EFFECT OF THE IMPOUNDMENT OF DAM CASCADE IN JINSHA RIVER AND INCREASED WATER LEVEL OF THE THREE GORGES RESERVOIR ON THE DISTRIBUTION AND ABUNDANCE OF PSEUDOLAUBUCA ENGRAULIS (NICHOLS) LARVAE IN THE UPPER MAINSTEM OF THE YANGTZE RIVER

Ichthyoplankton were collected in the upper mainstem of the Yangtze River at Yibin, Zhuyang and Mudong in 2009, 2012 and 2015 to determine the occurrence dynamic and abundance of Pseudolaubuca engraulis (Nichols) larvae. The abundance of P. engraulis larval peaked from mid-May in 2009 and 2012 (pre-impoundment of dam cascade in the Jinsha River), while few larvae occurred before late-June and the abundance peaked from mid-July in 2015 (post-impoundment of the dam cascade). Hypolimnetic and clean water discharge from the dam cascade are critical factors on delaying spawning seasons and peak occurrence of the larval fish. The abundance of P. engraulis larvae showed an apparent increase at Yibin and a slight increase at Zhuyang in 2015 comparing to that in 2012 and 2013, indicating that the fish may adapt to the altered habitat environments by the dam cascade. Abundance of P. engraulis larvae at Mudong showed a significant increase during 2009 to 2015. The increase of the fish is probably related to the increasing operational water level (from 156 m to 175 m) of the Three Gorges Reservoir (TGR), which created slow flow habitats at Mudong section and then offered a favorable nursery for the larvae. Based on the results of this study, it is suggested that the temporal-spatial variation of P. engraulis larvae was jointly controlled by the upstream dam cascade and downstream TGR. Meanwhile, we suggest that the " River-Reservoir” ecosystem, constrained by the tail of reservoir (even all the reservoir) and its upstream river section, may have similar functions as the river-floodplain ecosystem in natural systems, and is valuable to the successful recruitment of fishes which produce drifting-egg.