1. Introduction
Non-native species are a primary threat to global biodiversity and the economy (Early et al., 2016). They can cause biodiversity loss, food web impacts, and economic damage through competitive interactions, predation, parasites, and habitat alteration (Carey & Wahl, 2010; Houde, Wilson & Neff, 2015). Freshwater ecosystems are the most heavily impacted by non-native species, and the most frequent alien group is fish (Carey & Wahl, 2010; Houde, Wilson & Neff, 2015). Fishers, fisheries scientists, and natural resource managers must face the complex task of minimizing any economic and ecological losses due to alien fish invasions (Strayer, 2010; Liu, He, Chen & Olden, 2017; Wan, Jiang & Zhan, 2017; Gu, Yu, et al., 2019).
Thousands of freshwater fish species around the globe have been introduced to new environments either intentionally or inadvertently (Olden, Kennard & Pusey, 2008). In China, hundreds of non-native fish species are used for aquaculture and the ornamental trade (Li, Dong, Li & Wang, 2007; Gu, Hu, Xu, Wei, Luo, et al., 2018). Moreover, rapid trade and economic development in China has placed increasing attention on the need to prevent the establishment of invasive species (Ding et al., 2008; Liu et al., 2019). To date, at least 439 non-native freshwater fish species, representing 22 orders, 67 families and 256 genera, have been recorded in Chinese freshwaters (Xiong, Sui, Liang & Chen, 2015). A portion of the alien fish species have established self-sustaining populations in nature, though fewer have become invasive; even so, successful fish invaders pose serious threats to the country’s economy, human health, and native species (Hu, 2015; Gu, Hu, Xu, Wei, Luo, et al., 2018, Hu, Dong, Hao & Gu, 2020).
The distribution of invasive non-native species in China has a clear geographic and ecological bias, as they appear to be prevalent in the southern provinces, particularly Guangdong, Hainan, Guangxi, and Fujian (Wan, Jiang & Zhan, 2017; Hu, Dong, Hao & Gu, 2020). This phenomenon can also be related to non-native fish species in the region. South China accounts for about 81% of the country’s total aquaculture production of non-native fish, and is a significant trading center for ornamental fish; numerous non-native fish species have been introduced, farmed, sold, discarded, or have escaped in the region (Xiong, Sui, Liang & Chen, 2015; Gu, Hu, Xu, Wei, Luo, et al., 2018). The relatively warm climate and complex network of rivers that allow aquaculture production of non-native fishes also supports their survival and spread in natural waters (Radhakrishnan et al., 2011; Gu, Yu, et al., 2019). According to field records and past distribution data on non-native aquatic species, provided by China’s Ministry of Agriculture and Rural Affairs, approximately 40 non-native fish species are commonly found in natural freshwaters across China ((Li, Dong, Li & Wang, 2007; Hu, 2015; Gu, Hu, Xu, Wei, Luo, et al., 2018; Hu, Dong, Hao & Gu, 2019). However, more than 10 non-native fish species are frequently found in the rivers of South China and many of them can be considered successful invaders, including the Nile tilapia Oreochromis niloticus (Linnaeus), redbelly tilapia Coptodon zillii(Gervais), North African catfish Clarias gariepinus (Burchell), mrigal carp Cirrhinus mrigala (Hamilton), jaguar cichlidParachromis managuensis (Günther), and suckermouth catfish Hypostomus  sp. (Wang, Shi, Ma & Li, 2012; Gu, Mu, et al., 2012; Zhu et al., 2016; Gu, Hu, Xu, Wei, Luo, et al., 2018; Yu, Gu, et al., 2019). To date, systematic monitoring and analysis of the species composition of non-native fishes in South China are still lacking, including determination of the species composition and distribution patterns in particular rivers (Gu, Hu, Xu, Wei, Luo, et al., 2018; Hu, Dong, Hao & Gu, 2020).
Invasion by a non-native species progresses in four stages: introduction, establishment, spread and impact (Lockwood, Hoopes & Marchetti, 2007). This process is influenced by the biological characteristics of a species and the environmental conditions in the new habitat (Kolar & Lodge, 2001; Early et al., 2016). Three key steps lead to establishment of a non-native species: introduction, dispersal (either natural dispersal or anthropogenic spread), and survival (Richardson et al., 2000; Wan, Jiang & Zhan, 2017). The introduction and anthropogenic spread of a non-native fish species usually relates to human activity, and different vectors involve species with different characteristics, the frequency of introductions, number of discrete release events, and number of released individuals also differences, which illustrated as “propagule pressure” (Liu & Li, 2009; Lin, Gao & Zhan, 2015; Xiong, Sui, Liang & Chen, 2015; Gu, Hu, Xu, Wei, Luo, et al., 2018; Xu & Qiang, 2018). The vector of introduction is significant in the ingress of non-native fish species into the rivers of South China. Furthermore, non-native species must overcome multiple barriers to dispersal and ultimately survive in a novel environment, especially as regards the food resources and abiotic conditions of the new habitat (e.g. temperature, salinity, dissolved oxygen, hydrological regime) (Kolar & Lodge, 2001; Lockwood, Hoopes & Marchetti, 2007; Liu, Li, et al., 2014; Early et al., 2016; Gu, Hu, Xu, Wei, Luo, et al., 2018). Although some successful fish invaders can present a rapid realized niche shift in the invaded range, many non-native species survive most easily in an ecosystem that is similar to their native habitat (Shipton, Tweddle, & Watts, 2008; Parravicini et al., 2015; Hill, Gallardo & Terblanche, 2017). Furthermore, a non-native species’ trophic position influences their survival capacity outside their native range in a context of limited food resources (Marchetti, Light, Moyle & Viers, 2004; Gido & Franssen, 2007). To determine what characteristics have promoted the dispersal and survival of non-native fish species in South China rivers, we considered the introduction vector, the native habitat and trophic position of the non-native fish species, and the local fish fauna as important to their current distribution.
Fish distribution patterns are related to variations of species composition and biomass, especially differences among the most common species (Liu, He, Chen & Olden, 2017). Variations in the composition and biomass of fish species are typically impacted by biogeographic conditions, their biological characteristics, the existing environmental conditions, and regional socioeconomic factors (Taniguchi, Rahel, Novinger & Gerow, 1998; Kornis, Sharma & Zander, 2013). While biotic interactions influence fish abundance and assemblage structure in streams, they seem to play only a small role in comparison to the abiotic conditions (Moyle et al., 2003). For many non-native fish species, geographical condition and temperature not only determine their introduction, culture and trade, but also determine their survival, predation, growing, reproduction, and spread in natural waters (Shipton, Tweddle & Watts, 2008; Albert & Crampton,2010; Hill, Gallardo & Terblanche, 2017; Gu, Yu, Yang, Xu, Wei, et al., 2018, Yu, Gu, et al., 2019). We focused on geographic isolation and temperature as potentially two key limitations to the population distribution and biomass of non-native fish species in the rivers of South China, besides the biotic environmental and socioeconomic factors (Gu, Hu, Xu, Wei, Luo, et al., 2018).
In the present study, we record the species composition and distribution patterns of non-native fish species, and explore the potential factors underlying variation in their composition, in eight main rivers of South China, using survey data collected from 2016 to 2018. Specifically, we aimed to: 1) characterize the non-native fish species in the main rivers of South China, including their introduction vector, native habitat, trophic position, and fish fauna; 2) determine differences in the distribution patterns of non-native fish species across the rivers; and 3) highlight whether geographic isolation and temperature strongly supports the non-native fishes’ dispersal and distribution variation.