Empirical prediction model for egg production rate of a planktonic copepod, Calanus sinicus, estimated from reproductive index using formalin-preserved samples in Sagami Bay, Japan

Abstract

Calanus sinicus is one of the most dominant planktonic copepods and plays a key role as a secondary pro-ducer in the mid-latitude coastal shelf areas of the Northwestern Pacific. In this study, we investigated seasonal changes in in situ egg production rate (EPR) of C. sinicus collected from March 2014 to February 2015 in Sagami Bay, Japan. Simultaneously, we collected formalin-preserved samples to measure reproductive index (RI), which was calculated as the proportion of individuals with matured gonad to total females in each sample. Monthly averages of EPR and RI varied from 2.4 to 44.9 eggs female−1 day−1 and 0% to 75.9%, respectively. RI showed a significant positive effect on EPR. Moreover, EPR decreased with increasing temperature, which was the inverse of the effect of temperature on EPR reported in a previous study in the Seto Inland Sea. The difference between the studies suggests that the local population may adapt to different temperature environments in each location. Using a stepwise multiple regression model, EPR estimates for C. sinicus in Sagami Bay were explained by the average water temperature and RI with high prob-ability (R2=0.71). Although local water temperature ranges must be considered before applying our model to other ar-eas, the model would be useful to reconstruct past secondary production of C. sinicus from preserved samples collected in other neighboring coastal areas.