Seasonal size composition and abundance distribution of Euphausia pacifica in relation to environmental factors in the southern Yellow Sea

The seasonal size structure and spatial abundance distributions of Euphausia pacifica populations were investigated in the central part of southern Yellow Sea from August 2009 to May 2010.The abundance and biomass of E.pacifica were higher in spring and summer,and lower in autumn and winter.The mean abundance and biomass（calculated by carbon）were 74.94 ind./m;and 8.23 mg/m;,respectively.Females with total length（TL）ranging between 10 and 19 mm in summer had a substantial contribution to the population biomass,whereas larvae of TL of 3–7 mm in spring were the main contributor to the population abundance.The sex ratio（female:male）showed a female bias in four seasons.Its value peaked in summer,and then decreased in autumn,spring,and winter successively.Cohort analysis revealed that the length-frequency distribution of E.pacifica could be characterized as one group with large animals（mean TL>12 mm）accompanied by one or two subgroups of small individuals（mean TL<7 mm）.Regarding the spatial distribution,juveniles and adults of E.pacifica tend to concentrate in relatively deep water with low temperature（～11℃）and high salinity（>32）,whereas its larvae showed more abundance in inshore water with rich chlorophyll a,low salinity（<32）,and warm temperature（>11℃）,especially in summer and autumn.Associations changed seasonally between stage-specific abundance and environmental factors.