Deep-Sea Research I 50 (2003) 593–610 Feeding habits of Dall’s porpoises (Phocoenoides dalli) in the subarctic North Pacific and the Bering Sea basin and the impact of predation on mesopelagic micronekton Hiroshi Ohizumi a, *, Toshiaki Kuramochi b , Tsunemi Kubodera b , Motoi Yoshioka c , Nobuyuki Miyazaki d a Ocean Research Institute, The University of Tokyo, 1-15-1 Minamidai, Nakano, Tokyo 164-8639, Japan b Department of Zoology, The National Science Museum, 3-23-1 Hyakunincho, Shinjuku, Tokyo 169-0073, Japan c Faculty of Bioresources, Mie University, 1515 Kamihama, Tsu, Mie 514-8507, Japan d Otsuchi Marine Research Center, Ocean Research Institute, The University of Tokyo, 2-106-1 Akahama, Otsuchi, Iwate 028-1102, Japan Received 8 April 2002; received in revised form 26 November 2002; accepted 28 January 2003 Abstract We investigated the stomach contents of Dall’s porpoises collected in pelagic waters spanning most of their range in the North Pacific and the Bering Sea. Analysis revealed the porpoises fed mainly on myctophid fishes in the subarctic North Pacific and on gonatid squids as well as myctophid fishes in the Bering Sea. Most of the prey items were mesopelagic micronekton, primarily fishes and squids that migrate vertically to shallower waters at night. Stomach content was greater during twilight hours, suggesting the porpoises foraged actively on myctophids at night in shallower waters. Stomach contents were strongly characterized by local mesopelagic prey fauna, and prey species selectivity was not apparent. The annual consumption by Dall’s porpoises was estimated to be 2.0–2.8 million tons, or 4.7–6.5% of the biomass of mesopelagic fishes in the subarctic North Pacific, and may account for approximately 24–33% of the overall mortality of mesopelagic micronekton, especially myctophids. Myctophids are also common, but less important, prey of other subarctic predators. Dall’s porpoises are likely the primary consumers of myctophids in the subarctic North Pacific. Since myctophids are the major component of the mesotrophic level, the trophic relationship between myctophids and Dall’s porpoises is thought to be an important pathway of mass and energy in the pelagic food web in the subarctic North Pacific. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: Phocoenoides dalli; Feeding habits; Myctophids; Gonatids; Prey consumption; Subarctic North Pacific; Bering Sea basin 1. Introduction Dolphins and porpoises are top trophic-level predators in marine food webs. This is a well- known concept, but details of prey items and food consumption have not been evaluated for most *Corresponding author. Cetacean Population Biology Sec- tion, National Research Institute of Far Seas Fisheries, 5-7-1 Orido, Shimizu, Shizuoka 424-8633, Japan. E-mail address: ohizumi@affrc.go.jp (H. Ohizumi). 0967-0637/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0967-0637(03)00033-5