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 infre quently encountered species. Consequently, its behavior and habitat re quirements are largely unknown. Evidence of the trophic level and diet of M. bidens was obtained through a comparison of delta C-13 and delt a N-15 values for this species with those for eight other cetaceans an d the basking shark. Cetorhinus maximus. The delta C-13 value for M. b idens (-18.5 +/- 1.1 parts per thousand) was similar to that for all o ther cetaceans (-18.7 to -17.2 parts per thousand) except the sperm wh ale, Physeter macrocephalus, and the blue whale, Balaenoptera musculus (-22.8 and -20.1 parts per thousand, respectively). At least a portio n 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 t he samples. Nitrogen isotope values appear to reflect trophic position . Based on deltaN-15, organisms can be ordered along a trophic continu um bracketed by the planktivorous basking shark (9.9 parts per thousan d) and the piscivorous white-beaked dolphin, Lagenorhynchus albirostri s (16.2 parts per thousand). The trophic position of M. bidens (deltaN -15 = 11.7 parts per thousand) is intermediate between those of the py gmy 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 speci es 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 d iet that has a large contribution from small offshore squid, deltaN-15 = 9.3 parts per thousand.