Beaked whale dive behavior and acoustic detection range off Louisiana using three-dimensional acoustic tracking

Understanding abundance and trends of beaked whales in the heavily industrialized Gulf of America (formerly Gulf of Mexico), is critical for management but challenging with visual-based distance-sampling due to their elusive surface behavior. Acoustic-based distance-sampling methods rely on accurate modeling of detection probability as a function of distance from a recorder, requiring population-specific diving and acoustic behavior parameters, which is currently lacking for Gulf populations. To address this, we used passive acoustic tracking with two 4-channel High-Frequency Acoustic Recording Packages (HARPs) deployed off Louisiana (~1100 m depth) in 2021. Echolocation clicks detected on both recorders were localized in 3D to characterize acoustic and diving behavior. These data informed a Monte Carlo cue-based simulation to estimate the probability of detection by a near-seafloor single-sensor HARP. A trial-based approach also estimated detection probability as a function of range to a single-channel sensor deployed at the site. Results show species-specific differences. Goose-beaked whales (Ziphius cavirostris), were detected for longer periods during foraging dives (n = 24 dives, mean: 20.5 min; range: 7-42) compared with Blainville's (Mesoplodon densirostris, n = 2 dives, 13.6 min; 11-16) and Gervais' (Mesoplodon europaeus, n = 24 dives, 12.7 min; 7-19) beaked whales. Maximum dive depths also differed, with some goose-beaked whales foraging at or near the seafloor. Descent and ascent rates were similar within species but differed among them (1.34/1.40 m/s for goose-beaked and 1.15/1.19 m/s for Gervais' beaked whales). Source level and broadband directivity index were estimated at 225 dBpp re 1 μPa-1m and 26 dB for goose-beaked whales, and 218 dBpp re 1 μPa-1m and 20 dB for Gervais' beaked whales. Estimates were not possible for Blainville's beaked whales due to limited data. In both the Monte Carlo simulation and trial-based approach, detection probability declined sharply with ranges, reflecting the highly directional beam of beaked whale echolocation clicks.