Trophic relationships between sperm whales and jumbo squid using stable isotopes of C and N

The trophic position and the predator-prey relationship between the sperm whale Physeter macrocephalus and the jumbo squid Dosidicus gigas were examined by measuring stable isotope ratios of carbon and nitrogen. Skin samples of sperm whales and muscle samples of small and large jumbo squid were collected between 1996 and 1999 in the Gulf of California. Gender determination through molecular analysis and field identification of size were used to identify adult male, female and immature male sperm whales. The stable isotope ratios of C and N of females and immature males were significantly different from those of adult male sperm whales; however, between females and immature males they did not differ significantly. The δ¹³C and δ¹⁵N values of females and immature males were higher than large jumbo squid by 1.1 ‰ and 2.7 ‰ respectively, suggesting a predator-prey relationship between them. A low isotopic interannual variation among the years 1997 to 1999 was observed in the isotopic signature of females and males. Adult males exhibited a lower isotopic signature than females and immature males, and did not show a trophic relationship with D. gigas. We hypothesized that the stable isotopic signature of mature males reflected their diet from an earlier high-latitude feeding ground. This study shows that stable isotope analysis of sloughed skin samples from free-ranging sperm whales can be an alternative method to stomach content and fecal analyses for evaluating trophic relationships.