Ontogenetic Changes in Mercury and Stable Isotope Ratios in Pacific White-Sided Dolphins (Lagenorhynchus obliquidens) Stranded in Hokkaido, Japan: Comparison with Harbour and Dall’s Porpoises

The Pacific white-sided dolphin (PWD) is a small cetacean that is hunted for human consumption in central Japan, and is frequently stranded on the coast of Hokkaido, northern Japan. However, little information is available on the Hg contamination and ecology of PWDs. We investigated the ontogenetic changes in Hg distribution, δ13C, and δ15N, and δ18O values in PWDs stranded in Hokkaido, and compared with those in harbour and Dall’s porpoises stranded in the same region. Hg concentration in the liver of mature PWDs was markedly higher than that in the muscle and other tissues of mature PWDs. Hg concentrations in the liver and muscle increased sharply when body lengths (BLs) exceeded 2.0 m, at which PWDs attain maturity. Hg concentrations in the muscle of all mature PWDs exceeded 0.4 μg/wet g, which is the Hg guideline for fish and shellfish set by the Japanese government, and those in most livers of mature PWDs exceeded 60 μg/wet g, which is the threshold of moderate Hg toxicity for marine mammals. The δ15N values in muscles were higher in the calves than in the immature PWDs, possibly due to extensive nursing of 15N-enriched milk. Significant positive correlations were found between δ13C and δ15N values and between log THg concentrations and δ15N values in the muscle of weaned males, but not of weaned females. Weak positive correlations between δ18O values and BLs and between δ13C and δ18O values were found in weaned males, whereas weaned females showed weak negative correlations between the two. These differences between weaned males and females may be due to the differences in habitats and prey species. The 40% confidence ellipse using δ13C and δ15N values segregated PWDs from Dall’s and harbour porpoises, suggesting a difference in their spatial niches.