Distribution and population structure of Thysanoessa inspinata and its dominance among euphausiids off northeastern Japan

Abstract

Distribution and population structure of Thysanoessa inspinata were investigated off northeastern Japan and compared with those of Euphausia pacifica and other Thysanoessa species. Thysanoessa inspinata were distributed as widely as E. pacifica, but their abundance in the transitional water mass area (TWA) was conspicuously lower than in the cooler Oyashio water mass area (OWA). Another three Subarctic species of Thysanoessa occurred restrictively in the OWA, but rarely occurred during summer-autumn when sea surface temperature increased up to nearly 20°C. Thysanoessa inspinata was distributed in surface waters at night in the OWA in spring, but in the middle layers during summer-autumn; in contrast, E. pacifica occurred near the surface throughout the year. Euphausia pacifica was >7 times more abundant than T. inspinata in the OWA, whereas T. inspinata was >90 of the total numbers of Thysanoessa spp. Temperature tolerances may strongly affect the degree of dominance of euphausiids and explain why the TWA is the southern limit of T. inspinata distribution, perhaps by restricting its access to the food-rich surface waters. Growth of males of T. inspinata was slower than that of females. Evidence from this study corroborates that E. pacifica is the key euphausiid species for food of surface-migratory Pacific saury during summer-autumn. © 2011 The Author.