Reproductive Strategy Of Calanus Sinicus In The Southern Yellow Sea

In the Southern Yellow Sea, where Calanus sinicus spawn continuously all through the year, seasonal variation of egg production rate (EPR) is bimodal, with maximum attained in spring and autumn, when surface and bottom water temperature is optimal (10-23℃). Egg production is most active from April to July, when prosome length and dry weight of females are also highest of the year. After July, EPR, prosome length and dry weight decreases dramatically, and minimized in October. Due to lack of December and February data, the autumn to winter variation of EPR and body size is not quite clear. The second peak of egg production observed in November is less intense than the main reproductive peak in spring. Dry weight in November is larger than that in January; while the prosome length in January is higher than that in November or March.Geographical variation of EPR tended to be jointly affected by all environmental factors. It is hard to determine the specific effect from one certain factor, say temperature or food availability. In summer, C. sinicus congregates in the Yellow Sea Cold Water Mass (YSCWM); where adults almost stop diel vertical migration and egg production and concentrates in the cold bottom layer. This strategy of avoiding high temperature most possibly benefits the population. In the onboard incubation during cruises, when temperature exceeded 26 ℃, spawning ability and hatching rate were depressed and many abnormal embryos were observed. In the south of the Southern Yellow Sea and north of the East China Sea, where Kuroshio water intruded into the continental shelf area in a northwestward direction, the cold bottom water was substituted by warm pelagic water. The C. sinicus population in this area was found less concentrated, but egg production was more active than that inside the YSCWM.In winter, Huanghai Warm Currents (HWC) intrudes from the southeast into the Southern Yellow Sea carrying with warm and less nutritious pelagic water. The areas affected by HWC were characterized by low primary production and chlorophyll concentration. Correspondingly, EPR of C. sinicus was higher nearshore, but lower in central areas.EPR is not only an index of reproduction, but is also useful in many aspects of marine ecology research. EPR and pigment ingestion rate were investigated in the Bohai Sea in relation to phytoplankton assemblages, and their quotient was considered as an indicator for the feeding habit of C. sinicus. It was suggested that 78.7% and 97.9% of egg production were contributed by phytoplankton feeding in May and June, but during the autumn phytoplankton bloom (October), the proportion was only 46.0%. The phytoplankton assemblage may also affect pigment ingestion rate and contribution rate to egg production. High proportion of dinoflagelates, either Ceratium or Noctiluca, may depress pigment ingestion rate and phytoplankton feeding of C. sinicus. Hatching success was found to be lower in October, when density of diatoms cells was extremely high. For diel spawning rhythm had been established, egg mortality (i.e. prey yield for fish larvae) could be estimated by comparing the potential fecundity and variation of standing stock of eggs in the water column. Our results suggested that 80% of eggs produced by C. sinicus and more than 50% of eggs produced by Paracalanus parvus are lost after being spawned into the water column, equal to 10.7 mgC m^-2 d^-1 prey yield for fish larvae.