Age-related variation in reproductive potential and influence on recruitment of western sand lance Ammodytes japonicus in the Seto Inland Sea, western Japan

Abstract

Assessment of total egg production as a proxy for reproductive potential is critical for understanding the mechanisms of recruitment variability and implementing appropriate stock management. This study determined whether the temporal variation in the estimated egg production index (EPI) of spawning western sand lance (Ammodytes japonicus) collected in the Bisan Strait in the central Seto Inland Sea in Japan explains its recruitment variability from 2009 to 2019. Although the maturation rates of individuals aged 1 year varied among sampling years, they were positively correlated with mean standard length at aestivation onset in summer. The negative relationship between the mean standard length and density of aestivating young-of-the-year fish at aestivation onset suggests that growth was dependent on density during the spring foraging period. All specimens aged 2 and 3 years were mature, and relative fecundity was significantly higher than that in specimens aged 1 year. The EPI of spawning individuals was markedly reduced in recent years and was associated with the decreased density of fish aged 2 and 3 years. In addition, the number of recruit fish was significantly associated with the EPI of spawning fish. However, the density of spawning fish in winter did not adequately explain the variation in recruitment. Thus, our findings suggest that sand lance aged 1 year have reduced reproductive potential, and that the causes of the recent decline in sand lance stocks might be partly attributable to a decline in the abundance of older fish.