Effects of Predation on Telemetry-Based Survival Estimates: Insights from a Study on Endangered Atlantic Salmon Smolts

Acoustic telemetry is increasing being used to estimate population-level survival rates. However, these estimates may be impacted by detection efficiency of receivers and are reliant on the assumption that telemetry data represent the movements of the tagged (i.e. targeted) fish. Predation on tagged fish confounds survival estimates and, unlike the issue of detection efficiency, methods to deal with predation have yet to be developed. In an effort to incorporate predation into survival estimates, a suite of eleven summary migration metrics were compared between Atlantic salmon smolts (Salmo salar) and striped bass (Morone saxatilis) in 2008 and 2011. Cluster analyses revealed that 7% to 27% of tags implanted in smolts exhibited migration patterns similar to striped bass, which could be interpreted as evidence of predation. The “fate” of smolt tags detected exiting the study site was re-assigned as “predated / died” and subsequently, estimates of survival were adjusted accordingly. Compared to a traditional mark-recapture model, the cluster analysis-adjusted approach reduced estimated survival from 51.8% to 43.6% and from 36.4% to 22.7%. It is unclear whether model-based approaches can resolve all issues associated with the complicating effects of predation. Limitations of this technique will be discussed, including recommendations for future efforts.