Bottom substrate attributes relative to bedform morphology of spawning site of Chinese sturgeon Acipenser sinensis below the Gezhouba dam

Characteristics of bottom substrates have caused wide concernas to their importance for sturgeon spawning ground requirements. Substrate requirements for spawning of Chinese sturgeon Acipenser sinensis have not yet been well studied because of the difficulties in investigating on a large and fast flowing river such as the Yangtze River. During 2007 and 2008, the underwater video surveys on the present spawning ground of A. sinensis were successfully conducted. Two kinds of substrates were documented: one stable infrastructure consisting of large boulders and cobbles mixed with pebbles and gravels, the other comprise sediments in transportation, especially coarse and fine sands. The large boulders (size range 20-50 cm), cobbles and pebbles (size range 10-20 cm), pebbles or gravels are estimated to account for 50, 30, 20% of the available substrates of the infrastructure on spawning ground according to our underwater video survey points, respectively. The transporting sands can be observed in most area, while the river sand embedded or covered the large structural compounds just occur in certain area and the substrate where spawning occurs annually and occasionally were similarly 'clean' of fine sediment. The eggs (embryos) of Chinese sturgeon have been observed 'hiding' in or adhering to the interstitial spaces between the clean large materials e. g. boulders, cobbles and pebbles during the 2007/2008 study period. Therefore the degree of cover of the larger particles by sand (embeddedness), which will directly affect the spawning habitat and subsequently influence survival of the new hatched larvae, were assessed by determining the topographic data in relation to water velocity. The multivariable correlation analysis and binary logistic regression analysis showed that the comparably topographic characteristics which had been confirmed to be suitable at the historic and present spawning sites would eventually contribute to the substrate deposition. The embeddedness of the sediment are obviously lower in shallow water with higher water velocity than in the deeper water with slower water current (P < 0.01) and the sites where the embryo occur can be predicted with the three physical variables: embeddedness of the sediment, water velocity and river bottom elevation. The bedform morphology with an adverse slope (channel slope adverse to the bottom elevation grads) formed by larger blocks of substrate, will improve the water turbulence at high current to carry the suspended or embedded fine sediments away from the spawning site and (in turn) provide enough interstitial spaces for effective egg deposition and incubation. Therefore the substrate requirements for the spawning habitat of Chinese sturgeon should be described and assessed more than just rocky and gravelly substrate, taking the embeddedness of sediment in well consider. The concepts of detailed description of the substrate requirements and their relationship with the overall bedform morphology has been shown to be essential to provide benefit for the rehabilitation of the spawning ground while fostering the conservation of the endangered species.