Seasonal productivity drives aggregations of killer whales and other cetaceans over submarine canyons of the Bremer Sub-Basin, south-western Australia.

Cetaceans are iconic predators that serve as important indicators of marine ecosystem health. The Bremer Sub-Basin, south-western Australia, supports a diverse cetacean community including the largest documented aggregation of killer whales (Orcinus orca) in Australian waters. Knowledge of cetacean distributions is critical for managing the area's thriving ecotourism industry, yet is largely sporadic. Here we combined aerial with opportunistic ship-borne surveys during 2015–2017 to describe the occurrence of multiple cetacean species on a regional scale. We used generalised estimating equations to model variation in killer whale relative density as a function of both static and dynamic covariates, including seabed depth, slope, and chlorophyll a concentration, while accounting for autocorrelation. Encountered cetacean groups included: killer (n = 177), sperm (n = 69), long-finned pilot (n = 29), false killer (n = 2), and strap-toothed beaked (n = 1) whales, as well as bottlenose (n = 12) and common (n = 5) dolphins. Killer whale numbers peaked in areas of low temperatures and high primary productivity, likely due to seasonal upwelling of nutrient-rich waters supporting high prey biomass. The best predictive model highlighted potential killer whale 'hotspots' in the Henry, Hood, Pallinup and Bremer Canyons. This study demonstrates the value of abundance data from platforms of opportunity for marine planning and wildlife management in the open ocean. Every summer, a remote area 50 km off Bremer Bay, Western Australia, attracts many dozens of killer whales in what are currently the largest recorded numbers of the predator in Australian waters. New data collected aboard vessels show that this hotspot may be one of many found across an extensive network of underwater canyons where food is likely plentiful. Mapping whale aggregations is a key step in supporting and managing the region's thriving wildlife tourism industry. Photograph by Rebecca Wellard, Curtin University and Project ORCA. [ABSTRACT FROM AUTHOR]