Abundance estimates of killer whales at subantarctic Marion Island

Killer whales Orcinus orca are apex predators known to have important effects on marine ecosystems. A fundamental step towards understanding their role in ecosystems, and vital for their informed management and conservation, is the rigorous estimation of their abundance. Studies concerning this species have used mark-recapture analytical techniques to estimate abundance, but enumeration of identifiable individuals is more common. This study estimated the abundance of killer whales occurring inshore at subantarctic Marion Island. Mark-recapture analyses were performed using nearly 10 000 photographs taken from 2006 to 2009. Using careful quality control criteria, we identified 37 ind. The evident capture heterogeneity violates the underlying assumptions of the open population POPAN parameterization in the software program MARK we initially used. We thus used the simpler Chapman modified Lincoln-Petersen estimator, calculating a population size of 37 ind. (95% CI = 29 to 44) for the period 2006 to 2007 and 32 ind. (95% CI = 30 to 33) for 2007 to 2008. Both estimates are close to the catalogue size, suggesting that enumeration is an accurate measure of abundance in this study. Our results are comparable to recent abundance estimates for the neighbouring Crozet Archipelago (similar to 1000 km due east). No rigorous approach has been used previously to estimate the abundance of killer whales at Marion Island. This estimate provides a foundation for further research related to the sociality and potential ecological impact of this population of killer whales in the Southern Ocean.