Comparison of visual and passive acoustic estimates of beaked whale density off El Hierro, Canary Islands

Funding: P.A., D.M., K.G., T.A.M., A.C., and field work were funded by Office of Navel Research award #N000141612973 and research conducted under Spanish Ministry permit #28606/2016 and Canary government permit # 421373/2016. T.A.M. is thankful for partial support by CEAUL (funded by Fundação para a Ciência e a Tecnologia, Portugal, through the project UIDB/00006/2020) and time under the ACCURATE project funded by the U.S. Navy Living Marine Resources program (Contract #N3943019C2176).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 s - 1 (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.Publisher PDFPeer reviewe