Feeding ecology of gray whales inferred from stable-carbon and nitrogen isotopic analysis of baleen plates

Stable-carbon and nitrogen isotopic compositions (delta C-13, delta N-15) of baleen plates of two juvenile and four adult gray whales (Eschrichtius robustus) were examined. High variance in isotopic composition of baleen plates was detected among individuals and, unlike other migratory species of baleen whales examined isotopically, delta C-13 and delta N-15 values of most whales showed no regular oscillations. Only one baleen plate reflected the assumed principal Arctic prey (ampeliscid amphipods) during growth on the summer grounds. The rate of baleen growth inferred for one of the juveniles in the last 5 months of life (4.7 mm/week) was similar to the rate calculated previously for a rehabilitating gray whale calf. Autumn corresponded to the timing of the formation of lowest delta N-15 values measured along plates. We estimate that the baleen length in all adult gray whales recorded around a year of information (1.3 +/- 0.3; Mean +/- SD). This short period of dietary integration precludes long time series analyses in this species and reflects the extensive wear on baleen plates due to benthic foraging.