NOAA-NMFS, Santa Cruz; Ilysa S. Iglesias, NOAA-NMFS, Santa Cruz; Brendan M. Lehman, NOAA-NMFS, Santa Cruz, and David D. Huff, NOAA-NMFS NWFSC, Newport
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