摘要：In the Sacramento-San Joaquin Delta, several salmonid species are listed as threatened or endangered. One potential cause of lower juvenile salmonid survival may be predation by striped bass (Morone saxatilis) and other piscivores. Acoustic telemetry is routinely used to estimate salmonid behavior and survival by releasing and detecting tagged juvenile salmonids with the assumption acoustic-tagged salmon are not consumed by predators. If this assumption is violated, behavior and survival estimates may be misinterpreted. A key consideration is the time taken by an acoustic tag to pass through the digestive tract of a predatory fish. All tagged dead juvenile Chinook salmon (Oncorhynchus tshawytscha) placed into the Tracy Fish Collection Facility (TFCF) primary channel were verified as being consumed by a predator and the tags evacuated from the predator’s gut, via evaluation of two-dimensional (2D) tracks and detection signal patterns from single and multiple hydrophones. Tracks and signal patterns simultaneously showed after time of consumption. Salmon were traveling around the primary channel, moving both with and against flow in both a linear and non-linear manner indicative of a free-swimming fish. Given salmon were dead prior to consumption, we feel confident they were inside a predatory fish. Further support for this was provided by two previously tagged striped bass with active tags that consumed tagged dead salmon from our study and an unknown predator that consumed two tagged dead salmon from our study at points about 30 min apart. Mean tag evacuation time was 1.8 days, ranging from 1.2 to 2.7 days (N = 14, SD = 0.49). Although not significant, we found a suggestive linear relationship (r 2 = 0.23; df = 12; P < 0.08) between mean water temperature during tag retention and tag evacuation time. We have strong evidence in the ability to confirm predation and measure the rate at which free-swimming predatory fishes digestively pass acoustic tags that were implanted in Chinook salmon within the TFCF primary channel. Our results have possible application to other areas/systems depending on the physical characteristics of the site, design of the hydrophone array, and methods used.
