移入河川におけるオイカワの豊富さと藻食に対するアユの影響

抄録

Nonbenthic fishes were investigated in 16 rivers in the Chubu and Tohoku regions, into which pale chub Zacco platypus had been introduced. Pale chub coexisted predominantly with Japanese dace Tribolodon hakonensis, followed by ayu Plecoglossus altivelis altivelis, and were abundant in rivers lacking the latter. Although pale chub, ayu, Japanese dace, dark chub Nipponocypris temminckii and Amur minnow Rhynchocypris lagowskii steindachneri all fed on both benthic algae and invertebrates, the percentage of algae in the diet of pale chub was less when ayu were present. However, the population abundance and diet of pale chub were not affected by the other nonbenthic species. Although pale chub and other cyprinids rarely fed on cyanobacteria, a main food item of ayu, pale chub was considered to be negatively affected by ayu through interference, exploitative competition and indirect effects via changes in algal composition.