An acoustic survey of beaked whale distribution at São Tomé and Príncipe, Gulf of Guinea, using an unmanned surface vessel

Abstract

The first records of beaked whales, including Cuvier’s beaked whale Ziphius cavirostris, are reported from an acoustic baseline survey in the territorial waters of the Democratic Republic of São Tomé and Príncipe, Gulf of Guinea, West Africa. The survey was carried out between late 2018 and early 2019 using an AutoNaut unmanned surface vessel (USV), which is propelled by the motion of waves and operated remotely via a satellite link. The USV towed a hydrophone array on a cable, and over the course of 75 days the USV completed 1 772 km of acoustic survey effort. Beaked whale encounters were not uncommon and occurred on 28 of the 75 survey days. They were widespread on the shelf edge and in oceanic habitats above the abyssal plain. The average water depth at which acoustic detections were recorded was 2 519 (SD 555) m (n = 43). Beaked whales were evenly distributed across areas with different seabed characteristics: the number of encounters recorded in areas characterised by seabed gradient and ruggedness did not differ from that expected after accounting for survey effort. The majority of acoustic detections were attributable to Cuvier’s beaked whales, based on spectral and temporal characteristics of click-train vocalisations. The results provide baseline information on the distribution of beaked whales and suggest that passive acoustic methods using a small USV are well-suited to surveys of this species group, possibly because beaked whales, which are sensitive to disturbance associated with underwater sound sources, are less likely to avoid small, quiet vessels than larger vessels.