Niche differentiation of bottlenose dolphin species in South Australia revealed by stable isotopes and stomach contents

Context Animal species with overlapping ranges are common worldwide, but how these species coexist is less obvious. Dolphins are protected in Australia and yet little information is available on their resource use which is essential for successful management and conservation. Aim The aim of this study was to determine the degree of overlap in diet and feeding ecology of the two Tursiops spp. that have overlapping ranges in South Australia. Methods Stomach content (91 Tursiops sp. and 14 T. truncatus) and stable isotope (δ ¹³C, δ ¹⁵N) analyses (39 Tursiops sp. and 14 T. truncatus) were conducted. Carcasses of dolphins were opportunistically collected between 1974 and 2005. Diet was quantified by frequency of occurrence (FOO) and numerical abundance (NA) of prey. Key results Delta ¹³C from teeth revealed distinct differences in primary source of carbon, corresponding to coastal (Tursiops sp. n=39, X̄=-12.24, s.d.=1.32) and offshore habitats (T. truncatus n=14, X̄=14.21, s.d.=0.55). Differences in 15N revealed Tursiops sp. (X̄=11.66, s.d.=0.58) feeds at a lower trophic level than T. truncatus (X̄=14.29, s.d.=0.88). Stomach content analyses for Tursiops sp. corroborated stable isotope results. There was a significant difference between the diets of Tursiops sp. from north Spencer Gulf and south Spencer Gulf (ANOSIM R=0.249, P=0.001). Prey were generally demersal in habit with the most important from the cephalopod families Octopodidae, Sepiidae and Loliginidae and fish families Carangidae, Clupeidae, Terapontidae and Apogonidae. For T. truncatus there were insufficient stomach contents for assessment. Conclusions South Australian bottlenose dolphin species exhibit distinct niche differentiation with clear evidence of regional variation in the diet of Tursiops sp. Implications The between and within species diet differentiation demonstrated, highlights the importance of regional management. Such results are internationally significant as coastal and offshore forms of Tursiops spp. occur worldwide.