Niche partitioning among social clusters of a resident estuarine apex predator

Conspecifics may vary in their space use and diet leading to niche partitioning within populations. In social species, such partitioning may correspond to social structure as closely associated individuals likely encounter the same resources. This study investigated whether space use and diet varied among social clusters of a resident estuarine population of Indo-Pacific bottlenose dolphins. Dolphin photo-identification and behavioral data, as well as tissue samples for stable isotope analysis, were collected during boat-based surveys in the Peel-Harvey Estuary, Western Australia. Potential dolphin prey species were also collected for stable isotope analyses. Six mixing models, one assuming an invariant diet and others allowing for variation in diet according to sex, age class, and/or social cluster, were fitted to the data. The model with social cluster was the best fit and estimated detritivorous fish as the main dietary source for social clusters whose core activity space covered the eastern shores of the estuary and the rivers. These clusters occupied the lowest trophic position in the dolphin population. Benthic omnivores and carnivores contributed most to the diet of clusters whose core activity space included the two estuary entrances. These clusters occupied the highest trophic position. Clusters with core activity space located in the estuary basins reflected the overall mean contributions of fish feeding guilds to dolphin diet in this population. Detritivores, omnivores and herbivores, and benthic omnivores and carnivores each contributed approximately a third and water column species the remainder to the annual fish biomass removed from the estuary by the dolphin population. We conclude that dolphins share resources with fishers and piscivorous birds within the estuary. SIGNIFICANCE STATEMENT: This study identified intra-population resource partitioning according to social structure in a resident estuarine dolphin population. The heterogeneity in space use and diet among social clusters may result in individuals being susceptible to different pressures and threats. The dolphins’ foraging behavior and trophic interactions identified them as an apex predator in the Peel-Harvey Estuary, with their collective minimum annual food intake (~ 200,000 kg) exceeding the annual fish biomass removed by commercial fishers. As top predators in the system, dolphins may suppress prey populations through consumption and as agents of intimidation by changing prey distribution and behavior. This study provides scientific basis for recognizing dolphins as an important component of the Peel-Harvey Estuary ecosystem.