Population Viability Analysis and Comparison of Two Monitoring Strategies for Bottlenose Dolphins (Tursiops truncatus) in the Shannon Estuary, Ireland, to Inform Management

Photo-identification has been used to monitor the resident bottlenose dolphin (Tursiops truncatus) population in the Lower River Shannon Special Area of Conservation in Ireland as part of European Union obligations since 1997. A discovery curve using data collected by the Irish Whale and Dolphin Group (IWDG) between 2011 and 2015 suggested all animals in the population were captured during this period, providing an abundance of 145 extant individuals. The IWDG photo-identification catalogue was compared with an independently derived photo-identification catalogue compiled by the National Parks and Wildlife Service during 2015 to identify whether any individuals were not captured by the IWDG during that year; there were none. Mark-recapture analysis of both photo-identification catalogues indicated that the higher survey effort provided a higher and more precise abundance estimate, and the estimate of 145 individuals in the population was supported. Population Viability Analysis using this most up-to-date abundance estimate and new data on life-history characteristics were used to explore population status under a range of potential scenarios. A baseline model showed slowly declining dynamics with a growth rate (r) of -0.007. An increase in adult mortality from 3.8 to 7.0% (e.g., due to bycatch) led to a steep decline in population size. Similar results were obtained when the effects of an infrequent catastrophic event (e.g., a large oil spill) were simulated. Sensitivity analysis revealed the importance of female reproductive capacity for population viability. These results will help local managers to understand the implications of events that may affect this small, genetically discrete resident population. The current monitoring strategy may be insufficient to obtain a thorough understanding of population dynamics; hence, higher survey effort is recommended to gain a better knowledge of key demographic parameters which may affect population viability, especially those limiting female reproductive capacity.