Assessment of growth patterns of amphidromous fish larvae in alternative estuarine nursery habitats along a developed coast

Abstract

The placement of artificial structures in coastal areas can adversely impact the life histories of larval and juvenile fish by reducing suitable nursery areas. However, little is known about the responses of them to such habitat loss. The ayu Plecoglossus altivelis altivelis is an amphidromous fish that spends its early ontogenetic stages in suitable nurseries, the surf zone, within 2 km from river mouths. To evaluate the possible impact of habitat loss on larval and juvenile ayu, we collected 50 juveniles from the Takahashi River, Okayama, Japan, on 19 April 2018, where more than 95% of the coastal outlet area is covered with artificial structures. Otolith Sr/Ca- and increment analyses were conducted to examine the habitat use and growth rates of each individual. We also collected nine larvae on 27 December 2018 from a semi-natural surf zone, which is located 5 km from the Takahashi River mouth, in order to evaluate their growth rate. The 50 juveniles spent approximately one-third of their marine life stage in estuaries with brackish water, suggesting that the estuaries are the only nursery grounds remaining in this area. Among the 50 juveniles, specimens that experienced lower salinities showed higher growth rates and larger body sizes in the larval stage (1–80 days old). However, a large variation in growth rates was also observed among the specimens that grew in estuaries, probably owing to environmental instability in the area. While the difference was not statistically significant, the nine larvae from the semi-natural surf zone showed higher growth rates than the 50 juveniles from the Takahashi River. Our results indicate that the estuaries can be an alternative habitat, but that they might not be a better habitat than the regular surf zone habitat.