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.