COS 245-5 - Niche partitioning of beaked whales (family Ziphiidae) during the summer in the western North Atlantic

Abstract: A species niche can be measured spatially, temporally, and trophically. Ecosystems have a limited amount of resources, thus as species diversity increases there is an augmented need for niche partitioning. In the pelagic ocean, air-breathing marine mammals can forage near the surface or at deep depths, with beaked whales (family Ziphiidae) comprising a large fraction of those that forage at depth. Beaked whales are cryptic and visual species identification is often difficult, however passive acoustic monitoring is now a reliable tool in distinguishing species based on their foraging clicks. From 28 June to 25 August 2016, two broadscale shipboard cetacean abundance surveys, part of the Atlantic Marine Assessment Program for Protected Species, were conducted across the western North Atlantic shelf break using towed passive acoustic arrays. In addition, fixed bottom-mounted High-frequency Acoustic Recording Packages (HARPs), collected continuous data at 11 sites along the shelf break from Georges Bank in the north, to Blake Plateau in the south. The survey effort was subdivided into 11 regions, each containing one HARP site to facilitate cross-platform comparisons. Classification trees were created within R (4.2.2) to examine spatial and temporal partitioning of beaked whale species. Trophic partitioning was examined by estimating the depths of foraging click events using a combination of R and Matlab (R2019a) scripts, with comparisons to the seafloor depth. Five species of beaked whales were detected: Cuvier’s (Ziphius cavirostris), Blainville’s (Mesoplodon densirostris), Gervais’ (Mesoplodon europaeus), Sowerby’s (Mesoplodon bidens), and True’s (Mesoplodon mirus). Beaked whales were present at 10 of the 11 HARP sites, with as many as three species co-occurring on the same day. Beaked whales were commonly present at the daily scale, ranging from 21 - 98% days present across sites. The spatial distribution of beaked whale species (n = 394 acoustic encounters) appeared to be associated with ocean currents; for example, Blainville’s and Gervais’ were only detected in the Gulf Stream. Cuvier’s were present in all nodes of the spatial classification tree, confirming their cosmopolitan distribution. Blainville’s and Sowerby’s were found to exclusively forage in proximity to the seafloor, while Cuvier’s, Gervais’, and True’s also foraged in the water column (depths estimated for 192 acoustic encounters). Passive acoustic data provides detailed insights into beaked whale species presence, and a better understanding of how these cryptic deep diving species utilize the ocean and divide their foraging efforts in space and time.