Nonnative fish in the Pearl River Delta: A comprehensive study on their salinity adaptability and potential distribution area in brackish water

The Pearl River Delta (PRD) has undergone significant environmental changes resulting from anthropogenic activities, with bio-invasion emerging as one of the most consequential byproducts. Despite these apparent transformations, our understanding of nonnative fish species and their adaptability to the environment remains incomplete, compounded by the complexity of the river network. To address this, we conducted a literature survey to screen the nonnative fish species. Subsequently, we analyzed the fish's salinity adaptation capability, spatial variations in alpha diversity, and trophic level. Thirdly, we simulated the distribution of salinity under conditions of salinity intrusion. Our findings identified 25 nonnative fish species, phylogenetically belonging to nine orders, with three being translocated species within China and the remaining 22 being exotic introductions. Although most of the nonnative fish are freshwater species and exhibit species-specific differences in salinity tolerance, they generally can adapt to brackish water, with an average salinity tolerance of 23.74 ppt. The diversity of nonnative fish declined as the relative distance decreased from the upstream of the Pearl River Estuary (PRE) to its mouth in the sea (p < 0.01). Spatial analysis unveiled pronounced heterogeneity in salinity distribution, particularly in the east PRE of the LingDing Bay. Notably, under salinity intrusion circumstances, a gap region between salinity levels of 5.0 ppt and 15.0 ppt, spanning 3111.2 km2, emerged as a potential adaptation zone for nonnative fish. By emphasizing salinity as a key factor, our study may contribute to elucidating invasion mechanisms and enhancing the assessment of invasion risks in estuarine environments.