Differences in stable isotope ratios of carbon and nitrogen between long-finned pilot whales (Globicephala melas) and their primary prey in the 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 pilot whales stranded or caught in fishing gear in two locations in the western north Atlantic. Samples of the principal pilot whale prey species, long-finned squid and a secondarily important species, Atlantic mackerel, were collected for stable isotope analysis from three areas in the western north Atlantic. The stable carbon and nitrogen ratios from the mackerel and squid samples did not differ between areas. However, carbon ratios differed between the two prey species, while the nitrogen 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 assumed for cetaceans. The differences between carbon ratios among prey species and whale tissues suggest that mackerel comprise a significant proportion of the diet of pilot whales. (C) 1997 International Council for the Exploration of the Sea.