Diet Preference of Orcaella Brevirostris (Irrawaddy Dolphin) from the Bay of Brunei, as Indicated by Stable Isotope Analysis

The Irrawaddy dolphin (Orcaella brevirostris) is a euryhaline cetacean that inhabits inshore waters throughout the Indo-Pacific region, including the Bay of Brunei, a semi-enclosed estuarine system along the northeastern coast of Sarawak, East Malaysia. This study presents the first application of stable isotope analysis to investigate the diet of the Irrawaddy dolphin in Malaysian waters. Skin samples from free-ranging Irrawaddy dolphins and muscle tissues from locally caught fish and invertebrates were analyzed for δ¹³C and δ¹⁵N values. Prey items were grouped into five ecological categories, and dietary contributions were estimated using a Bayesian mixing model (MixSIAR). The results revealed that juvenile piscivores, particularly Scomberoides commersonnianus and Sphyraena forsteri, contributed the most to the dolphins’ diet. These prey species are abundant in nearshore estuarine habitats, highlighting the ecological importance of estuarine systems as foraging grounds for Irrawaddy dolphins. The findings underscore the species’ trophic reliance on fish assemblages and reinforce the need to conserve estuarine habitats to sustain both prey availability and dolphin populations in the Bay of Brunei.