Sensitivity of density estimates to off-axis beam pattern assumptions in deep-diving odontocetes

Density estimation from passive acoustic monitoring requires knowledge of the sound production rates and signal characteristics of animal calls. This study explores the impact of the sound radiation pattern of echolocation clicks on density estimation for two deep-diving species, sperm whales (Physeter macrocephalus) and dense-beaked whales (Mesoplodon densirostris). The likelihood of receiving a click from within the narrow half-power biosonar beam at a randomly positioned receiver is small, and the study demonstrates that the majority of clicks are likely to be detected from outside this main beam. However, the beam pattern, particularly for large angles to the animal's axis, is poorly known for most toothed whales, leading to major uncertainties in the detection probability. Using a simulated cue counting scenario with different beam pattern assumptions (n = 2 for sperm whales, n = 3 for beaked whales) combined with tag-derived movement, orientation, and sound production data, this study demonstrates that uncertainty in the beam pattern can bias density estimates from passive acoustic monitoring, especially in low-noise environments. This effect decreases in noisy environments due to reduced detection of off-axis clicks.