Within organisms, the complex pathways of biosynthesis 

 can alter the isotope ratios in the end products relative to 

 starting materials. The distribution of carbon isotopes has been 

 studied by several authors (DeNiro & Epstein, 1978; Jones 

 etai, 1981; Tieszen et al., 1983; Mizutani & Wada. 1988). 

 Muscle tissue tends to closely approximate diet whereas 

 keratinous proteins (hair, feathers, and hooves) are typically 

 enriched by 2-3 "/,„ relative to diet. Schelle/ a/. (1989b) found 

 that keratin in baleen averaged about one part per thousand 

 heavier than muscle, which in turn was about 6 7,),) heavier than 

 lipids. Polarbears, which are l-2trophicIevelsabovebowhead 

 whales, also show an enrichment in keratin 5"C of 1-2 "/„, 

 relative to the whales. As more and more studies are performed 

 on ecosystem processes, the usefulness of stable isotope ratios 

 as tracers has become increasingly evident. 



Background 



The initial work on this project commenced in 1985 and 

 sought to establish the significance of the eastern Alaskan 

 Beaufort Sea in the annual energy budget of bowhead whales. 

 One approach to answering this question was to use the 

 geographical differences in the stable isotope ratios (carbon 

 and nitrogen) in whales and their prey organisms as natural 

 tracers of food sources. 



Natural history investigations of the large baleen whales 

 present formidable problems due to the difficulties in observing 

 the animals in their natural environments. Schellt'?fl/. ( 1989a) 

 demonstrated, however, that bowhead whales have marked 

 annual oscillations in stable carbon and nitrogen isotope ratios 

 along the length of the baleen plates in the mouth. These 

 oscillations result from the annual migration of the animals 

 from wintering grounds in the Bering Sea to the summering 

 areas of the Canadian Beaufort Sea. Zooplankton along the 

 migrational path have differing isotopic ratios of carbon and 

 nitrogen, which are reflected in the composition of the keratin 

 in the continuously growing baleen plates. Since up to 20 years 

 feeding record may be present in the plate of a large bowhead 

 whale, considerable insight may be gained on the natural 

 history of the whales and their habitat usage. We have reported 

 (ScheW etal., 1989a.b;Saupec/«/.. 1989) on the isotopic ratios 

 in zooplankton prey that produce the large variations in 

 B. mysticetiis and a revised growth rate for B. inystlcetus, 

 detemiined through isotopic aging techniques. 



The stable isotope abundances in baleen oscillate in a 

 regular pattern along the length of the plate in response to the 

 compositional changes in the whale food (zooplankton) as the 

 animal migrates. The isotope ratios in the baleen — and 

 especially in the muscle and visceral fat of animals killedin the 

 spring compared to those killed in fall — show that the greatest 

 amount of food consumed by B. mysticetiis matches the isotopic 

 abundances typical of prey species in the western and southern 

 areas of the migratory range. The average "C isotope value in 

 visceral fat and mu.scle tissue from spring-killed B. mysticetiis 

 was enriched by 2.1 "/(,,, relative to two fall-killed animals, 

 implying that a major fraction of the total carbon of the animal 

 was derived from the western and southern parts of their annual 

 range. Although it is impossible to accurately estimate the 



relative amounts of food that the whales obtain from the 

 Beaufort versus Chukchi versus Bering Seas (because of the 

 close similarity of zooplankton isotope ratios in the Bering and 

 Chukchi), these data contrast with previous feeding scenarios 

 that suggested that bowheads fed in the summer in the eastern 

 Beaufort Sea and relied almost entirely on stored reserves for 

 the winter (Lowry & Frost, 1984). 



The isotopic data from three adult whales analyzed indicate 

 that these large whales have an average isotopic composition 

 derived from prey obtained almost entirely in the western and 

 southern parts of their range (Schell ??«/., 1989b). This might 

 mean that the eastern Beaufort Sea is not nearly as important a 

 feeding area for this segment of the population as the western 

 Chukchi and the Bering Seas. 



The findings listed above are based on a limited number of 

 whale samples. Nevertheless, the resuhs are sufficiently contrary 

 to previously accepted growth rates and feeding scenarios that 

 it is important that the data base be expanded to substantiate or 

 disprove the indicated findings. The work performed on the 

 cruise in 1988 sought to expand the zooplankton data from 

 around the range of the bowhead, especially from the missing 

 areas in the western Chukchi and the northwest Bering Seas, 

 and to provide further insight regarding the cause of the 

 isotopic shift between the southwestern and northeastern 

 segments of the migratory range. The data collected on this 

 cruise are part of the necessary samples required to fill the data 

 gaps in the natural history of important marine mammals living 

 in waters shared by the United States and the Soviet Union. 



Objectives of the 1988 Akademik Korolev Cruise 



Our overall goal was to use the isotopic gradients in the 

 Bering-Chukchi-Beaufort Seas to determine the habitat 

 dependencies and feeding strategies of the bowhead whale. By 

 comparing the carbon isotope ratios in bowhead tissues with 

 that in their prey organisms along the migratory route, we can 

 establish, at least qualitatively, the importance of the various 

 habitats to the animals. The objectives of this expedition were 

 to; 



/. Obtain zooplankton for isotope analysis from the 

 western (USSR) sector of the Bering and Chukchi Seas for 

 comparison with zooplankton from eastern waters. 



2. Interpret and synthesize new data in context with past 

 findings to confirm or deny current interpretations of bowhead 

 whale natural history with special reference to the role of the 

 Bering-Chukchi Seas as feeding habitat. 



Methods 



Zooplankton were collected using ring nets with 505 mesh 

 at the stations shown on Fig. 1 and listed in Table 1. Upon 

 collection, samples were sorted to major taxonomic groups and 

 to species where identification was feasible in the field. Samples 

 were then frozen for later processing in the laboratory at 

 Fairbanks. Procedures for sample handling and mass 

 spectrometry are described in Schell et at. ( 1987). Milligram 

 amounts of dried zooplankton were ground with CuO ( 750 mg) 

 and sealed into evacuated quartz tubes. Following combustion 



178 



