Small dams fragment assemblages of diadromous and freshwater decapods in Hong Kong lowland streams

Dam construction has fragmented and substantially altered streams globally, obstructing migrations between coastal and freshwater habitats by diadromous animals. We undertook a territory-wide survey of decapod species (shrimps and crabs) in Hong Kong, southern China, examining spatial and seasonal variability in assemblage composition, and the impacts of barriers, across 24 lowland streams. Thirteen diadromous and 10 primary (i.e., non-migratory) freshwater decapod species were recorded, considerably more than reported elsewhere on the Chinese mainland. While some decapods are proficient climbers, six diadromous species were confined to unobstructed streams. Dams (0.3-8.7 m high) reduced total richness, but had stronger effects on diadromous species (mean richness fell from 4.9 to 3.2 species). Mean species richness of both total and diadromous decapods were lowest in streams with dams > 2 m tall (reductions of 6.4 to 3.8 and 4.9 to 2.2, respectively). Decapod assemblage structure was significantly different above and below dams, reflecting the restriction of primary freshwater species to reaches above dams, and diadromous species to reaches downstream of dams. Our findings underscore the need for improved knowledge of the diversity of diadromous animals in China and tropical East Asia, as well as better understanding of mitigation measures to improve dam passage by these animals.