Seasonal Spatial Densities of Cetaceans in the Southern Salish Sea and on Swiftsure Bank

Abstract

Knowledge of seasonal density and distribution of cetaceans in the TMX marine shipping area was identified as a data gap by the Canada Energy Regulator. 29 systematic repeated boat-based surveys were conducted across all seasons from August 2020 to August 2024, covering the southern Salish Sea and Swiftsure Bank. Surveys were designed to detect multiple cetacean species. Eight different cetacean species were detected. There were sufficient sightings of humpback whales, harbour porpoises, and Dall’s porpoises to inform density and abundance estimation, accounting for the detectability of each species. Spatial modelling was used to produce effort-corrected, season-specific density maps throughout the Canadian portions of the TMX marine shipping area. All three species showed distinct seasonal and spatial density patterns. Highest abundance of humpback whales occurred in fall. Areas of highest density for humpback whales included Swiftsure Bank in all seasons, and central and western Juan de Fuca Strait in summer and fall. Harbour porpoises were the most frequently sighted species in all seasons. Areas of highest density of harbour porpoises included Haro Strait, Boundary Pass, and eastern Juan de Fuca Strait year-round. There were also high-density areas in the southern Strait of Georgia in winter, spring, and fall. Dall’s porpoise abundance was highest in winter. Dall’s porpoise density was highest in the southern Strait of Georgia in fall and winter, and in western Juan de Fuca Strait in spring and summer. This multi-year study provides the first year-round spatially-explicit estimates of the seasonal distribution and abundance of humpback whales, harbour porpoises and Dall’s porpoises in the southern Salish Sea and on Swiftsure Bank. These results are intended to inform a subsequent planned assessment of vessel strike risk. The seasonal density layers can inform ecosystem studies, recovery planning, environmental response planning, and impact assessments of threats, such as entanglement in fishing gear and underwater noise. The temporal variability observed in cetacean distributions within the TMX marine shipping area underscores the need for management strategies that are aligned with seasonal density patterns rather than relying on data from a single season or year.