Evaluation of the diet of Sowerby's beaked whale, Mesoplodon bidens, based on isotopic comparisons among northwestern Atlantic cetaceans

Abstract

Sowerby's beaked whale, Mesoplodon bidens, is a poorly known and infrequently encountered species. Consequently, its behavior and habitat requirements are largely unknown. Evidence of the trophic level and diet of M. bidens was obtained through a comparison of δ13C and δ15N values for this species with those for eight other cetaceans and the basking shark, Cetorhinus maximus. The δ13C value for M. bidens (−18.5 ± 1.1‰) was similar to that for all other cetaceans (−18.7 to −17.2‰) except the sperm whale, Physeter macrocephalus, and the blue whale, Balaenoptera musculus (−22.8 and −20.1‰, respectively). At least a portion of the range of δ13C values for the cetaceans and the basking shark is thought to be related to variations in the lipid content of the samples. Nitrogen isotope values appear to reflect trophic position. Based on δ15N, organisms can be ordered along a trophic continuum bracketed by the planktivorous basking shark (9.9‰) and the piscivorous white-beaked dolphin, Lagenorhynchus albirostris (16.2‰). The trophic position of M. bidens (δ15N = 11.7‰) is intermediate between those of the pygmy sperm whale, Kogia breviceps (δ15N = 11.9‰), and the sperm whale, Physeter macrocephalus (δ15N = 11.1‰), and thus supports previous suggestions that these species feed at similar trophic positions. The estimated δ15N value of the diet of M. bidens, 8.7‰, is consistent with a diet that has a large contribution from small offshore squid, δ15N = 9.3‰.