Search

Gathering information on the diet composition of baleen whales at different temporal scales is useful for investigating their feeding habits. Bryde's whales (Balaenoptera edeni) and sei whales (B. borealis) are known to feed on crustaceans and pelagic fish in their western North Pacific feeding grounds; however, their feeding habits before arriving in the feeding area are not known. This study clarified the feeding habits of these two whale species by simultaneously analyzing nitrogen and carbon stable isotope ratios and stomach contents of individuals captured in the western North Pacific in 2015 and 2016. A stable isotope Bayesian mixed-model was used to estimate prey composition. The results showed clear temporal differences in the feeding habits of both whales. Specifically, Bryde's whales fed primarily on krill and pelagic and mesopelagic fishes as they migrated northwards, and then fed exclusively on krill during the summer. Sei whales fed on copepods when they arrived in the subarctic region before shifting to pelagic fish species in the summer. The differences in the feeding habits of these two whale species are attributed to their latitudinal segregation and the availability of prey species, especially in sei whales, which migrate to the subarctic region where copepods are abundant. • Feedings of North Pacific Bryde's and sei whales. • Different time scales of feeding habits from stomach contents and stable isotope. • δ13C and δ15N were used in the stable isotope analysis. • Feeding habits differed depending on the analytical methods. • Whales change their feeding habits within a season. [ABSTRACT FROM AUTHOR]

A study of common minke and Bryde's whales was conducted in the western North Pacific in the 2000 and 2001 summer seasons to estimate prey selection of cetaceans as this is an important parameter in ecosystem models. Whale sighting and sampling surveys and prey surveys using quantitative echosounder and mid-water trawl were carried out concurrently in the study. Biomasses of Japanese anchovy, walleye pollock and krill, which were major prey species of common minke and Bryde's whales, were estimated using an echosounder. The results suggested that common minke whale showed prey selection for Japanese anchovy while they seemed to avoid krill in both the offshore and coastal regions and walleye pollock in the continental shelf region. Selection for shoaling pelagic fish was similar to that in the eastern North Atlantic. Bryde's whale showed selection for Japanese anchovy in August 2000 and July 2001, while it showed prey selection for krill in May and June in 2001. [ABSTRACT FROM AUTHOR]