Foraging preferences of an apex marine predator revealed through stomach content and stable isotope analyses

Insights into the food habits of predators are essential for maintaining healthy predator populations and the functioning of ecosystems. Stomach content and stable isotope analyses were used to investigate the foraging habits of an apex predator, the Indo-Pacific bottlenose dolphin (Tursiops aduncus) in south-western Australia. A total of 2,594 prey items from 26 families were identified from the stomachs of 10 deceased stranded dolphins. Fish otoliths from stomach contents were used to identify fish to family or species level. Ninety-three percent of identified stomach contents were perciforme fishes, however, perciformes comprised only 30% of the catch during prey sampling. Gobiidae species, small fish generally <100 mm in total length, were the most prevalent family identified in dolphin stomachs, accounting for 82% of identified prey, yet Gobiidae accounted for 12.7% of the catch during prey sampling. For stable isotope analyses, tissue samples from 14 free-ranging dolphins were analyzed for nitrogen (δ¹⁵N) and carbon (δ¹³C) ratios. From stable isotope analyses and boat-based dolphin photo-identification surveys (n = 339, 2007–2011), results indicated niche differentiation between coastal and inshore (bay and estuarine habitat) dolphins. Carbon signatures showed that coastal dolphins had a more pelagic diet compared to a benthic diet observed in the inshore dolphins. Whereas, nitrogen signatures of inshore dolphins showed higher nitrogen levels than coastal dolphins, likely attributed to feeding on enriched prey typical of estuarian environments. Overall, these results indicated that bottlenose dolphins in the study area were selective foragers and that their foraging is specialized by the habitats most frequently used.