Acoustic‐based estimates of Cuvier's beaked whale (<i>Ziphius cavirostris</i>) density and abundance along the U.S. West Coast from drifting hydrophone recorders

Abstract

Abstract An acoustic survey of Cuvier's beaked whales ( Ziphius cavirostris ) was conducted off the U.S. West Coast in August and September 2016 using drifting recorder systems with a vertical array of two hydrophones at a depth of ~110 m. Recorders were deployed 22 times to representatively cover a 1,058,000 km 2 study area from the shelf break to ~556 km offshore. Vertical angles to echolocation pulses were measured using the signal time‐difference‐of‐arrival on the two hydrophones. Echolocation pulses of Cuvier's beaked whales were identified from their arrival angles (always from below the array) and unique acoustic characteristics. The density and abundance of Cuvier's beaked whales were estimated using a group‐based point‐transect analysis with 2 min time snapshots. The area effectively surveyed was estimated using a maximum simulated likelihood approach to fit the observed distribution of signal arrival angles. The acoustic availability of whales during their dive cycle was estimated from the duration of acoustic encounters using a mark‐recapture approach. Overall, Cuvier's beaked whales were present during 0.60% of snapshots, and their estimated average density is 5.12 animals per 1,000 km 2 ( CV = 0.27). Their estimated abundance in the study area is 5,454 individuals (95% credibility intervals: 3,151 to 8,907).