d 15 N and d 13 C in skin biopsy samples : a note on their applicability for examining the relative trophic level in three rorqual species
Preliminary stable nitrogen and carbon isotope analysis was undertaken to investigate whether the resulting data support current knowledge of diet as obtained by conventional approaches. Blue (Balaenoptera musculus), fin (B. physalus) and Bryde's (B. edeni) whales co-occur temporally and are known to feed in the Gulf of California, Mexico. Isotope measurements were taken from: known prey (three euphausiids and four sardine samples); skin biopsies (two for each whale species); and from faeces (one blue and three fin whale samples). Although the sample size was small, the range of ë15N values obtained was consistent with prior knowledge of the whales feeding habits, with values increasing in the order: blue (x=12.9), fin (x=15.4) and Bryde's whales (x=15.8). The low value for the blue whale confirms its known stenophagous habit. The closeness of d15N values for fin and Bryde's whales coincides with the known icthyophagous habits of the Bryde's whale and the more generalist fin whale which feeds on both fish and zooplankton. The difference in ë13C values for fin (x=16.0) and Bryde's whales (x=18.1) suggests that although they feed at the same trophic level, they might use different food sources or feeding sites. Results of ë15N suggest that fin and Bryde's whales share the same relative trophic level, blue whales and juvenile sardines (S. sagax) share a lower position, followed by the euphausiid (Nematocelis difficilis) and fin whale faeces, and at the lowest level blue whale faeces.