4.3. Two factors most affecting distributions of the non-native fish species
The eight rivers surveyed could be divided to two big assemblages, the Continent rivers and Hainan Island rivers, according to the amount ratio, which reflects the geographical differences between continents and islands (Fig. 3). The non-native fish species composition, numbers of species, and dominant species differed between the two assemblages of rivers (Tables 1, 2; Fig. 2). Of the 5-most-common non-native fish species found in the rivers, the amount ratios of 3 species significantly differed between the Hainan Island rivers and Continent rivers (Fig. 5). The presence, survival, and spread of non-native fishes in specific natural waters usually results from intentional introductions, escapes of cultured fish, or disposals and ensuing self-spread (Kolar & Lodge, 2001). This progression may be determined by local aquaculture conditions, market requirements, and policy support related to the geographical situation (Liu, Li, et al., 2014; Lin, Gao & Zhan, 2015). Hainan Island has become one of China’s most important hatchery and culture areas for many non-native fish species owing to its relatively warm temperatures and similarities to the native habitats of the species reared (Fishery Department of the Ministry of Agriculture and Rural Affairs of China, 2019). As such, a greater number of non-native fish species was found on the island than was collected in the rivers on the continent, and several of these, namely the streaked prochilod, sutchi catfish, redhead cichlid and blackspot barb, were found only on Hainan Island (Table 2). The island’s isolation may block the spread of cultured non-native freshwater fishes after they escape or are discarded (Zambrano, Martinez, Menezes & Peterson, 2006; Albert & Crampton, 2010). Thus, it seems that geographical isolation is an important factor distinguishing the species composition between the Hainan Island and the Continent rivers.
The ratio of non-native fish species to total fish species collected allowed further grouping of the Continent rivers into Continent coastal rivers (JJ and MYJ), the Pearl River system 1 (BJ and DJ), and the Pearl River system 2 (XJ) (Fig. 3). Temperature also importantly affected the distribution patterns of the two widely distributed and common species—the Nile tilapia and redbelly tilapia (Figs. 6, 7). A warm climate makes the river basins of Hainan Island and the Continent coastal area important regions for Nile tilapia production (Zhu, Lu & Huang, 2008; Zhou, Wang, Cao, Wan & Li, 2016; Yang, 2016; Fishery Department of Ministry of Agriculture and Rural Affairs of China, 2019; Gu, Yu, et al., 2019). Nile tilapia are indeed the most important cultured freshwater fish in these basins: 84.29% of freshwater fish production on Hainan Island constitutes tilapia farming, and tilapia are also the most important cultured fish in the basins surrounding JJ and MYJ, which produces ~18% of China’s total tilapia production, while the Nile tilapia and its hybrids contribute to >90% of the tilapia production (Zhu, Lu & Huang, 2008; Yang, 2016). The same climatic conditions in these river systems that promotes the culture of Nile tilapia also supports its establishment and spread once it enters natural waters (Gu, Yu, Xu, Wei, Mu, et al., 2018). Furthermore, the Nile tilapia is a strong competitor in the warmer water because of its larger body size and faster grow rate (Gu, Yu, Xu, Wei, Mu, et al., 2018; Gu, Yu, et al., 2019). Nile tilapia was the most common species in both the Continent coastal rivers and Hainan Island rivers, and could be recognized as an absolute dominant fish species in those river assemblages (Figs. 3, 4, 6). However, Nile tilapia is sensitive to temperature, with mortalities increasing at temperatures below 12°C, thus its abundance is less in colder rivers (Gu, Yu, Xu, Wei, Mu, et al., 2018) (Fig. 6). Although the second-most-abundant species, the redbelly tilapia is not popular in aquaculture owing to its small size and slow growth; however, it can tolerate lower temperatures than the Nile tilapia, and was thus more abundant in colder rivers and could be recognized as an absolute dominant fish species in the DJ and BJ (Figs. 3, 7). Previously, Gu, Yu, Xu, Wei, Mu, et al., (2018) similarly found that the distribution patterns of these two most common invasive tilapia species were related to temperature conditions. Likewise, Yu, Gu, et al. (2019) noted similar circumstances for the distributions of the invasive mrigal carp and native mud carp Cirrhinus molitorella .