The freshwater river and lake system supports high biodiversity and various ecosystem services, making it one of the most valuable ecosystems in the world. However, human activities, such as river-lake isolation and overfishing, have posed great threats to its health and sustainable development.    

In the Yangtze basin, seasonal connection strategies, including filling water and accommodating fries, have been implemented for the purposes of flood control, biodiversity protection and water improvement. The Yangtze River and its related lakes have been under a "ten-year fishing ban" since January 2020. Quantitative studies on how the structure and function of lake ecosystems respond to these management measures are needed.   

In a study published in Ecological Indicators, researchers led by Prof. WANG Kexiong and Prof. XU Jun from the Institute of Hydrobiology (IHB) of the Chinese Academy of Sciences revealed the risk of food web simplification and biodiversity decline in sluice-controlled lakes during the fishing ban, and suggested to restore the fish migration by improving the connectivity in autumn and winter.   

The researchers selected two oxbow lakes for comparative study. The lakes (Tian-e-zhou and He-wang-miao) located in the Jing Jiang section of the middle reaches of the Yangtze River, and were only 67 km apart and highly similar in climate and biome. They both introduced Yangtze Finless Porpoise for ex-situ conservation, but their management modes varied.    

The researchers first compared the aquatic ecosystems of the two oxbow lakes in Ecopath. The results showed that the flood-season-connected and longest-protected Tian-e-zhou system was more developed, but it had a shorter energy path length and lower community evenness.  

They then found that after the fishing ban, the biomass of adult migratory fish such as Silver Carp, Bighead Carp, Grass Carp, and Bream, increase greatly in the year-round connected He-wang-miao system, and the ecosystem structure is also simplified, similar to Tian-e-zhou. The reason could be that the 10 cm mesh net at the lower mouth allows small fish to flow in but restricts large fish from wintering out, which is similar to the effect of seasonal connection of sluice-controlled lakes.   

To find a solution to such food web simplification, the researchers conducted trophodynamic simulations in Ecosim. The results indicated that the maturity and complexity could be improved simultaneously when large individuals of river-lake migratory fish are removed from the system. Hence, increasing autumn-winter connectivity and driving large river-lake migratory fish back to the main stream may mitigate the risk of food web simplification. 

These findings complement seasonal connecting methods, may enhance the effectiveness of the “ten-year fishing ban” in Yangtze River and benefit the conservation of floodplain lakes worldwide.