Effects of mangrove structure on fish distribution patterns and predation risks

This study examined the effects of mangrove structure on fish distribution patterns and predation risks in southern Japan, utilizing field experiments in which artificial units mimicked mangrove root structure and canopy shade in a mangrove estuary. Fishes responded strongly to the different levels of root structure complexity but not to canopy shade, fish species richness and abundance being consistently higher in the units with roots than in those without roots, regardless of shade presence/absence. With increasing root density, fish abundance increased considerably, although with decreasing fish body size. A tethering experiment, employing the above units and the most abundant local fish Apogon amboinensis as representative of small fishes, revealed that the predation mortality rates of small fishes decrease with increasing root density. The results suggested that the sheltering effects provided by root structure against predators may be one of the most important factors determining the distribution patterns of mangrove-associated fishes, and at least partly explain the greater abundance of small-sized fishes in microhabitats with greater root densities. •Fish abundance was higher in artificial unit with roots than in unit without roots.•Fishes did not respond to presence/absence of canopy shade.•Fish abundance increased with increasing root density.•Predation mortality rates of small fishes decreased with increasing root density.•Sheltering effect of root structure may be responsible for small fish distributions.