An evaluation of hot-iron branding as a permanent marking method for adult New Zealand sea lions, Phocarctos hookeri

Abstract

Context Studies of the population and behavioural ecology of pinnipeds require the ability to identify individuals over periods ranging from a single season to an entire lifetime. Aims The aims of this research were to examine the efficacy of hot-iron branding as a permanent marking technique including the legibility of marks over time and comparing estimates of survival for animals marked with brands versus flipper tags. Methods Adult female New Zealand sea lions (n = 135) aged between 4 and 24 years of age were hot-iron branded with four-digit numbers during the austral summer of 2000. Key results Ten years on, 100% of animals still alive could be identified from these brands. Over the 10-year research period, it was observed that the skin of fully healed individual brands could, on occasion, become lacerated due to injuries received from shark bites and/or bites from other sea lions, removing or temporarily reducing the legibility of single characters of some brands. However, these animals were still identifiable when all digits were considered – and scars could become an identifying mark in their own right. Key conclusions Survival estimates derived from branded versus tagged-only individuals were similar, although the variance associated with tagged-only survival estimates was higher, giving less robust estimates. This is likely a result of higher resight probabilities observed for branded individuals. Resighting of tags requires a close approach with a high associated level of disturbance to both the marked animal and those associated with it, especially when considered over the lifetime of the animal, while brands can be read from a considerable distance with little or no disturbance. Implications Thus, hot-iron branding can be an effective method for permanently identifying sea lions that provides robust parameter estimates, causes low disturbance in the resighting process, and does not compromise survival.