Larval drift and dispersal are critical processes that affect recruitment success for many riverine fishes. Hypolimnetic discharge from the Three Gorges Dam (TGD) lowers river temperature and reduces downstream nutrients, inducing distinct shifts in habitat conditions downstream of the dam. The inflow of major tributaries buffers these influences and creates physiochemical gradients according to the distance from the dam. We investigated the abundance, feeding, and growth of larvae of four major Chinese carps in three sections of the middle Yangtze River. Water temperature and transparency showed clear spatial gradients. Larvae in the river section closest to the dam tended to be lower in abundance and temporally delayed peak abundance and showed lower feeding intensity, poorer condition, and slower growth than those further from the dam. Our results demonstrate that physiochemical gradients influenced by the TGD have strong effects on abundance, feeding, and growth of the drifting larvae. We recommend that river sections farther from the TGD, particularly around the mouth of Poyang Lake, should become high-priority conservation areas to enhance protection of critical aquatic species.

Drifting larval fish assemblages were investigated at three sections in the middle reach of the Yangtze River downstream of the Three Gorges Dam to reveal recovery gradients and to identify the major environmental factors influencing their temporal and spatial patterns. Larval fish in the river section closest to the dam showed lower abundance, later occurrence, narrower temporal niche breadth, and more pairs of dominant species with high niche overlap than those further from the dam. Water temperature, transparency, and discharge significantly influenced the larval fish assemblages. They showed longitudinal gradients of increased water temperature, decreased transparency, and increased discharge in a downstream direction. Our results suggested that hypolimnetic and clean water discharge from the dam heavily impacted larval fish assemblage in the river section near the dam. With an increased distance to the dam and the input of some major tributaries, this impact was gradually buffered, thus forming the longitudinal recovery gradients. We suggest that priority for conservation should shift from river sections closer to the dam to those further from the dam.

Landscape pattern plays an important role in structuring of larval fish assemblages in large river basins. The lower reach of the Yangtze River includes the river section from the mouth of Poyang Lake, the largest freshwater lake in China, downward to the estuary. We hypothesized that river-lake connectivity at the upstream portion and tidal intrusion at the downstream portion are significant drivers of larval fish assemblages. Larval fish were collected monthly from April 2012 through September 2013 in three locations at Anqing (AQ), Nanjing (NJ), and Jingjiang (JJ). Fifty-six taxa of larval fish were identified with 52 of them determined of species. Larval fish were highly abundant from April through August; the number of species and the abundance were much higher at AQ than at NJ and JJ; the bloom of larval fish at JJ tended to be later than at NJ and AQ. We suggest that the larger number of species and higher abundance of larval fish at AQ reflect the effects of the connectivity of Poyang Lake with the river upstream of AQ, and the delayed bloom of larval fish and the occurrence of estuarine species at JJ reflect the influences of tidal intrusion from the estuary.