Body Length, Stable Carbon, and Nitrogen Isotope Ratios and Mercury Levels in Common Minke Whales Stranded Along the Coast of Hokkaido, Japan

We quantified the stable isotope ratios of carbon (delta C-13) and nitrogen (delta N-15) as well as the total mercury (Hg) concentration in muscle samples from 20 common minke whales (Balaenoptera acuto-rostrata) stranded along the coast of Hokkaido, Japan. We investigated the relationship between the quantified values and the body length (BL) of the whales to examine the BL at which ontogenetic changes, such as nursing and weaning, occur. The whales investigated consisted of 12 calves (BL <= 5 m), including newborns, and eight sexually mature whales (7.2 to 10.2 in BL). The delta N-15 values in the calves were in the range of 11.2 to 13.8(SIC) and were fitted to a quadratic function (F-10 = 4.588, R-2 = 0.505, p = 0.0423). The delta N-15-enriched peak was calculated to be 4.0 m BL, which may correspond with the onset of weaning. In contrast, the delta N-15 values of the mature whales showed no particular change regardless of growth (10.8 to 12.4(SIC)). The (delta C-13 values in the calves showed no particular pattern (-18.3 to -20.0(SIC)), probably due to the large variation in the lipid concentration in milk, while those in the mature whales tended to decrease with growth (F-6 = 3.400, R-2 = 0.362, p = 0.115). The Hg concentrations in the muscle samples were trace but increased linearly with an increase in BL (F-18 = 87.68, R-2 = 0.830, p < 0.0001). The delta N-15 and Hg levels of mature whales were lower than respective levels observed in odontocetes stranded along the coast of Hokkaido, suggesting that mature whales feed mainly on zooplankton and small fish containing low Hg concentrations at low trophic positions.