Abstract
West Coast Transient (WCT) killer whales ( Orcinus orca rectipinnus ) hunt marine mammals along the west coast of North America from California to southeastern Alaska. However, little information is available on their seasonal use of nearshore areas and the relative importance of different parts of their range. Using sighting data from 2016 to 2023, we assessed their seasonal distribution and abundance in the inside waters of Vancouver Island (the Salish Sea and North Island Waters). WCT killer whales were seen an average of 250 days throughout each year. They were most frequently observed in the Salish Sea from March to September but had fewer sightings in July of even years when pink salmon returns were low. In contrast, occurrences of WCT killer whales in the North Island Waters peaked in September but were significantly lower overall. Peak occurrences in both areas corresponded with the seasonal presence of seals and sea lions, their primary prey. Overall, ~70% of the coastal subset of the WCT population occurred annually in a region that accounts for a relatively small portion of their range. This high concentration highlights the importance of the inside waters for their survival and emphasizes the need to assess other key areas within their range.