UC Santa Cruz

The Irrawaddy dolphin (Orcaella brevirostris) is an imperiled Southeast Asian coastal and freshwater mammal species. Throughout their range, Irrawaddy dolphins face numerous anthropogenic threats including gillnet entanglement, habitat degradation, and boat disturbance (e.g. noise). Five subpopulations, four of which are freshwater, are classified as Critically Endangered on the International Union for the Conservation of Nature’s (IUCN) Red List of Threatened Species and the species as a whole is classified as Vulnerable. It is thought that most subpopulations are small and declining. Little is known about this species’ habitat preferences, life history, or movement. The subpopulation in the Gulf of Thailand is one that has yet to be evaluated and thus provides an ideal research subject to increase the body of knowledge on this rare and little-studies species. In Chapter 1, we analyzed stable isotopes of carbon and nitrogen using ANOVA and MANOVA to investigate regional subpopulation structure and ontogenetic patterns in the Gulf of Thailand and the Andaman Sea. We further used a mixing model (MixSIAR) to determine diet in two sites in the eastern Gulf of Thailand by comparing stable isotopes of dolphins to those of potential prey species. Limited age and sex structure was discovered. However, stable isotope ratios in Irrawaddy dolphins varied significantly by region (eastern Gulf, northern Gulf, western Gulf, Andaman Sea), suggesting distinct geographic groups. Mixing models revealed that the group within the eastern Gulf is primarily piscivorous, consuming mainly ponyfish, mackerel, gizzardshad, and scad, and secondarily crustacean-eating. Cephalopods make up a smaller proportion of the overall diet. Slight differences were found between the two study sites within the eastern Gulf, suggesting weak sub-regional structure, perhaps driven by resource partitioning. In Chapter 2, we used a hurdle model to evaluate the habitat of Irrawaddy dolphins in the eastern Gulf of Thailand. We found that depth and temperature are significant predictors of dolphin presence and group size, respectively. Depth had a positive relationship with dolphin presence probability up to around 10.5 meters, at which the relationship became negative. Dolphin group size was significantly negatively correlated with temperature. We further identified two primary locations of dolphin occurrence probability (combined high presence and large group size probabilities), one large region in the center of the study area and a smaller region in the south. Finally, in Chapter 3, we used binomial and negative binomial GLMs to model fishing effort in three locations in the eastern Gulf of Thailand. We further compared these results to the results from Chapter 2 to determine where dolphins were more likely to encounter fishing vessels and gear.