Patterns of stable carbon and nitrogen isotopes in the baleen of common minke whale Balaenoptera acutorostrata from the western North Pacific

Stable carbon and nitrogen isotope ratios were determined in the baleen plates of 17 common minke whales Balaenoptera acutorostrata from the north-western Pacific Ocean off Japan, as well as prey species (krill Euphausia pacifica, Japanese anchovy Engraulis japonicus and Pacific saury Cololabis saira) collected in the stomach contents, to investigate the trophic relationship between the minke whales and their prey. A few delta N-15-depleted peaks occurred along the length of baleen plates for 10 males irrespective of stomach content (anchovies and sauries). Similar delta N-15-depleted peaks were also found for one female and two immature individuals. It was likely that these delta N-15-depleted peaks formed in early summer. The stable nitrogen isotope ratio (delta N-15) values in Pacific saury (9.3 +/- 1.4 parts per thousand) did not differ significantly from that in Japanese anchovy (8.8 +/- 0.9 parts per thousand). In contrast, delta N-15 in krill (7.2 +/- 0.5 parts per thousand in July and 8.0 +/- 0.2 parts per thousand. in September) were significantly lower than in the Pacific saury. Thus, these peaks may reflect the dietary change from krill to fishes in the feeding migration of the whales. Growth rate of the baleen plate was estimated to be 129 mm/y, and it appeared that a dietary record of about 1.4 years remained in the baleen plate. For two immature whales, the maximum value of delta N-15 occurred at the tip of baleen. This delta N-15 enrichment may possibly be useful for discriminating weanlings and older whales.