Abstract:
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 (~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.
