Population Demographics of Killer Whales (Orcinus orca) in Western Australia

Understanding population demographics is crucial for the effective conservation of species. This is particularly important for apex predators, such as the killer whale (Orcinus orca), which play important roles in maintaining the structure and function of ecosystems. In this study, we used capture-mark-recapture (CMR) modeling to assess killer whale demographics at two distinct aggregation sites in Western Australia: the Ningaloo Reef (NW) and Bremer Sub-basin (SW). Using 12 years of photo-identification (photo-ID) data from the NW and 6 years from the SW, we applied open population models (POPAN and Cormack–Jolly–Seber) to estimate the abundance (N_total), apparent survival (Phi), capture probability (p), and probability of entry (pent) of killer whales inhabiting these key areas. Abundance estimates in the NW, which combined females, males, and individuals of unknown sex (N_total = 47 ± 2, 95% CI = 43–52), revealed a small, well-marked, and frequently resighted population with high apparent survival (Phi_female = 0.98 ± 0.016, 95% CI = 0.91–0.99; Phi_male = 0.88 ± 0.060, 95% CI = 0.71–0.96; Phi_unknown = 0.82 ± 0.064, 95% CI = 0.66–0.92). In contrast, abundance estimates in the SW, which separated sexed adults (N_total = 102 ± 7, 95% CI = 88–117) from the unknown sex group, suggested a much larger and more transient population with variable apparent survival (Phi_female = 0.98 ± 0.001, 95% CI = 0.97–0.98; Phi_male = 0.82 ± 0.012, 95% CI = 0.79–0.84; Phi_unknown(T) = 0.65 ± 0.177, 95% CI = 0.29–0.90; Phi_unknown(R) = 0.84 ± 0.185, 95% CI = 0.26–0.99). Variation in population demography between the two study areas likely reflects divergent ecological and evolutionary histories shaped by both genetic and environmental factors. Additionally, differences in habitat accessibility, individual distinctiveness, and sampling design may have influenced detectability and model outcomes. These results provide vital baseline demographic data for killer whales in Australian waters and highlight the need for standardized long-term monitoring to inform conservation management strategies.