Spatial distribution of Euphausia pacifica (Euphausiacea: Crustacea) in the Yellow Sea

Abstract

Euphausia pacifica were collected in the Yellow Sea in summer (August, 1997) and winter (February, 1998), and their distribution was investigated in terms of the developmental stages in relation to environmental factors (temperature, salinity and chlorophyll a). In summer, the water column was highly stratified with a thermocline between 10-30 m depth, whereas the water column was well mixed in winter. Seasonal variation in temperature was large, ranging between 6.3 and 28.8°C in summer and 3.9 and 12.2°C in winter. Chlorophyll a concentration changed little seasonally but a high concentration was observed in coastal areas. Salinity in the two seasons varied little spatio-temporally. In the studied area and sampling period, E.pacifica was the most dominant euphausiid species (99.7 and 99.8% in summer and winter, respectively), and comprised adults and juveniles (74.6% in summer and 41.9% in winter), furcilia (17.5 and 44.9%) and calyptopis (7.9 and 13.1%). The spatial distribution of the E.pacifica population varied seasonally depending on developmental stage and appeared to be related primarily to the seawater temperature and secondly to the chlorophyll a concentration, but not to salinity. In summer, adults were abundant in areas below 10°C, and furcilia and calyptopis above 9°C. In winter, adults were confined to areas between 7 and 10°C, and furcilia and calyptopis, to areas between 6 and 8°C. Furcilia and calyptopis were concentrated in the vicinity, or at the center of chlorophyll a-rich water masses, whereas adults seemed to avoid those water masses.