THE STATUS OF THE EARLY-STAGE FISH RESOURCES AND HYDROLOGIC INFLUENCING CONDITIONS IN THE YICHANG SECTION IN THE MIDDLE REACHES OF THE YANGTZE RIVER

In order to reveal the current status of the early-stage fish resources in the middle reaches of the Yangtze River, we conducted surveys in Yidu reach from May to July in 2017 and 2018. Sampling nets included a semi-conical net and a conical net. A total of 21120 eggs and 2123 larvae were collected. A total of 37 species in 9 families and 5 orders were identified using both morphological and molecular methods. Among which, 29 and 27 species were identified in eggs and larvae, respectively. We estimated that 124.45×10⁸ ind eggs and 16.43×10⁸ ind larvae drifted through Yidu reach in 2017, while 101.07×10⁸ ind eggs and 8.29×10⁸ ind larvae drifted through Yidu reach in 2018. The four major Chinese carps and Hemiculter bleekeri were the dominant species in eggs while Hemiculter bleekeri and Pseudolanbuca engraulis were the dominant species in larvae. According to the analysis of development period and velocity, the eggs of the four major Chinese carps came from the spawning site in Yichang section between the lower reaches of the Three Gorges Dam and the Yidu section, and the larvae came from the spawning site above the Three Gorges Dam. Compared with data in 2009 and 2010, the runoff of fish eggs through Yidu section increased by 85.3% in 2017 and 2018, and the runoff of eggs of the four major Chinese carps increased by about 13 times compared with data from 2005 to 2012. Redundancy Analysis showed that flow charge and daily flow velocity were main environmental factors affecting reproductive activities of drifting eggs species. The results showed that after the impoundment of the Three Gorges Reservoir, a large number of fish eggs and larvae were replenished to the middle reaches of the Yangtze River. Compared with the data from 2005 to 2012, spawning size of pelagic eggs below the Gezhou Dam increased significantly which is related to the implementation of a series of protective measures, such as the breeding and releasing of the major four Chinese carps, the ecological regulation to promote the natural breeding of the major four Chinese carps and the management of banning fishing in the middle reaches of the Yangtze River in recent years. In addition to the gradual implementation in the Yangtze river’s comprehensive ban on fishing, we also suggest restoring the connectivity between rivers and lakes, protecting and restoring natural environment conditions of floodplains, releasing parental fish of economically important species, as well as carrying out ecological regulation, to effectively protect fish resources in the middle reaches of the Yangtze River.