Effects of environmental modification due to weirs on freshwater fish communities

We surveyed fish communities and environmental characteristics above and below the weir set in the lowest reaches of each four rivers in northern region of Ibaraki Prefecture in August of 2014. of 16 fish species captured, seven were diadromous fishes, seven were freshwater fishes, and two were peripheral fish species. Diversity index (H') of fishes above weirs was significantly lower than those of below weirs in case for total fishes and di-adromous fishes. The best model of GLM based on AIC indicated that presence of weir had a negative significant effect on diversity index in case for total fishes and diadromous fishes. The parameters (slope of standardized partial regression coefficient) incorporated with best GLM model to account for population number of diadromous fish species are: water depths ( + ), current velocities (-), height of weir (-), and area of study site as an offset for Plecoglossus altivelis altivelis; presence of weir ( + ), water depths ( + ), and height of weir (-) for Gymnogobius petschi'liensis; presence of weir (-), water depths ( + ), and current velocities ( + ) for Rhinogobius nagoyae; presence of weir (-), current velocities (-), height of weir ( + ), and area of study site as an offset for Cottus reinii. Bray-Curtis dissimilarity index (BC_d) of fish species composition between any possible combinations of study sites positively correlated with Euclidian distance based on two environmental characteristics (mean water depths and current velocities) in case for total fishes and diadro-mous fishes, and Pearson's correlation coefficient was highly significant for diadromous fishes. These results suggest that remarkable decline in diversity and population number of diadromous fishes above weirs would stem from not only inhibition of upstream migration by weirs, but also weir-induced anthropogenic modification of instream habitat features for diadromous fishes.