Random swims: An evaluation of acoustic telemetry thresholds for reef-shark behavior and residency

Abstract Background Passive acoustic telemetry (AT) is a method used to quantify residency within an array of receivers, but this technology has limitations for capturing complex behaviors in reef sharks: pulse intervals and detection range drop-offs. This study addressed residency calculation methodologies by examining visitation qualifier functions (thresholds) in commonly used R packages (e.g., VTrack). Methods Random walk models simulated the mismatch between detections on acoustic receivers and real shark movement by testing 30-minute, 1-hour, 2-hour and 24-hour visit thresholds to compare gaps between shark detections at different transmitter settings (1- and 5-minute intervals). We also modeled tracks of transient sharks to show how these animals may interact with a passive acoustic receiver differently than resident individuals. Results Our results suggested that longer transmitter pulse intervals (1–5 minutes standard for sharks and larger fish) require shorter visit thresholds (< 30 minutes) to reduce variability in residency time. Consequently, using thresholds of less than 2 hours increased the number of counted visits that stemmed from the same events. Similarly, the 5-minute interval also predicted greater elapsed residency times than did the real path. Our directional walks sent transient sharks through a receiver at 0–1 and 1–2 meters per second; under these scenarios, transmitters were unlikely to ping twice (default minimum visit qualifier) if 5-minute pulse intervals were set on their transmitters (16.4%), whereas 1-minute intervals did frequently (84.2%), indicating that a 5-minute interval may misrepresent residency time for transient animals. Conclusions Thresholds and detection qualifiers manually set during passive acoustic surveys can bias residency and visitation results, and careful consideration should be applied on the basis of the life history (residential or transient) of the target species.