Seasonal habitat use by warmwater fishes in a braided river, southwestern Japan: effects of spatiotemporal thermal heterogeneity

In braided rivers, networks of groundwater-fed and surface-water dominated channels cause a high variability in stream water temperature in space and time. Water temperature of groundwater-fed channels is cooler than that of surface-water dominated channels in summer, and vice versa in winter. Fishes in braided channels may exploit such spatiotemporal thermal heterogeneity through seasonal movements. In this study, we examined seasonal habitat use by two warmwater fishes, pale chub (Opsariichthys platypus) and freshwater goby (Rhinogobius nagoyae), in a braided river in southwestern Japan. We used two connecting channels having contrasting thermal regimes (groundwater-fed vs. surface-water dominated channels), and conducted bimonthly surveys over a year on spatial variations of the two species' densities and characteristics of their habitat across the two channels. Pale chub density was higher in the groundwater channel than in the surface-water channel in winter and vice versa in summer. These spatial variations of pale chub density were related to water temperature both in winter and summer, suggesting their seasonal movement between the two channels to seek better thermal conditions. Freshwater goby density was also higher in the groundwater channel than in the surface-water channel in winter, and positively related to water temperature. Our results suggest that groundwater seeps can provide important winter habitats for warmwater fishes and the close proximity of channels having contrasting thermal regimes is an important aspect of braided rivers for persistence of diverse fish assemblages.