Effects of dam removal on fish community structure and spatial distribution in Heishui River

Dam removal causes a series of changes in river connectivity, hydrodynamics and river landforms, which directly affects the structure and spatial distribution of fish community. The Heishui River is a first-level tributary on the left bank of the Jinsha River with four power stations built on it totally. Due to the necessity of tributary habitat restoration, the fourth-level dam Laomuhe Dam was demolished in December 2019. In order to ascertain the effects of the low-head dam removal on the fish community, the changes in fish community structure and ecological type, dominant species and catch per unit effort and community similarity between different river sections were analyzed in combining with the fish survey at 10 points in the lower reaches of the 60.4 km Heishui River in June 2018 and June 2019. The results showed that the dominant species were still mainly Cyprinidae and Cobitidae after the dam was demolished six months ago, but the species and abundance increased overall. The ecological type was dominated by omnivorous, preferring to running water and spawning slightly viscous fish. The spatial distribution of fish community changes obviously after dam removal. Fish in natural river sections near the estuary migrated to the upstream and downstream of the original dam site. Some native fish in the upper reaches of the Yangtze River colonized near the original dam site, and the fish community structures in each section after dam removal tended to be homogeneous. In general, removing the Laomu River Dam has positive effect on the fish community structure and spatial distribution in the lower Heishui River.