The Impact Mechanism of Inter-Basin Water Transfer on Fish Assemblages Was Revealed Using a Model of Terminal Reservoirs.

Understanding the impact mechanism of inter-basin water transfer projects (IBWTs) on fish assemblages is critical for conserving aquatic ecosystems and managing water transfer. However, direct evidence of the impact mechanism of IBWTs on fish assemblages is absent. Fish assemblages in newly built terminal reservoirs originate and assemble with diverted individuals through water transfer in IBWTs, providing a valuable model for mechanistic research. This study sampled fish quarterly using a combination net in three terminal reservoirs along the Eastern Route of the South-to-North Water Transfer Project (ER-SNWTP) from July 2021 to May 2022. We investigated assemblage composition and structure, calculated alpha and beta diversity indices, and analyzed the effects of environmental factors on fish assemblages. A total of 35 species were collected and indicated which species could be diverted accompanying water transfers along the ER-SNWTP. Fish assemblages were dominated by eurytopic species, but showed clear seasonal differences. The distance from Dongping Lake, operation time of the reservoirs, and water quality affected diversity indices, composition and structure of the fish assemblages in the reservoirs. Fish assemblages in the terminal reservoirs were unstable and periodically disturbed by the water transfers of the ER-SNWTP. Our results indicate that fish diverted by water transfer are likely to be the main cause of the impact of IBWTs on fish assemblages, and the distance and number of water transfers, and nutrient loading of diverted water are crucial factors affecting the impact of IBWTs. Our study provides a theoretical understanding of the ecological impacts of IBWTs by introducing fish and periodically disturbing fish assemblage through water transfer and highlights the necessity of scheming transfer programs to avoid overlapping with the spawning peaks of some fish with special guilds, such as small-sized fish. In terminal reservoirs, the fish assemblages are not and may have no possibility to reach a stability like a natural one, and then their management should be oriented toward water quality regulation, such as rational harvesting and replenishment of biomanipulation fish to enhance their capacity for water quality regulation.