Stream gobies of the genus Rhinogobius may use paddy systems as a nursery habitat during their amphidromous migration.

Abstract

A previous study has shown that an amphidromous goby, Rhinogobius mizunoi, in the Daimyoujin River, Shikoku, is a landlocked population. However, this river has no standing water body (e.g., dam reservoir), which is necessary for landlocked migration as a nursery habitat for their larvae. Furthermore, the flow in the lowest reach of this river is ephemeral (i.e., surface flow is usually disconnected from the sea). Therefore, we inferred that amphidromous gobies of the genus Rhinogobius in this river use paddy-associated aquatic systems behind the river channel for their landlocked migration. To examine this idea, we monitored drifting larvae and ascending juveniles of the gobies in agricultural ditches connecting the Daimyoujin River channel with the paddy systems. As a result, we confirmed that (1) drifting larvae emigrated to the paddy systems from the river channel through the ditches. We also found (2) small juvenile gobies that could be assumed to be ascending age-0 individuals in the ditches. Our one-year monitoring confirmed that (3) surface water of the study river rarely reached the sea, suggesting that ordinary amphidromous migration using the sea is almost impossible for the gobies in this river. However, our fish survey confirmed that (4) the perennial-flow reach of this river was inhabited by abundant amphidromous gobies. These lines of circumstantial evidence support our idea that the amphidromous gobies in the Daimyoujin River use paddy-associated aquatic systems for their landlocked migration. Such a landlocked migration using paddy systems may be common for gobies of the genus Rhinogobius, as most streams and rivers in Japan are usually accompanied by paddy-associated aquatic networks.