淡水エビ類の分布に与える河川横断工作物の影響

Abstract

Freshwater shrimps were collected from March 2022 to January 2024 at 156 sites in the Yahagi River system and its surroundings, located in central Honshu, Japan. In 2023, we investigated the seasonal changes in the occurrence of freshwater shrimps in the Hiromi River within the river system. Our findings reveal that freshwater shrimps were widely distributed in the area. Alien shrimps were found alongside native species at many sites, with the former often being abundant. However, only native shrimps inhabited the upper reaches of certain rivers. In these rivers, the upstream distribution of Palaemon paucidens was replaced by Neocaridina downstream. Additionally, only native P. paucidens and Paratya spp. were collected in the ponds at the Expo 2005 Aichi Commemorative Park in 2009 and 2011. However, in 2022-2023, alien Neocaridina shrimps were also observed alongside the two native species. In the Hiromi River, the distribution of freshwater shrimps exhibited seasonal variations across the weirs. Our observations suggest that the weirs in the Hiromi River do not pose significant obstacles to upstream migration for the shrimps. Consequently, it appears unlikely that weirs would effectively control the migration of alien shrimps. Therefore, considering the ineffectiveness of weirs in controlling shrimp migration, it may not be necessary to assess the impact of fish passages on these structures with regard to freshwater shrimp migration. However, it is crucial to install fish passages for the benefit of an endangered native loach species, Lefua echigonia, which inhabits the area and requires such passages for upstream movement.