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

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 km2 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 km2 (CV = 0.27). Their estimated abundance in the study area is 5,454 individuals (95% credibility intervals: 3,151 to 8,907).

Beaked whale dive cycle estimation based on acoustic data from drifting recording systems

“…Drifting recorders typically do not record a full record of echolocation from a foraging dive, even if the animals are relatively nearby. Barlow et al (2021) estimate that the acoustic availability time for Cuvier's beaked whale (17.2 min) is less than half of the expected echolocation period for this species in their study area (39.2 min). Echolocation signals from beaked whales are highly directional (Zimmer et al, 2005) and signals are typically lost when animals are not oriented towards the hydrophone.…”

“… Note : (1) Barlow et al (2020) and Barlow et al (2021), (2) Baird et al (2008), (3) Tyack et al (2006), and (4) Aguilar de Soto et al (2020). …”

“…Dive cycle duration is an important parameter in estimating acoustic availability, which is used in acoustic estimates of population density (Barlow et al, 2021), and may be an important descriptor of beaked whale life history strategies. Previous estimates of dive cycles required data from tagged animals; however, only a few of the 24 species of beaked have been tagged.…”

“…The dive cycle duration of beaked whales has been estimated as the sum of the deep dive duration and the inter-deep-dive interval (IDDI; Tyack et al, 2006) or as the inverse of the mean dive rate (the mean number of deep dives divided by the hours of observation; Schorr et al, 2014). The dive cycle duration is an important parameter in estimating acoustic availability for acoustic-based abundance estimation methods (Barlow et al, 2021).…”

Current global population size, post-whaling trend and historical trajectory of sperm whales

“…Acoustic surveys are becoming more common, and procedures for analysis have been developed in recent years so that they seem to produce more precise estimates than visual surveys with comparable effort 34 . Methods for surveying acoustically from autonomous vessels and drifting buoys are under development 48 , 49 and these should considerably reduce costs and the feasibility of large-scale acoustic surveys in a wide range of habitats and conditions. Satellite imagery is another potential solution to the problem of surveying far offshore 50 .…”