Ontogenetic feeding migration of the euphausiid Euphausia pacifica in the East Sea (Japan Sea) in autumn: a stable isotope approach

We investigated the ontogenetic feeding migration of Euphausia pacifica within the epipelagic zone (0-200 m depth) in the East Sea (Japan Sea) during the autumn using stable isotope analysis (SIA). SIA indicated that the ontogenetic stages of E. pacifica from egg to adulthood were closely related to the vertical profile of three cell-sized fractions of particulate organic matter (POM): pico-(<2 mu m), nano-(2-20 mu m) and microPOM (20-200 mu m). From nauplius to metanauplius, the delta C-13 and delta N-15 values of larvae were slightly lower than those of eggs, indicating the consumption of yolk reserves in the eggs. From calyptopis I, the first feeding stage to furcilia I, larvae mainly distributed in the surface layer and fed mainly on surface layer picoPOM. During developmental progress from CI to adulthood, the residence depth range deepened from the surface to deeper layers with the exception of juveniles, which stayed in the surface layer. The ontogenetic feeding migration pattern of E. pacifica may reduce intraspecific competition for food items, resulting in coexistence of the ontogenetic stages in the East Sea during the autumn.