An Example of Comprehensive Research on Little-Known Cetaceans: The Irrawaddy dolphin (Orcaella brevirostris) in the Eastern Gulf of Thailand

The Irrawaddy dolphin is a coastal and freshwater cetacean patchily distributed from eastern India to West Papua, Indonesia. Little is known about its ecology, limiting capacity for successful conservation. As recently as 2008, the IUCN altered its listing from Data Deficient to Vulnerable. We present a study design to obtain five key features of imperiled species ecology that we believe can be widely applied: current knowledge, abundance, diet, habitat, and potential interaction with humans. Current knowledge was determined with a literature review and discussion with experts. Between 2008 and 2014, we collected sightings, environment, and human use data along zig-zag transect lines. Distance 6 software was employed to estimate abundance from sightings and a hurdle model was performed to investigate relationships between dolphins and the environment. We determined the relative density of types of human use within the study area (e.g. consumptive such as fishing and non-consumptive such as recreation), assessed the relationship between human use and environment, and examined the overlap of dolphin habitat with human high-use areas. We studied diet and foraging history using stable isotopes of carbon and nitrogen in skin (n=22) and teeth (n=27) of stranded dolphins and muscle samples of cephalopods (2 species), crustaceans (2 species), and fish (8 species). Data from soft tissues was used to investigate the proportional contribution of each prey type to diet using a Bayesian mixing model, while we examined growth layer groups in teeth to assess ontogenetic diet variation using protected analysis of variance. This study is the first such comprehensive study on this species and contributes to general knowledge about the species, provides information that is relevant to conservation, and can serve as a template for future studies on little-known, threatened species.