Seasonal variability in the otolith and somatic size relationship of Japanese anchovy larvae: Counter effects of somatic growth and temperature

Abstract

Otolith microstructure analysis has played an essential role in understanding the recruitment mechanisms of fish. Despite the fundamental assumption of a close correlation between otolith size and somatic size in growth back-calculations, numerous studies have shown that somatic growth and temperature have negative and positive effects on otolith size at the same somatic size, respectively. However, few studies have investigated the effects of somatic growth and temperature on otolith and somatic size relationships at intra- and inter-cohort levels in the same study framework. The present study investigated the effects of somatic growth and temperature on the seasonal variability in the otolith and somatic size relationship in Japanese anchovy Engraulis japonicus larvae collected from multiple seasonal cohorts throughout the year in the Kii Channel, Japan. The otolith and somatic size relationships were strong within each seasonal cohort sample (R2 = 0.771–0.949), but the relationship became weaker when all the seasonal data were pooled (R2 = 0.571). Relative otolith size at a given somatic size (ROS) was negatively related to somatic growth rates within each seasonal cohort sample (i.e., at the intra-cohort level), but the relationship became weak when all the data were pooled. On the contrary, sea temperature had positive effects on ROS, which would explain the seasonal variability in the otolith and somatic size relationship. The multiple regression analysis showed that temperature influenced ROS almost six times more than somatic growth. Overall, the present study demonstrated that an uncoupling in the otolith size–somatic size relationship was generated by the counter effects of somatic growth and temperature. The observed counter effects should be taken into account in estimating past larval growth for larvae obtained from different environmental conditions.