Stable Isotope Ratios of Carbon, Nitrogen, Oxygen, and Mercury Concentrations in North Pacific Baleen Whales and the Comparison of Their Calves with Toothed Whale Calves

Abstract

We quantified 13C, 15N,18O, and Hg concentrations in the muscle of calf and immature humpback whales stranded along the coast of the North Pacific Ocean around Hokkaido, Japan, and investigated those changes owing to the lactation. Next, we compared these concentrations in stranded humpback whale calves with those in stranded fin whale and North Pacific right whale calves, and stranded calves from other species reported previously [1,2]. We further compared those concentrations in stranded fin whales with those in fin whales hunted from the North Atlantic and Antarctic Oceans. The δ13C value in humpback whale calves increased with body length (7.0-8.7 m), whereas the δ18O values tended to decrease. In contrast, a small δ15Nenriched peak was found in middle-sized calves. Humpback whale calves had trace Hg concentrations (≤0.05 μg/wet g), whereas these concentrations exceeded 0.10 μg/wet g in immature humpback whales. These changes in the δ13C, δ15N, and δ18O values and Hg concentrations in humpback whales could reflect a feeding shift from milk to solid foods. The δ13C and δ15N levels of calves, humpback and fin whales, and common minke whales reported previously [1] were similar, slightly higher than those of North Pacific right whales and significantly lower than those of killer whales [2]. These findings suggest that the δ13C and δ15N values in the milk and weaning solid foods of humpback, fin, and common minke whales are similar (opportunistic fish eaters), slightly different from North Pacific right whales (zooplankton eaters), and markedly different from killer whales (highest predator). Fin whales stranded in the North Pacific Ocean could be distinguished from fin whales hunted from the North Atlantic and Antarctic Oceans using δ13C, δ15N, and δ18O values. The δ18O values, combined with the δ13C and δ15N values could be an excellent proxy to discriminate fin whales from the three oceans.