Influence of a weir on the upward migration of ayu Plecoglossus altivelis altivelis in the Shonai River, Central Japan

Abstract

To clarify the influence of a weir located in the lower-reach of the Shonai River on the upward migration of ayu Plecoglossus altivelis altivelis, we conducted frequent sampling, otolith analysis by EPMA method, and combined analysis of abundance, size, and age at habitat change. Under the weir, ayu were first caught on April 5 and CPUE showed a maximum in mid-June. At the fishways in the weir, until mid-May, few fish were caught by set net. But in late May, CPUE increased and showed a maximum. Throughout the study period, larger individuals as caught under the weir were not observed in the fishways. Furthermore, ayu which spent approximately 80 days after entering into the river were caught in late May under the weir. From otolith analysis, individuals sampled at middle reach of the river, under the weir and in the fishways, peak time of entering the river was early April, late April and mid-May, respectively. Individuals that passed the fishways soon after arrival at the weir early in the upward migration season could go up to the middle reach of the river, while those which failed to continue the upward migration remained under the weir which was an unsuitable habitat.