Seasonal changes in spawning and distribution of Euphausia pacifica Hansen along the coastal areas off northeastern Japan

Abstract

Spawning and distribution of Euphausia pacifica were investigated bimonthly using Norpac and large cylindrical-conical nets along the coastal areas off northeastern Japan [Southeastern Hokkaido (44-43°N), Sanriku (38-41°N) and Joban (36-38°N) from March 1997 to February 1998. Copulated females, egg and calyptopis stage were abundant off Sanriku in spring and off southeastern Hokkaido in mid fall, but rare throughout the survey areas in late fall. They mainly occurred near the marginal area of Oyashio Current (T₁₀₀=5°C) throughout the year, Furcilia and immature stages tended to expand their distribution into warmer southern areas compared to younger stages (eggs and calyptopis stage) in each season. Copulated females, eggs, calyptopis, furcilia and immature stages were few or absent throughout the year except in mid fall off southeastern Hokkaido, where cold waters of the Oyashio (T₁₀₀<2°C) dominated. Small adults (≤15mm) occured in all survey areas including the areas off southeastern Hokkaido throughout the year, Large adults (>15mm), which are considered to be individuals that have overwintered, however, were few from August to December off Sanriku and Joban where the warmer waters of the transitional area (10°C≤T₁₀₀≤15°C) dominated. It is suggested that the geographical change in the distribution and spawning activity depend mainly on the seasonal changes of the water masses associated with the Oyashio Current.