INTRODUCTION
Despite extensive conservation measures, biodiversity loss is unavoidable, and freshwater biodiversity is currently in crisis (Reid et al., 2018; Cooke et al., 2021). The loss of freshwater biodiversity far exceeds that in terrestrial and marine environments (Tickner et al., 2020). Furthermore, freshwater fishes have gone extinct globally at a faster rate than other vertebrates, with freshwater species declining by 83% since 1970 (Dias et al., 2017; Harrison et al., 2018).
Fish diversity has received increasing attention as a result of the global biodiversity crisis. Fish are important consumers in the aquatic food chain (Villéger et al., 2017) and play a significant role in aquatic ecosystems (Su et al., 2021). Changes in the structural composition and distribution of fish communities can reflect the stability and health of aquatic ecosystems (Weigand et al., 2019). Unfortunately, China’s large population base, rapid economic development, and increased resource exploitation have resulted in serious threats to aquatic biodiversity, particularly fish diversity (Hu et al., 2009; Limburg et al., 2011; Chen, 2017).
The Yangtze River is China’s longest river, spanning 6300 km and originating in the Qinghai-Tibetan Plateau before flowing into the East China Sea. According to Ye et al. (2011) and Liu et al. (2021), the Yangtze River has diverse climatic conditions and high habitat heterogeneity. It has one of the most diverse areas for fish species in the Palearctic region with 416 fish species, 178 of which are endemic to the Yangtze Basin (Ye et al., 2011). Unfortunately, the fish population in the Yangtze River has significantly declined in recent decades, with over 90 fish species now endangered or seriously threatened. This decline is primarily due to human activities such as overfishing, habitat loss, climate change, and the introduction of invasive alien species (Blanchet et al., 2010; Ye et al., 2011; Liu et al., 2019; Chen et al., 2020; Su et al., 2021). Iconic species, such as Psephurus gladius and Tenualosa reevesii , are now considered functionally extinct (Zhang et al., 2020a; Zhang et al., 2020b).
The Chinese government implemented several measures to address biodiversity loss in the Yangtze River. These included a seasonal fishing moratorium in the Yangtze River Basin from to 2002–2016 and a one-year fishing ban on 332 aquatic natural reserves in the Yangtze River Basin, including the Chishui River, from 2017 to 2018 (COMA, 2019).
The Chishui River is a first-class tributary of the upper reaches of the Yangtze River, with a total length of 436.5 km and a watershed area of 21,010 km2. The main stream and major tributaries remain free of hydraulic facilities, rendering the river close to its natural state. The Chishui River supports a diverse and abundant fish population, with 143 fish species belonging to eight orders and 24 families. Since 2017, the Chishui River has been under a 10-year fishing ban, as mandated by the Ministry of Agriculture and Rural Affairs of the People’s Republic of China (2016). As of 2020, this ban has been extended to all major natural water bodies in the Yangtze River, as per the State Council of China (2018). Despite this ban, the fish community in the basin has changed significantly over the past decade (Liu et al., 2021). However, the dominant drivers of these changes remain unclear. Therefore, it is critical to investigate the factors that have contributed to changes in the fish communities of the Chishui River.
Using traditional methods to conduct systematic survey monitoring during the fishing ban period has been challenging. As a result, the development of new technologies to monitor fish diversity is critical for conservation. Environmental DNA (eDNA) is a non-invasive technology for detecting and identifying species based on DNA released into the environment. Because of its efficiency and speed, it has become an increasingly popular tool for large-scale aquatic biological surveys (Cowart et al., 2020).
There is no consensus on the factors influencing fish diversity and composition. Natural factors such as climate and geography are frequently thought to influence large-scale fish distribution patterns (Collen et al., 2014; Henriques et al., 2017). However, as human societies have developed, human drivers such as overfishing, exotic species introduction, and urbanization have played an increasing role in shaping fish diversity (Xiong et al., 2015; Geist and Hawkins, 2016; Chen et al., 2020). Given the pressures associated with the China’s rapid socioeconomic growth and changes in ecosystem environments on a spatial scale, determining the contributions of human and natural factors to fish diversity and community composition is crucial.
In this study, we utilized the eDNA method to assess the fish community composition of the entire Chishui River Basin and analyzed the response of fish diversity to drivers in waters dominated by different drivers. We used fish taxonomic units (OTU) to examine the alpha and beta diversities of fish at 52 sites in the Chishui River and tested the relationship between fish and different environmental and human factors to determine the drivers of fish community characteristics.