Determining the relative importance of catchment- and site-scale factors in structuring fish assemblages in small coastal streams

Coastal streams provide important habitat for many diadromous fish species, which migrate between freshwater and the sea during their life cycle. However, coastal systems are poorly studied in comparison to large, continental river systems. Furthermore, the relative importance of catchment- and site-scale factors in structuring lotic fish communities is largely unknown. In this study, I addressed these issues by surveying the fish fauna of small coastal streams on Sado Island (northwest Japan) and determining the relative importance of catchment- and site-scale factors to the structure of freshwater fish assemblages. In total, 14 freshwater fish species were collected from the 19 streams. All but one of the fish species collected were diadromous and 9 of the species (64%) were amphidromous, primarily represented by the goby group including the genera Rhinogobius and Gymnogobius. Variance partitioning analyses showed that catchment-scale factors (namely, stream discharge) were better predictors of both fish species richness and composition than site scale and spatial factors. These results indicate that discharge, which is directly linked to habitat stability, can have a major role in structuring coastal fish assemblages, likely because small coastal streams experience extreme discharge fluctuations associated with regional weather conditions.