Passive acoustic monitoring reveals spatiotemporal segregation of two fish-eating killer whale Orcinus orca populations in proposed critical habitat

Abstract

Competition for prey resources among ecologically similar populations that occur in sympatry can be reduced by spatiotemporal resource partitioning. Understanding patterns of habitat use of cetaceans can be difficult since they are highly mobile and can have large home ranges. We used passive acoustic monitoring at 15 sites along the coast of Washington State, USA, to assess habitat use patterns of 2 sympatric populations of fish-eating killer whales Orcinus orca: northern residents (NRKW) and southern residents (SRKW). This area is part of the ocean distributions of a number of important runs of Chinook salmon Oncorhyncus tshawytscha, the preferred prey of both populations, and is proposed critical habitat for SRKW. We compared monthly occurrence of both populations at recorder locations grouped by their proximity to the Strait of Juan de Fuca to the north and the Columbia River to the south in one analysis and by their distance from shore in a second analysis. NRKW and SRKW were detected throughout the year with spring and fall peaks in occurrence. The northernmost sites accounted for 93% of NRKW detections, while less than half of SRKW detections were at these sites. SRKW were most frequently detected at nearshore sites (83% of detections), while the majority of NRKW detections were at mid-shelf and deep sites (94% of detections). This study provides further information about the habitat use of these resident killer whale populations with implications for their management and conservation.