DIFFERENCES IN STABLE-ISOTOPE RATIOS OF CARBON AND NITROGEN BETWEEN LONG-FINNED PILOT WHALES (GLOBICEPHALA-MELAS) AND THEIR PRIMARY PREY INTHE WESTERN NORTH-ATLANTIC

Carbon (C-13/C-12) and nitrogen (N-15/N-14) stable isotope ratios were measured in skin and muscle samples from free-ranging long-finned pil ot whales stranded or caught in fishing gear in two locations in the w estern north Atlantic. Samples of the principal pilot whale prey speci es, long-finned squid and a secondarily important species, Atlantic ma ckerel, were collected for stable isotope analysis from three areas in the western north Atlantic. The stable carbon and nitrogen ratios fro m the mackerel and squid samples did not differ between areas. However , carbon ratios differed between the two prey species, while the nitro gen ratios did not. The difference between the stable nitrogen isotope ratios for prey and predator suggests trophic enrichment of 1.1 to 1. 7 parts per thousand, values substantially lower than previously assum ed for cetaceans. The differences between carbon ratios among prey spe cies and whale tissues suggest that mackerel comprise a significant pr oportion of the diet of pilot whales. (C) 1997 International Council f or the Exploration of the Sea.