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.