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Andrew Schultz USBR, Kevin Kumagai HTI
U.S. Bureau of Reclamation, Tracy Fish Collection Facility,
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 striped bass predation. Acoustic tags are routinely used to estimate survival by releasing and detecting tagged juvenile salmonids with the assumption that tagged salmonids are not consumed by other fishes. If this assumption is violated, salmonid 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. To measure evacuation rates, acoustically tagged juvenile Chinook salmon were euthanized, then tethered and fed to free swimming (acoustically tagged and wild) striped bass in the primary channel of the Tracy Fish Collection Facility. Confirmation of time at predation event was determined by analysis of 2D acoustics tracks. The amount of time required for an acoustic tag to pass through the digestive tract of a striped bass estimated the evacuation rate. Mean tag evacuation time was 1.8 d (n = 14; SD = 0.49; range = 1.2 to 2.7). Mean tagged salmon was 146.6 mm FL (n = 14; SD = 7.6; range = 131 to 165). Results indicated free swimming striped bass evacuate small acoustic tags (0.5 g) relatively quickly in 22°C water. By comparison, a study investigating predation events using larger tags (1.1 g) on relatively sedentary laboratory fish held in 16°C had greater
evacuation times. This feasibility study demonstrated acoustic tags can provide information about predation and gut evacuation in free‐swimming predatory fish. As a result, the Bureau of Reclamation extended testing of predation and gut evacuation rates to include temperature variation and food availability levels in the Tracy Fish Collection Facility Primary Channel, starting
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