Exposure risk of alcids to marine vessel-associated oil pollution in Western Canada

Oiling risk from vessel traffic is an increasing threat to marine birds in the Pacific region of Canada. We defined exposure risk as the joint likelihood that oil pollution and sensitive marine birds occur in the same area at the same time. We developed and applied two standardized overlap metrics-the Utilization Distribution Overlap Index (UDOI) and Intersect Intensity (II)-to identify where and when marine birds are most exposed to oil pollution within the Canadian Pacific Exclusive Economic Zone (EEZ). We used Kernel Density Estimation (KDE) based on at-sea transect and coastal swath/point-count data (compiled from 2000 to 2022) to estimate seasonal (winter: Oct-Mar; summer: Apr-Sep) relative distributions for six alcid species: Ancient Murrelet (ANMU), Marbled Murrelet (MAMU), Cassin's Auklet (CAAU), Common Murre (COMU), Rhinoceros Auklet (RHAU), and Pigeon Guillemot (PIGU), accounting for spatially and temporally heterogeneous effort. We also used KDEs based on AIS-based vessel traffic, maritime infrastructure, and shipwreck location data to estimate relative likelihood of oil pollution. UDOI results showed that vessel traffic is the dominant source of exposure risk for alcids, with strong seasonal and species-specific variation. Exposure was consistently high for COMU and MAMU in both seasons, and consistently low for CAAU. Risk was elevated in winter for ANMU, COMU, and MAMU, and in summer for RHAU and PIGU, largely due to seasonal range contraction into the Salish Sea, where vessel traffic and coastal infrastructure are concentrated. II maps revealed persistent hotspots in the Salish Sea, particularly Haro Strait and eastern Juan de Fuca Strait, for most species in both seasons. Western Juan de Fuca and Georgia Straits, and Queen Charlotte and Johnstone Straits, were also identified as important seasonal hotspots, especially during summer.