Decadal scale change in the feeding habits of sei whales in the western North Pacific
off Japan
Kenji Konishi, Tatsuya Isoda and Tsutomu Tamura
Institute of Cetacean Research, Tokyo, Japan. Email: konishi@cetacean.jp
Sei whale Balaenoptera borealis is the dominant baleen whale species in the western
North Pacific and preys upon a variety of species from copepods to pelagic fishes in the
subarctic zone in summer. The authors have examined the stomach contents of sei whales
(n=1174) from 2002 to 2014 obtained in the Japanese Whale Research Program between
Japan’s coast to 170°E (JARPNII) to investigate changes in prey composition. The
stomach contents were removed, weighed and the prey species were identified to the
lowest taxa possible. The main prey species were Neocalanus copepods, Euphausiids,
and pelagic fishes (Engraulis japonicus, Scomber japonicas and S. australasicus). In the
early part of the research period before 2010, E. Japonicus was the dominant prey species
in addition to the copepods and Euphausiids, then the occurrence of anchovy decreased
with later years and was finally replaced by Scomber spp. in the early 2010’s. In 2014
Japanese sardine Sardinops melanostictus was, for the first time, found as one of the main
prey species during the survey years. Body length distribution of E. Japonicus showed
modes of over 100mm in the early years of survey period, with dispersed peaks in later
years, suggesting the change of merged aggregation pattern in anchovy. The body length
distribution in mackerels shows the opposite pattern to anchovy with a stable mode
around 150-200mm after 2011. This drastic shift of fish prey species in sei whale seems
highly dependent upon the change of food availability, which is the most important factor
related to prey composition.