矢作川水系などにおけるヒメドロムシ科の生息状況

Abstract

In the Yahagi River, multiple dam constructions and flood-control operations are thought to have reduced riverbed disturbance in the middle reaches, causing various abnormalities in aquatic organisms. The main objective of this study was to identify species of the family Elmidae (Insecta, Coleoptera) that can serve as indicators of riverbed disturbance by examining their distribution and habitats, and to provide baseline data for evaluating riverbed disturbance in future projects such as the planned sediment bypass tunnel and sediment replenishment experiments. From 2023 to 2025, time-constrained sampling, quantitative sampling, hyporheic excavation sampling, and light-trap sampling were conducted in the Yahagi River system and nearby rivers, Honshu, Japan. In addition, specimens collected between 2001 and 2023 were examined. As a result, a total of 23 species in 13 genera were collected, including five endangered species belonging to five genera. Many species were widely distributed within the Yahagi River system; however, some were restricted to downstream areas, while others were limited to small tributaries. One species showing a highly localized distribution in relation to dams was Zaitzevia nitida, which was collected only upstream of Yahagi Dam. Based on this distribution pattern and comparisons with the Nagara River system, Zaitzevia nitida is considered to prefer habitats with strong riverbed disturbance. In hyporheic excavation sampling, larvae of Stenelmis vulgaris, Ordobrevia gotoi, Optioservus spp., Neoriohelmis kurosawai, and Zaitzevia spp. were relatively abundant or were collected only by this method compared with standard benthic sampling on the riverbed. These results suggest that the larvae of these taxa are hyporheic invertebrates inhabiting interstitial spaces within riverbed sediments. In the Yahagi River system, the five endangered species were found across a wide range of both the main channel and tributaries. Light-trap sampling often attracted some species that are unlikely to inhabit nearby rivers. Therefore, light trapping is inferred to be less suitable for investigating species distribution and habitat compared with sampling methods conducted directly within river water.