Journal of Oceanography, Vol. 57 (No. 3), pp. 361-375, 2001
Jun Nishikawa*, Shuhei Nishida, Masatoshi Moku, Kiyotaka Hidaka and Kouichi Kawaguchi
Ocean Research Institute, The University of Tokyo, Minamidai, Nakano-ku, Tokyo 164-8639, Japan
(Received 9 August 2000; in revised form 7 December 2000; accepted 7 December 2000)
Abstract: The biomass, abundance, and vertical distribution of micronekton, including cnidarians, mysids, euphausiids, decapods, thaliaceans, and fishes, were studied on the basis of samples collected with an 8-m2 opening-closing rectangular midwater trawl (RMT-8, mesh size: 4.5 mm) at three stations in the subarctic Pacific (the western subarctic gyre, the central Subarctic, and the Gulf of Alaska) and one station in the oceanic Bering Sea. The total biomass in the 0-1000 m water column ranged from 2.9 to 5.1 gDW m-2. Except for primary consumers that showed highly variable biomass (thaliaceans and euphausiids), biomass was highest in the oceanic Bering Sea followed by the central (boundary between eastern and western gyres), western gyre, and eastern Gulf of Alaska. The biomass compositions by higher taxa were basically similar between regions: fishes were most dominant, followed by cnidarians at all stations, except for the marked predominance of thaliaceans in the Gulf of Alaska. High biomasses of gelatinous animals (31% of overall dry weight), occasionally comparable to those of fishes and crustaceans, suggest their potential importance in the subarctic Pacific. Characteristics in vertical patterns of micronekton biomass common in all stations were: (1) a mesopelagic peak around 500-600 m both day and night, (2) a layer of low biomass in the cold intermediate water and/or in the upper mesopelagic zone, (3) a nighttime shift of biomass to upper layers, and (4) an highly variable biomass of epipelagic/interzonal migrants (euphausiids and thaliaceans).