2.1. Sampling locations
Field surveys of fish were carried out from 2016 to 2018 at 50 sites on rivers in three provinces (Hainan, Guangdong, and Guangxi) of South China. Fish were systematically collected from eight main rivers: Wanquanhe River (WQH), Nandujiang River (NDJ), Changhuajiang River (CHJ), Jianjiang River (JJ), Moyangjiang River (MYJ), Dongjiang River (DJ), Beijiang River (BJ), and Xijiang River (XJ) (Fig. 1). The lengths of the sampled rivers are 160 km, 330 km, 232 km, 230 km, 199 km, 500 km, 445 km, and 2,100 km, respectively; the estimated catchment areas of these rivers are 3,693 km2, 7,177 km2, 5,070 km2, 9,464 km2, 6,091 km2, 35,340 km2, 46,710 km2, and 353,120 km2, respectively (Pan, 1991). Each of the basins of the eight rivers has a warm climate, abundantĀ rainfall, diverse native fish species, and flourishing aquaculture operations (Huang, 2012). However, the WQH, NDJ and CHJ are located in the tropics and have relatively warmer temperatures (Huang, 2012). Furthermore, the study rivers are considered to suffer severe invasions of non-native fish species (Gu et al., 2018; Hu et al., 2020).