Survival, Mark Change and Capture Heterogeneity of Bottlenose Dolphins (Tursiops truncatus) in the Shannon Estuary, Ireland

Kim Ellen Ludwig

Demographic parameters, especially adult survival rates, provide important information for the conservation of long-lived species such as bottlenose dolphins (Tursiops truncatus), and can be estimated using mark-recapture (MR) modelling. However, equal catchability and identifiability of individuals is an underlying assumption of MR-modelling, and failure to comply with it can result in a negative bias of survival estimates. Here, photo-identification data and Cormack-Jolly-Seber models were used to estimate survival rates of a genetically distinct bottlenose dolphin population in the Shannon Estuary, Ireland, considering capture heterogeneity between individuals caused by mark severity and site fidelity variability. Individual identifiability over time was assessed by calculating mark changing rates. Survival rates were constant over time and comparable with those of other bottlenose dolphin populations. Lower capture probabilities and survival rates were found for individuals with low sighting rates, which were the result of both low mark severity and, presumably, non-random temporal migration. The latter was suggested due to remarkably high encounter rates with Shannon dolphins in a nearby coastal area. Therefore, future surveys should be extended to this area to further investigate migration patterns. Additionally, high gain rates of dorsal fin indentations support a continuous high effort for photo-ID data collection.