Population estimates and range of movement of Irrawaddy dolphins (Orcaella brevirostris) in Bintulu, Malaysian Borneo

Cetacean surveys were conducted using line transect and photo-identification methodology to determine the abundance and range of Irrawaddy dolphins along the coastal waters of Bintulu, Malaysia. During surveys from June... [ view full abstract ]

Cetacean surveys were conducted using line transect and photo-identification methodology to determine the abundance and range of Irrawaddy dolphins along the coastal waters of Bintulu, Malaysia. During surveys from June 2008-September 2013, dolphins were photographed and group size and behavior were recorded. Locations of re-sighted dolphins were mapped and ranges of movement were measured using ArcGIS.

After filtering for photo quality and distinctiveness of dorsal fins, the best mark-recapture estimate for Irrawaddy dolphins based on a weighted mean of estimates derived from individuals represented by photos of their left sides and right sides  was 189 (CV = 23%, 95% CI 122-292). These estimates overlap well with those derived from line transect analysis using DISTANCE (170, CV=38% 95% CI 81-356). A number of individuals showed a high-degree of site fidelity. Juveniles and calves were observed during 33 of the 77 sightings of Irrawaddy dolphins. Feeding and probable feeding behavior were observed during 44.2% of the sightings.

The high rate of site fidelity, coupled with the dolphins’ utilization of the entire study area and observations of calves and feeding demonstrate that these coastal waters are important habitat for the species. The baseline data presented here are crucial for the evaluation of the conservation status of Irrawaddy dolphins in Sarawak and for the design of effective management strategies in an area of coastline that is undergoing rapid agricultural and industrial growth, including coastal oil palm plantations, a new deep water port and industrial complex situated between a national park and a fishing village.