Potential impact of predation by larval Spanish mackerel on larval anchovy in the central Seto Inland Sea, Japan

Japanese anchovy is used as an essential dried fish material from the larval to adult stages. In the central Seto Inland Sea, Japan, the catch of larval anchovy has markedly decreased to <3.9% of the maximum recorded in 2002 since 2013; however, the reason causing this reduction has not been well understood. The abundance of recruit fish, including larvae and early juveniles, has decreased in the last decade, despite abundant eggs, suggesting that the majority of larvae do not survive before recruitment. In contrast, the stock of Japanese Spanish mackerel, whose larvae are the major predator of larval anchovy, has increased in the Seto Inland Sea. It is hypothesized that an increase in the density of Spanish mackerel may have a top-down control on the decrease in anchovy recruitment by an increase in predation opportunities. In this study, we investigated the abundance of Spanish mackerel and anchovy larvae using a bongo net in the field in 2018 and 2019. The average densities of larvae in late May were 1.5-3.3 individuals (inds)/100 m3 and 1058-1346 inds/100 m3 for the Spanish mackerel and the anchovy, respectively; both were higher than those in 2002-2005. We constructed a Stella model, simulating the growth and survival of larval anchovy until they reached the commercial sizes by taking into account consumption by larval Spanish mackerel. The model suggested that the consumption of larval anchovy by larval Spanish mackerel accounted for <4% of the initial abundance of anchovy in 2018, which was not greater than that in 2005. In contrast, the reduction in the growth rates of larval anchovy due to reduced maternal conditions can adversely affect their survival. Thus, the results did not fully support the hypothesis mentioned above.

Automated summary

Since 2013, the catch of Japanese anchovy larvae in the Seto Inland Sea has decreased significantly. Despite abundant eggs, the abundance of recruit fish has also decreased, suggesting high mortality rates before recruitment. The increased density of larval Spanish mackerel, a major predator, may be contributing to the decrease in anchovy recruitment by increasing predation opportunities, although the results did not fully support this hypothesis.