Comparison of visual and passive acoustic estimates of beaked whale density off El Hierro, Canary Islands
Date
2023Abstract
Passive acoustic monitoring (PAM) offers considerable potential for density estimation of cryptic cetaceans, such as
beaked whales. However, comparative studies on the accuracy of PAM density estimates from these species are
lacking. Concurrent, low-cost drifting PAM, with SoundTraps suspended at 200 m depth, and land-based sightings,
were conducted off the Canary Islands. Beaked whale density was estimated using a cue-count method, with click
production rate and the probability of click detection derived from digital acoustic recording tags (DTags), and distance sampling techniques, adapted to fixed-point visual surveys. Of 32 870 detections obtained throughout 206 h of
PAM recordings, 68% were classified as “certain” beaked whale clicks. Acoustic detection probability was 0.15
[coefficient variation (CV) 0.24] and click production rate was 0.46 clicks s1 (CV 0.05). PAM density estimates
were in the range of 21.5 or 48.6 whales per 1000 km2 [CV 0.50 or 0.44, 95% confidence interval (CI) 20.7–22.4 or
47–50.9), depending on whether “uncertain” clicks were considered. Density estimates from concurrent sightings
resulted in 33.7 whales per 1000 km2 (CV 0.77, 95% CI 8.9–50.5). Cue-count PAM methods under application provide reliable estimates of beaked whale density, over relatively long time periods and in realistic scenarios, as these
match the concurrent density estimates obtained from visual observations