4.2. Factors influencing the non-native fish species composition
Vectors of fish translocation and introductions include the aquarium trade, aquaculture rearing, fisheries stocks, and biological control (Li, Dong, Li & Wang, 2007; Glozan, 2008; Xiong, Sui, Liang & Chen, 2015; Gu, Hu, Xu, Wei, Luo, et al., 2018). Of the 20 non-native fish species found in South China, 16 are aquaculture species, and 6 of the 7 invasive fish species were introduced for aquaculture rearing (Fig. 2b). Fish inadvertently introduced from aquaculture facilities typically have a strong ability to tolerate a range of environmental conditions, flexible habitat requirements, efficient reproductive strategies, and fast growth, as compared with fish species cultured for the ornamental trade (Canonico, Arthington, McCrary & Thieme, 2005; Martin, Valentine & Valentine, 2010). For example, Nile tilapia is one of the most popular cultured fishes and it has become perhaps the most widely distributed invasive species globally (Zambrano, Martinez, Menezes, & Peterson, 2006; Gu, Yu, et al., 2019). Therefore, vigilant management is needed to prevent the escape of cultured non-native fish species from aquaculture facilities.
All of the non-native fish species found in the eight rivers are native to tropical freshwaters, except for the channel catfish, which is native to North America, and all of the successful invasive fish species in these rivers of South China are native to the tropics (Li, Dong, Li & Wang, 2007; Hu, Dong, Hao & Gu, 2020) (Fig. 2c). The rivers surveyed in this study have the warmest climates in China and their environmental conditions are similar to the native habitats of the non-native fish species (Huang, 2012). Furthermore, among the eight rivers, more non-native fish species were found on Hainan Island (NDJ, WQH, CHJ), where the rivers are tropical and warmer (Table 1). Although the region of the study contains most of the non-native fish species cultured in China, many species native to colder areas, such as largemouth bassMicropterus salmoides , were not found in the rivers. This result suggests that, in this region, alien fish species originating from regions with similar geographical conditions will best survive and spread easily.
Thirteen of the non-native fish species found are omnivores, and 7 are predators (Fig. 2d). Of the 7 successfully invasive species, the North African catfish, marble goby, and jaguar cichlid are effective predators, whereas the Nile tilapia, redbelly tilapia, mrigal carp, and suckermouth catfish are typical omnivores. Fish predators may succeed in a new environment when their prey are not adjusted to their particular style of predation (Moyle & Light, 1996). Omnivores that forage on low-quality resources may succeed because such food resources are rarely limiting in aquatic systems (Moyle & Light, 1996; Gido & Franssen, 2007; Gu, Mu, et al., 2012; Gu, Hu, Xu, Wei, Luo, 2018). Accordingly, a species’ trophic position may be a key factor in its colonization and invasion capacity, and top predators and omnivores are most likely to be successful invaders in the rivers of South China.