EVALUATION OF THE DIET OF SOWERBYS BEAKED-WHALE, MESOPLODON-BIDENS, BASED ON ISOTOPIC COMPARISONS AMONG NORTHWESTERN ATLANTIC CETACEANS

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 delta C-13 and delta N-15 values for this species with those for eight other cetaceans and the basking shark. Cetorhinus maximus. The delta C-13 value for M. bidens (-18.5 +/- 1.1 parts per thousand) was similar to that for all other cetaceans (-18.7 to -17.2 parts per thousand) except the sperm whale, Physeter macrocephalus, and the blue whale, Balaenoptera musculus (-22.8 and -20.1 parts per thousand, respectively). At least a portion of the range of delta C-13 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 deltaN-15, organisms can be ordered along a trophic continuum bracketed by the planktivorous basking shark (9.9 parts per thousand) and the piscivorous white-beaked dolphin, Lagenorhynchus albirostris (16.2 parts per thousand). The trophic position of M. bidens (deltaN-15 = 11.7 parts per thousand) is intermediate between those of the pygmy sperm whale, Kogia breviceps (deltaN-15 = 11.9 parts per thousand), and the sperm whale, Physeter macrocephalus (deltaN-15 = 11.1 parts per thousand), and thus supports previous suggestions that these species feed at similar trophic positions. The estimated deltaN-15 value of the diet of M. bidens, 8.7 parts per thousand, is consistent with a diet that has a large contribution from small offshore squid, deltaN-15 = 9.3 parts per thousand.