Seasonal changes in the vertical distribution and population structure of Calanus sinicus and Calanus jashnovi (Copepoda: Calanoida) in Sagami Bay, Japan

Seasonal occurrences of Calanus sinicus and Calanus jashnovi were investigated using monthly samples covering 0–200 or 0–1000 m in central Sagami Bay, Japan, from May 2002 to January 2004. While copepodite stages I–V (CI–CV) of C. sinicus appeared at 0–200 m throughout the year, significant fractions of the CVs occurred below 200 m during spring–autumn, then disappeared in winter. The adults (CVIs) occurred at 0–200 m throughout the year, although they were also abundant below 200 m during autumn–winter. Respiration rates and carbon contents of C. sinicus were measured in July 2007, revealing that the mesopelagic CVs had lower oxygen consumption and higher carbon contents than those in the epipelagic CV–CVIs. These results indicate that C. sinicus has two ecologically-distinct populations in Sagami Bay: one exists in the epipelagic zone throughout the year, while the other diapauses at CV in the mesopelagic zone during spring–autumn and matures in winter. The C. jashnovi CVs occurred below 600 m in spring–autumn, while the CVIs occurred at 200–400 m during winter and the CI–CIVs occurred at 0–200 m during winter–spring. The lower abundance of the CI–CIVs and CVIs compared to the CVs imply that the C. jashnovi reproduced outside of the bay and were occasionally transported into the bay.