Center for Marine Biodiversity and Conservation

The endangered Southern Resident Killer Whale (SRKW) population is an icon of the Pacific Northeast. The population has experienced a 10 percent decline in population since 2005 and now number only 76 individuals, the lowest abundance in more than 30 years. The SRKWs have been shown to be food-limited with declines in survival and reproduction in years following low salmon availability. Diet studies conducted on the SRKWs have shown a strong prey preference for Chinook salmon (Oncorhynchus tshawytscha) in the summer, yet uncertainty remains about prey preferences and foraging behavior. To help fill this data gap, I used high-resolution aerial images to quantify and describe foraging behavior from a new perspective. Specifically, I reviewed images collected by a small unmanned hexacopter during five individual month-long field efforts between 2015 and 2017. From this collection 29 distinct foraging events were documented in 2,384 images that allowed for photogrammetry measurements of fish size, and species identification of fish preyed upon. The data show that there is a clear difference in the size of fish chased versus the size of fish confirmed to be captured and shared with other members of the pod, suggesting that the whales may be selectively targeting certain size or age classes of fish. Most of the fish observed within the study were determined to be Chinook salmon with the possibility of other salmonid species also being preyed upon. Of the 18 successful foraging events, prey-sharing occurred 88% of the time, with 62.5% of prey-sharing behavior occurring between mothers and calves. This knowledge is important to the successful management and protection of this unique and critically endangered population. Knowing targeted species and size classes of fish can allow for better fishing and recovery strategies, which may lead to increased foraging success of the SRKWs.