Collection of minimally invasive biopsy samples has become an important method to establish normal stable isotopes reference ranges in various wildlife species. Baseline data enhance the understanding of feeding ecology, habitat use, and potential food limitation in apparently healthy, free-ranging cetaceans. Epidermis and muscle were collected from subsistence-hunted northern Alaskan bowhead (n = 133 epidermis/134 muscle) and beluga whales (n = 42/49) and subsistence-hunted Russian gray whales (n = 25/17). Additional samples were obtained from gray whales stranded in California (n = 18/11) during mortality events (1999, 2000). Both ␦ 15 N and ␦ 13 C are trophic position and benthic/pelagic feeding indicators, respectively, in muscle and epidermis. Epidermis is generally enriched in 15 N over muscle, while epidermal 13 C is more depleted. Lipid extraction does not alter ␦ 15 N in either tissue, but affects epidermal ␦ 13 C. Nitrogen-15 is enriched in muscle, but not epidermis of stranded compared to subsistence-hunted gray whales, E173 E174 MARINE MAMMAL SCIENCE, VOL. 28, NO. 2, 2012 indicating probable protein catabolism and nutritional stress in stranded whales. Similarly, epidermal ␦ 13 C of harvested whales is lower than in stranded whales, suggesting depleted lipid stores and/or food limitation in stranded animals. Epidermal isotope signatures are similar in both present-day bowheads and in an ancient sample from the Northern Bering Sea region. Although only one specimen, this suggests trophic level of the ancient whale compares to modern bowheads after a millennium.

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Dramatic multiyear fluctuations in water temperature and seasonal sea ice extent and duration across the Bering-Chukchi continental shelf have occurred in this century, raising a pressing ecological question: Do such environmental changes alter marine production processes linking primary producers to upper trophic-level predators? We examined this question by comparing the blubber fatty acid (FA) composition and stable carbon isotope ratios of individual FA (δ 13 C FA ) of adult ringed seals (Pusa hispida), bearded seals (Erignathus barbatus), spotted seals (Phoca largha), and ribbon seals (Histriophoca fasciata), collectively known as "ice seals," sampled during an anomalously warm, low sea ice period in 2002-2005 in the Bering Sea and a subsequent cold, high sea ice period in 2007-2010. δ 13 C FA values, used to estimate the contribution to seals of carbon derived from sea ice algae (sympagic production) relative to that derived from water column phytoplankton (pelagic production), indicated that during the cold period, sympagic production accounted for 62-80% of the FA in the blubber of bearded seals, 51-62% in spotted seals, and 21-60% in ringed seals. Moreover, the δ 13 C FA values of bearded seals indicated a greater incorporation of sympagic FAs during the cold period than the warm period. This result provides the first empirical evidence of an ecosystem-scale effect of a putative change in sympagic production in the Western Arctic. The FA composition of ice seals showed clear evidence of resource partitioning among ringed, bearded, and spotted seals, and little niche separation between spotted and ribbon seals, which is consistent with previous studies. Despite interannual variability, the FA composition of ringed and bearded seals showed little evidence of differences in diet between the warm and cold periods. The findings that sympagic production contributes significantly to food webs supporting ice seals, and that the contribution apparently is less in warm years with low sea ice, raise an important concern: Will the projected warming and continuing loss of seasonal sea ice in the Arctic, and the associated decline of organic matter input from sympagic production, be compensated for by pelagic production to satisfy both pelagic and benthic carbon and energy needs?

Knowledge of humpback whale (Megaptera novaeangliae) foraging on feeding grounds is becoming increasingly important as the growing North Pacific population recovers from commercial whaling and consumes more prey, including economically important fishes. We explored spatial and temporal (interannual, within-season) variability in summer foraging by humpback whales along the eastern side of the Kodiak Archipelago as described by stable carbon (d 13 C) and nitrogen (d 15 N) isotope ratios of humpback whale skin (n = 118; 2004-2013). The trophic level (TL) of individual whales was calculated using basal food web d 15 N values collected within the study area. We found evidence for the existence of two subaggregations of humpback whales ("North," "South") on the feeding ground that fed at different TLs throughout the study period. Linear mixed models suggest that within an average year, Kodiak humpback whales forage at a consistent TL during the feeding season. TL estimates support mixed consumption of fish and zooplankton species in the "North" (mean AE SE; 3.3 AE 0.1) and predominant foraging on zooplankton in the "South" (3.0 AE 0.1). This trend appears to reflect spatial differences in prey availability, and thus, our results suggest North Pacific humpback whales may segregate on feeding aggregations and target discrete prey species.

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