Seasonal variation in abundance of Oithonidae copepods in the Kagoshima Bay [Japan]

Seasonal variations in abundance of Oithonidae copepods were investigated from zooplankton samples collected weekly to biweekly at the two different stations in the Kagoshima Bay, southern Japan. Oithonidae copepods showed the similar pattern between the two stations, which they occurred abundantly from April to September and decreased below 10 inds./cubic m during October to January. Abundance of each developmental stage was simultaneously fluctuated, indicating that it was a short development time. Nauplii, young copepodites and adult females were found over the study period, whereas adult males disappeared in some sampling periods between October and March. During this season, adult females and nauplii were at predominant stages. Females comprised more than 70% of adults, showing a longer life span than males. These results suggest that the decline of their abundance is caused by low rates of egg production, copepodite recruitment from nauplii and/or survival of young copepodites during the cold season although they reproduce throughout the year.