2021 Volume 24 Issue 1 Pages 39-50
Lake Biwaʼs inflowing rivers often experience drought in summer. Periods of drought can severely affect fishes by preventing their migration. This study examined the effect of observed drought on under-yearlings of an amphidromous goby, Rhinogobius sp. OR, which seasonally moves between Lake Biwa and its tributaries. A survey of under-yearling fish was conducted at the end of the upstream migration season in 2008, from September to November, in the downstream and midstream sections of six rivers that flow into Lake Biwa. To assess migration success from the downstream to midstream sites, we gauged the upstream migration index of under-yearling fish (=CPUE in the midstream site / CPUE in the downstream site, hereafter "UM-UY index") and determined a past upstream migration index (hereafter "UM-P index") for individuals except for under-yearlings. The UM-UY index was lower in the droughted rivers. Multiple regression analysis showed that the UM-UY index serve as an indicator for distance of drought. Additionally, the UM-P index and the UM-UY index were similar for most of the rivers. The Echi River, in particular, had very low values of both indices. However, the UM-P index and the UM-UY index were different in Ado and Inukami River, this suggested that the condition of drought differed depending on the year, and thus resulted in different indices affecting the upstream migration of under-yearling fish. On the other hand, the Echi River was considered to experience a severe drought every year that caused a significant impact on the up-stream migration of under-yearling fish. These results indicate that drought negatively affected the upstream migration of under-yearling fish and that the index of upstream migration could be a useful indicator to evaluate the scale of drought conditions in a river habitat.
