Random swims: an evaluation of acoustic telemetry thresholds for shark behavior and residency

Passive acoustic telemetry is a method used to quantify residency within an array of receivers, but this technology has limitations for capturing complex behaviors in sharks: pulse delays and detection range drop-offs in near-shore habitats. This study addressed residency calculation methodologies by examining visitation qualifier functions (thresholds) in commonly used R packages. Random walk models simulated the mismatch between detections on acoustic receivers and modeled shark movements, by testing 30-min, 1-h, 2-h and 24-h visit thresholds to compare gaps between shark detections at different transmitter settings (1- and 5-min delays). We also modeled tracks of transient sharks to show how these animals may interact with passive acoustic receivers differently than resident individuals. Our results suggested that longer transmitter (tag) pulse delays (1–5-min standard for sharks and larger fish) required short (< 30 min) visit thresholds, as they reduced variability in residency times. Consequently, using thresholds of less than 2 h increased the number of counted visits that stemmed from the same events. Similarly, the 5-min delay 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 m/s; under these scenarios, transmitters were unlikely to ping twice (default minimum visit qualifier) if 5-min pulse delays were set on their transmitters (16.4%), whereas 1-min delays did frequently (84.2%). This indicated that a 5-min delay may misrepresent residency time for transient sharks. 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.