| As one of six important Euphausiids species being commercially harvested, Euphausia pacifica Hansen is widely distributed in the North Pacific Ocean and its adjacent coastal waters. E. pacifica is the dominant zooplankton species and functional group in the Yellow sea and is regarded as one of the most important zooplankton species and functional group of China-GLOBEC (China-Global Ocean Ecosystem Dynamics) in the Yellow Sea ecosystem by virtue of its enormous abundance, large body size and the significant role it plays in the marine food chains. Its swarming habit has made it attractive to commercial fisheries. In the Yellow Sea ecosystem, E. pacifica is the most important food of adult anchovy, which is the main prey of most of the economic fishes. We suggest that the changes of distribution and abundance of E. pacifica could directly influence the whole Yellow Sea ecosystem.Patterns of spatial distribution of Euphausia pacifica in the southern Yellow Sea in spring (April) and autumn (October), 2006, were studied in this paper. In spring, E. pacifica population was mainly distributed in the sea area of 33°N-36°N and 50 m-75 m depth. Population (except for eggs) abundance was 152.90 ind. m-3. Egg abundance was very high (average abundance of the whole research area was 57.61 ind. m-3). Adult abundance was relatively low (average abundance was 0.35 ind. m-3). Larvae were the main proportion of population, which took up 90.85% of the whole population abundance. In autumn, population (except for eggs) abundance was 335.38 ind. m-3, which was twice that of spring. The population mainly distributed in the YSBCW area. Adult abundance was much higher than spring and its average abundance was 7.73 ind. m-3. Larvae and eggs were nearly absent from the studied area in October. Therefore, spring was the recruitment period of E. pacifica, and E. pacifica population was in a steady condition in autumn. Body length of E. pacifica adult in spring was higher than in autumn. In spring, adult total length was mostly within 13-18 mm. Adult total length was mainly from 9 mm to 13 mm in autumn.In April 2006, eggs could be collected throughout the whole water column. The different developmental stages of E. pacifica may show different diel vertical migration (DVM) behaviour. Adults showed DVM behaviour, which aggregated below the 50 m layer in daytime, and only a few adults went up during nighttime. Stages N2 and MN larvae, somehow, showed a reversed trend of DVM. Stages C3 began to show DVM behaviour and stages later than F2 took on obviously DVM behaviour. The chlorophyll a concentration gradually increased from bottom to surface. Therefore, the DVM of E. pacifica was probably related to its grazing activity.The variations of E. pacifica adult distribution was studied by six cruise surveys from September 2006 to August 2007. The distribution of E. pacifica adult was closely interrelated with seawater temperature. The fitting temperature of E. pacifica adult in the southern Yellow Sea was 8-16°C. The distribution of adult was wide in spring and winter and adult mostly stayed in the YSBCW area in summer and autumn. Additional, there was a higher adult abundance region near north of the Yangtze River estuary.We carried out seven in situ experiments on egg production from September 2006 to August 2007. The results showed that E. pacifica can reproduce from March to June in the southern Yellow Sea and April was the recruitment fastigium. The maximum brood size of individual female was 617 egg female-1 which occurred in April. Spawning behaviour was not found in August, September and December. E. pacifica may produce twice broods of eggs in our experiment in reproduction period. The brood size of E. pacifica was nearly related to its dry and wet weight. Adult could not spawn when its dry weight was lower than 5.0 mg and wet weight lower than 26 mg. E. pacifica adult got the maximum dry and wet weight in the recruitment fastigium. E. pacifica larvae developmental pathway was ascertained with net samples collected in April 2006. In the southern Yellow Sea, the development of E. pacifica larvae nearly followed the below pathway: Egg→Nauplius→Metanauplius→Calyptopis→Furcilia, F1 (0' 7, 1' 7)→Furcilia, F2 (1' 4'' 7, 3' 1'' 7)→Furcilia, F3 (5'' 7)→Furcilia, F4 (5'' 5)→Furcilia, F5 (5'' 3)→Furcilia, F6 (5'' 1). The developmental time of E. pacifica larvae under 4°C was obviously longer than in 15°C. Eggs can be successfully developed into stage C1 within 5.6 days under 15°C, however, it would take 16.1 days under 4°C.Results of five grazing experiments of E. pacifica adult at station S1-4 showed that in August and September, the ingestion rate on phytoplankton was very low. Adult mainly ingested the microzooplankton, and then the chlorophyll a concentration instead increased for the cascading trophic interactions. In December and March, the grazing activity on phytoplankton was so strong that the chlorophyll a concentration greatly reduced. Meanwhile E. pacifica may also probably ingested the microzooplankton in seawater.Three in situ metabolism experiments on adult E. pacifica were carried out in September, December 2006 and March 2007 at station S1-4. Results showed that oxygen consumption rate was 172.92μg ind.-1 d-1 in March, which was six times more than in September and December. The carbon consumption rate (CCR) and daily body carbon consumption capacity (DBCCC) in September and December were low. The metabolism of E. pacifica in March was very vigorous, with CCR and DBCCC value of 62.9μg C ind-1 d-1 and 2.70 % d-1, respectively. O:Ns of E. pacifica metabolism were 11.3 and 7.0 in September and December. Their metabolic substance was mostly protein. In March, the O:N of metabolism was 35.1. E. pacifica adult mainly metabolized lipid and carbohydrate.
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