Stable Isotope and Trace Element Status of Subsistence Hunted Bowhead and Beluga Whales in Alaska and Gray Whales in Chukotka
Bowhead whales (Balaena mysticetus), beluga whales (Delphinapterus leucas), and gray whales (Eschrichtius robustus) were sampled during Native subsistence hunts in northern Alaska and Chukotka, Russia. Muscle, epidermis, kidney and liver were analyzed for Ag, Cd, Cu, Se, Zn and total Hg (THg). Lumbar muscle was analyzed for stable carbon and nitrogen isotopes and body length was used as a proxy for age. Carbon-13 is significantly enriched in gray whales as compared to bowhead and beluga whales (p = <0.0001). The more enriched carbon-13 in gray whales is likely due to benthic feeding habits. Bowheads are mostly feeding in the water column on krill, displaying the more depleted carbon isotope signatures of the pelagic foodweb. Belugas take an intermediate position between bowhead and gray whale, suggesting that both pelagic and benthic foodwebs are important components in beluga prey selection. δ15N shows significant differences between the three cetacean species (p = <0.0001). The beluga, an odontocete (toothed whale), occupies the highest trophic level (δ15N = 16.8 ± 0.6‰). However, stable nitrogen isotope ratios indicate that bowheads are foraging on a higher trophic level than gray whales, thus pointing to differences in prey consumed (13.3 ± 0.6‰ and 12.0 ± 0.9 ‰ for bowheads and gray whales, respectively). Typical bowhead prey (e.g., euphausids and copepods) have higher nitrogen isotope ratios than benthic gammaridean amphipods that make up the majority of the gray whale diet (10.4 ± 1.2‰ and 7.9 ± 0.8‰ for euphausids and amphipods, respectively). Concentrations of trace elements in tissues of bowhead, beluga and gray whale are markedly different. Zn and Ag in liver and kidney are significantly higher in beluga than in both bowheads and gray whales (p = <0.0001). Hepatic and renal Cd is not statistically different between bowheads and belugas (15.1 ± 14.9 ppm and 10.2 ± 4.3 ppm for bowhead and beluga kidney, respectively), while gray whales have lower concentrations (1.2 ± 1.5 ppm for kidney). Cu is highest in belugas (25.0 ± 27.2 ppm in liver), followed by gray whales (18.9 ± 34.7 ppm in liver) and lowest concentrations are found in bowheads (9.1 ± 21.7 ppm in liver). Renal and hepatic Se and THg are higher in belugas than in both baleen whales (p = <0.0001). However, the molar ratio of Se to THg is higher in gray whales and bowheads than beluga and exceeds the ratio of 1:1 in all three species (127.8:1 ± 102.0, 121.6:1 ± 136.2 and 6.9:1 ± 6.4 for gray whale, bowhead and beluga liver, respectively). The Se to THg molar ratio in liver is negatively correlated to length in these whales. Hepatic Se has a strong positive correlation to hepatic THg in belugas and bowheads, but not in gray whales. Cd in kidney and liver and THg in liver were positively correlated to length in all three species.