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The distribution of Pacific euphausiids


A study has been made of the distribution of 59 euphausiid species in the Pacific Ocean, based upon oceanic surveys carried out from 1949 to 1961 by the Scripps Institution of Oceanography, California Cooperative Oceanic Fisheries Investigations (CalCOFI), and Pacific Oceanic Fisheries Investigations (POFI). Quantitative aspects of the vertical and horizontal distributions are presented. The species belonging to the genera Bentheuphausia and Thysanopoda are typically deep-living. Bentheuphausia ambloyops, Thysanopoda cornuta, and T. egregia are widely ranging bathypelagic species, the adults of which inhabit depths greater than 1,500 meters. Of the ten genera, six (Thysanopoda, Euphausia, Thysanoessa, Nematoscelis, Nematobrachion, and Stylocheiron)contain some species that inhabit a mesopelagic zone between depths of about 500 and 1,500 meters and other species that are characteristically epipelagic, living mainly above 500–700 meters. The epipelagic species are designated in accord with the analogous water masses inhabited. The Pacific water masses are (1) subarctic, situated to the north of the North Pacific Drift, including only the northern part of the California Current (typical species: Thysanoessa longipes, Tessarabrachion oculatus); (2) transitionzone, including the cooler part of the California Current south of 40°–45° N. and a belt extending westward to Japan in the region of the North Pacific Drift (typical species: Thysanoessa gregaria, Nematoscelis difficilis); (3) central, occupying the oceanic gyrals of mid-latitudes 15°–40° in both hemispheres (typical species: Nematoscelis atlantica, Euphausia brevis); and (4) equatorial, occupying a broad tropical belt in the eastern Pacific and a narrower belt to the west (typical species: Euphausia diomediae, E. distinguenda). Equatorial species occupy equatorial water masses in the Pacific and Indian oceans. A zone occupied by the transition-zone species Thysanoessa gregaria and Nematoscelis megalops (corresponding to N. difficilis of the Northern Hemisphere) occurs near 35°–45° S. Subantarctic and antarctic species are found south of the Subantarctic Convergence. The composite range of a tropical western Pacific group of species (e.g., Euphausia pseudogibba, E. fallax, E. sibogae) extends eastward across the Pacific in a zone 10°–20° S., while an eastern tropical group (e.g., Euphausia distinguenda, E. eximia) extends westward in a tongue having an axis near 10° N. Ekman’s zones of the Pacific littoral fauna each have a counterpart species in the coastal or boundary euphausiid fauna. Seasonal changes in euphausiid distribution are discussed with respect to five regions: (1) the Kuroshio and East China Sea, (2) the North Pacific Drift Current, (3) the California Current, (4) the Peru Current, and (5) the equatorial mid-Pacific.

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