Variation in reproductive parameters of round herring in the Pacific coastal waters of Japan

Abstract

This study assessed potential variability in batch fecundity and the spawning fraction of round herring Etrumeus micropus in the Pacific coastal waters of Japan. Fish were caught by a research vessel in the Kumano Nada (Kumano Sea) and Tosa Bay in February 2016, and other fish caught by commercial vessels in the Kumano Sea were collected between January 2019 and February 2020. The appearance of hydrated oocytes in the 2016 samples indicated that the fish spawned between midday and evening in the Kumano Sea, whereas they spawned from daytime to nightfall in Tosa Bay. The spawning fraction was estimated using the 2016 samples and was based on presence of post-ovulatory follicles and oocytes during oocyte maturation. The spawning fraction and spawning interval, respectively, were similar between the two areas: 0.17–0.26 and 3.9–5.9 d in the Kumano Sea, and 0.14–0.53 and 1.9–7.0 d in Tosa Bay. Batch fecundity was estimated by counting oocytes during oocyte maturation. The relationships of batch fecundity to body length varied significantly among the monthly samples. Batch fecundity at a given body length increased from January to May. This indicates that the acceleration in individual egg production would have contributed to the increase in egg abundance during the main spawning period. Relative batch fecundity (RBF) ranged from 76 to 142 eggs g−1 of ovary-free female. The 2016 samples showed that RBF was higher in Tosa Bay than in the Kumano Sea. The RBF increased from January to April in the Kumano Sea. A comparison of RBF values for the same month (February) differed significantly between samples in the three years, suggesting inter-annual variation in RBF. Our data provide essential information for understanding the spawning ecology of round herring and for allowing the egg production method to be applied to estimates of stock biomass.