UC San Diego

Food availability has been identified as a critical factor influencing the growth, individual health, and population dynamics of killer whales (Orcinus orca) in the coastal waters of the eastern North Pacific Ocean. Between 2014 and 2019 we conducted 113 hand-launched unmanned aerial system flights over mammal-eating Bigg’s killer whales around Vancouver Island, resulting in 20,545 aerial photographs of 95 individually identified animals. I conducted photogrammetric measurements from high-quality images in 91 individuals, a sample that ranged from first-year calves to mature adults of both sexes; this is the first study to estimate size, growth, and health of Bigg’s killer whales. Individual lengths ranged from a 2.4m neonate to an 8.3m 38-year-old male. Using a Richard’s growth curve model, I estimated asymptotic adult length at 6.4 ± 0.1m (standard error) in females and 7.3 ± 0.2m in males, as well as age of inflection at 14.2 ± 2.8 years in females and 18.4 ± 2.3 years in males. Comparison with sympatric salmon-eating Southern Resident killer whales found that both sexes of Bigg’s killer whale measured significantly longer than Southern Residents (female z-test P = 0.003; male z-test P = 0.093) but there was no significant difference in age of inflection between males (z-test, P = 0.53) or females (z-test, P = 0.45) between the two populations. Analysis of eye patch ratio (a proxy for body condition) revealed that all age/sex classes of Bigg’s killer whales were more robust than Southern Resident killer whales, and the difference was most significant when comparing calves (z-test, P < 0.0001) and juveniles (z-test, P < 0.0001) between the two populations. I propose that in the absence of major discrepancies in growth trends, morphometric divergences between the two populations are largely a function of prey availability.