Comparison of the seasonal variability in abundance of the copepod Pseudocalanus newmani in Lagoon Notoro-ko and a coastal area of the southwestern Okhotsk Sea

Replacement of the warm water of the Soya Warm Current (SWC) and the cold water of the East Sakhalin Current (ESC) occurs seasonally along the coast of the southwestern Okhotsk Sea, and sea ice covers the surface during winter. Pseudocalanus newmani is one of the dominant copepods in coastal waters of the northern hemisphere. To better understand the population dynamics of the copepod P. newmani in coastal areas of the southwestern Okhotsk Sea, this study compared the seasonal variation in P. newmani abundance in Lagoon Notoro-ko and a coastal area of the Okhotsk Sea with regard to developmental stage. We sampled P. newmani in the lagoon, including during the ice cover season, and the coastal waters. Pseudocalanus newmani was abundant at both sites in spring. During summer–fall, adults disappeared from the populations at both sites, whereas the early developmental stages were abundant and dominated the population. Total length of adult females decreased toward summer at both sites. Pseudocalanus newmani abundance in the lagoon increased in early winter, and larger females were found in the populations at both sites. These phenomena at both sites corresponded with seasonal variation in water temperature caused by seasonal water-mass replacement and sea ice.本研究では、南西部オホーツク海沿岸域におけるPseudocalanus newmaniの個体群動態を理解するために、オホーツク海に隣接する能取湖および湖外の沿岸域における本種現存量の発育段階毎の季節変動を比較した。本種は春季には能取湖および沿岸域で現存量が高かった。夏季から秋季にかけて、能取湖および沿岸域における成体は検出限界以下となったが、発育段階の若い個体は現存量が高く、個体群の中で優占した。この期間、成体雌の体長は減少した。初冬季には本種の現存量は再び増加し、大型雌の出現が認められた。これらの現象は、能取湖および沿岸域における季節的な水塊交替および海氷生成によって引き起こされる水温の季節変動と一致した