Estimating predation rates of stocked juvenile Chinook Salmon using novel acoustic predation transmitters

Abstract Objective Acoustic telemetry is a tool commonly used to better understand the fates of fish following a stocking event. Until recently, predation on stocked fish could only be inferred by observing and identifying abrupt changes in tag detection histories. Recently, a novel predation transmitter has provided better insight into how predation affects poststocking survival. The main objective of this study was to use these transmitters to estimate predation rates on stocked juvenile Chinook Salmon Oncorhynchus tshawytscha in a landlocked system (Lake Oahe, South Dakota). Our secondary objectives were to determine how quickly predation was occurring, the time of day of predation events, and if there were differences in size between predated and nonpredated fish. Methods In 2021 and 2022, we surgically implanted Innovasea V5D 180‐kHz transmitters in 110 juvenile Chinook Salmon. These tagged Chinook Salmon were stocked at two locations on Lake Oahe along with a large cohort of untagged individuals. The tagged Chinook Salmon were passively tracked using an array of Innovasea VR2W 180‐kHz acoustic receivers. Result Across the two study years, we observed a mean predation rate (±SE) of 32.9 ± 6% on the tagged Chinook Salmon. Predation on the tagged Chinook Salmon occurred relatively quickly, with a mean time to predation of 86.7 ± 10.8 h. Additionally, 70% of the observed predation occurred during the daytime. Interestingly, results indicated that total length did not differ between fish that were preyed upon or and those that were, but there was a difference in total weight between the two groups. Conclusion Our results provide insight into recruitment bottlenecks within this population and offer information to other researchers regarding the advantages and limitations of this novel technology. Understanding how to mitigate predation could help to guide future stocking strategies to ensure a better return on investment for costly stocking programs.