Fishery Resources, Environment, and Conservation in the Mississippi and Yangtze (Changjiang) River Basins

<em>Abstract</em>.—The Yangtze (Changjiang) River floodplain is one of the most important ecosystems in China, as well as in the world, but is seriously threatened by multiple factors. Thus, it is crucial and urgent to rehabilitate the river floodplain. This paper reviews ecological studies conducted on the Yangtze River floodplain and presents suggestions for conservation and rehabilitation. First, the Yangtze River system is briefly introduced. Formed 23 million years ago, the Yangtze River is ca. 6,300 km in length with a mean annual runoff of 9.6 × 10¹¹ m³. Thousands of floodplain lakes are distributed along the mid-lower Yangtze River, and the total area remains 15,770 km² at present. Such a river-lake complex ecosystem holds a unique and diverse biota, with ca. 400 hydrophytes and hygrophytes, ca. 170 mollusks, ca. 200 fishes, ca. 400 water birds, and endangered dolphins and porpoises. Second, main threats to the Yangtze River floodplain ecosystem are identified: (1) habitat loss, including river channelization, sharp shrinkage of lake area (ca. 10,000 km² since the 1950s), degradation of lakeshore zones, and sand overmining; (2) alternations of hydrological regimes, including construction of ca. 47,000 reservoirs in the whole basin and disconnection of most floodplain lakes from the main stem; (3) water pollution, including eutrophication, heavy metals, and organic pollutants; and (4) overexploitation of biological resources, including overfishing and intensive pen culture. Third, effects of river–lake disconnection on lake ecosystems are summarized on the basis of our studies in the past 20 years. It was found that (1) disconnection is one of the main causes of lake eutrophication; (2) species diversity, biomass, and production of macrophytes and macrobenthos reach maxima at some levels of intermediate river connectivity; (3) disconnection greatly reduces fish species richness of each habitat guild, and natural fish larvae is severely depleted; and (4) disconnection simplifies macroinvertebrate food web structure, and the trophic basis of the simplified food web is more heavily dependent on detritus in disconnected lakes. Last, conservation strategies are proposed. Since the Yangtze River floodplain is a huge integrated system, the biodiversity conservation must be conducted on the whole basin scale. By establishing species–area models of fishes, the minimum protected area of Yangtze-connected lakes is estimated to be ca. 14,400 km². It means that at least 8,900 km² of disconnected lakes should be reconnected with the Yangtze main stem, and ecohydrological operation of dams and sluices is the feasible approach. Based upon our preliminary studies on environmental flow requirements, the following measures are suggested: (1) lower water levels during spring to improve germination of macrophytes, and control rising rates of water levels during spring–summer to ensure development of macrophytes; and (2) open sluice gates to restore migration routes for juveniles migrating into lakes during April–September and for adults migrating back to the Yangtze main stem during November–December.