Vertical distribution and diel migration of euphausiids from Oyashio Current to Kuroshio area off northeastern Japan

Abstract

The vertical distribution of dominant euphausiid species was investigated in the offshore waters of north- eastern Japan in April and September 1998 and November 1999 using MOCNESS-I net samples. In total, 32 eu- phausiid species in eight genera were identified in the study area. Subarctic Euphausia pacifica and Thysanoessa in- spinata occurred over a wide range, although mainly in the Oyashio and cooler transitional areas, and the former made a relative contribution of 78% to the total numerical abundance of euphausiids. At night, these species tended to stay near the surface in the Oyashio area and in deeper layers in warmer areas. During the daytime, they tended to stay in deeper layers in the warmer areas, although E. pacifica stayed in deeper layers even in the Oyashio area where the chlorophyll a concentration was low. T. inspinata did not perform clear diel vertical migration. Except for these subarc- tic species, five warm-water species of Euphausia were abundant among the epipelagic euphausiids. At night, these species tended to stay at a wide range of depths from the surface to subsurface layers in the warmer Kuroshio area, but were concentrated near the surface in the transitional area. During the daytime, they stayed in deeper layers in the Kuroshio area. Diel vertical migration of euphausiids seems to be strongly affected by water temperature, but other fac- tors such as phytoplankton abundance may also be important. The possible effects of diel vertical migration of eu- phausiids on euphausiid fisheries are discussed.