Nonnative Fish in the Pearl River Delta: A Comprehensive Study on Their Salinity Adaptability and Potential Distribution Area in Brackish Water

After extensive aquaculture, urbanization, and industrial development, the Pearl River Delta (PRD) has undergone profound environmental changes, with biological invasion emerging as a consequential anthropogenic byproduct, carrying significant implications for natural ecosystems, human health, and the economy. Despite these transformations, our understanding of nonnative fish species and their adaptability to the dynamic environmental conditions within the intricate river network of the PRD remains incomplete. To address this, we conducted a systematic survey of nonnative fish species in PRD waters through literature reviews. Then we analyzed the salinity adaptation of these fish, coupled with a spatial examination of salinity distribution, to comprehensively understand the potential invasion mechanisms of exotic fish in the region. Our findings identified 25 nonnative fish species in PRD waters, with three being translocated species within China and the remaining 22 being exotic introductions. Phylogenetically, these fish are broadly distributed among several orders, with Cichliformes, Perciformes, Cypriniformes, and Siluriformes emerging as the predominant orders. Despite most of the nonnative fish being freshwater species and exhibiting species-specific differences in salinity adaptability, they generally demonstrated an average salinity tolerance of 23.74 ppt, allowing them to adjust to brackish water. The diversity of species 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, featuring a sizable area with brackish water. 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.