Resolving the Trophic Relations of Cryptic Species: An Example Using Stable Isotope Analysis of Dolphin Teeth

Kylie Owen,Kate Charlton-Robb,Ross Thompson,Yan Ropert-Coudert

doi:10.1371/journal.pone.0016457

Abstract

Understanding the foraging ecology and diet of animals can play a crucial role in conservation of a species. This is particularly true where species are cryptic and coexist in environments where observing feeding behaviour directly is difficult. Here we present the first information on the foraging ecology of a recently identified species of dolphin (Southern Australian bottlenose dolphin (SABD)) and comparisons to the common bottlenose dolphin (CBD) in Victoria, Australia, using stable isotope analysis of teeth. Stable isotope signatures differed significantly between SABD and CBD for both δ13C (−14.4‰ vs. −15.5‰ respectively) and δ15N (15.9‰ vs. 15.0‰ respectively), suggesting that the two species forage in different areas and consume different prey. This finding supports genetic and morphological data indicating that SABD are distinct from CBD. In Victoria, the SABD is divided into two distinct populations, one in the large drowned river system of Port Phillip Bay and the other in a series of coastal lakes and lagoons called the Gippsland Lakes. Within the SABD species, population differences were apparent. The Port Phillip Bay population displayed a significantly higher δ15N than the Gippsland Lakes population (17.0‰ vs. 15.5‰), suggesting that the Port Phillip Bay population may feed at a higher trophic level - a result which is supported by analysis of local food chains. Important future work is required to further understand the foraging ecology and diet of this newly described, endemic, and potentially endangered species of dolphin.

Highlights

Understanding the foraging ecology of a species or population can strongly impact on the ability to accurately conserve the species or population of concern
The Port Phillip Bay potential prey items had higher average d13C (217.6% vs. 218.1%) and a lower average d15N (12.5% vs. 14.2%) compared to the Gippsland Lakes potential prey items (Figure 2). For both the common bottlenose dolphin (CBD) and the south Australian bottlenose dolphins (SABD) no significant effect of age of the animal or year of sample collection was found in correlation to isotope values (Table 2 and Table 3)
The CBD had significantly lower values for d13C (215.5% vs. 214.4%) (F1, 31 = 14.515, p,0.001) and d15N (15.0% vs. 15.9%) (F1, 31 = 4.980, p = 0.033) compared to the SABD (Figure 2)

Summary

Introduction

Understanding the foraging ecology of a species or population can strongly impact on the ability to accurately conserve the species or population of concern. Populations of killer whales in the northwest Pacific exhibit distinct differences in dietary preferences, with resident populations targeting salmon, while transient populations forage on other marine mammals and sea birds [5]. This distinction in foraging ecology has long been used as an argument for differences in the way that the two populations should be managed [6] as well as an additional line of evidence supporting divergence of two populations as separate species [7]

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