Cross-Scale Anthropogenic Threats Jointly Drive Declines in China's Estuarine Fish Assemblages Over the Past Half-Century

Identifying multi-scale threats in the Anthropocene to vulnerable yet crucial ecosystems can inform the development of effective conservation and restoration actions. Estuarine fish assemblages, which are vital for ecosystem functioning and serve as key indicators of ecosystem health, face a multitude of anthropogenic threats operating across local, regional, and global scales. However, due to a lack of standardized, long-term, and spatially extensive datasets, the trends in estuarine fish assemblages and their responses to multi-scale anthropogenic threats remain unclear. We construct a dataset spanning over half a century for fish assemblages across different life stages and the multi-scale anthropogenic threats they face in three major estuaries of China, aiming to identify patterns of abundance and richness, and to validate the relationships between widely recognized anthropogenic threats and changes in these patterns. The results indicate that, over the past half-century, fish assemblages in all three major estuaries in China have experienced declines in abundance or remained at persistently low levels, while richness remained relatively stable. At the local scale, increased fishing intensity in estuaries and adjacent coastal areas directly reduced adult fish abundance. Regionally, water and sediment regulation measures implemented following the construction of upstream dams and reservoirs raised freshwater input and altered sediment delivery to estuaries during the fish spawning period, indirectly depressing the abundance of fish eggs and larvae. Higher sea surface temperatures (SST), driven by global warming, corresponded to greater species richness across estuaries. However, this effect was not consistently observed within individual estuaries, indicating regional variation in thermal responses. Although current protection and restoration efforts addressing threats at individual scales are gradually being implemented, we recommend a coordinated, multi-scale management strategy to safeguard the future of fish assemblages in estuaries of China and many other estuarine systems globally against escalating local–global threats.