Distribution and life history of Euphausia pacifica off northeastern Japan
The abundance, spawning, growth pattern and life span of Euphausia pacifica were examined using widely collected samples off the Japanese coast of the North Pacific. Adults were widely distributed in the Oyashio area [OW; temperature at 100-m depth (T100) ≤ 5°C] and the transitional area (5°C < T100 ≤ 15°C) throughout the year, but the occurrence of large adults (>15 mm) was limited to OW and the cold waters of the transitional area (CW; 5°C < T100 ≤ 10°C). Spawning occurred in OW and the transitional area throughout the year, especially in CW in spring but rarely in winter. Two modal cohorts of adults were consistently recognized in OW and CW. Cohorts of small males and females (10–11 mm length), which newly appeared in spring, grew remarkably to about 17 mm from the following spring to early summer, followed by a long period of little growth during spring–winter. Then, both male and female cohorts grew slowly and steadily after summer to a maximum of 18–19 mm by the following spring. The cohort of males disappeared after spring, but the females (about 20 mm length) were still being observed up until late summer. If the cohort of small adults is assumed to develop mainly from the spring hatching, the life span of male and female E. pacifica can be estimated to be 24 and 28 months, respectively, off northeastern Japan.