3.1. Species composition
The total catch from the eight rivers comprised 98,887 individual fish, of total weight 6,525.65 kg. The total catch per river survey averaged 815.70 kg (range 542.62–1,388.86 kg), and the average number of fish caught per river was 12,361 (range 6,579–28,293) (Table 1). The number of fish species caught in each river averaged 84 and ranged from 55 to 136.
A total of 11,832 individual non-native fish, of total weight 1,531.90 kg, were caught in the eight rivers. Twenty non-native fish species were found: Nile tilapia, redbelly tilapia, suckermouth catfish, mrigal carp, North African catfish, marble goby Oxyeleotris marmorata(Bleeker), rohu Labeo rohita (Hamilton), streaked prochilodProchilodus lineatus (Valenciennes), jaguar cichlid, Mozambique tilapia Oreochromis mossambicus (Peters), blue tilapiaOreochromis aureus (Steindachner), red pacu Piaractus brachypomus (Cuvier), sutchi catfish Pangasianodon hypophthalmus(Sauvage), redhead cichlid theraps maculicauda Cichlasoma synspilum (Regan), blackspot barb Dawkinsia filamentosa(Valenciennes), channel catfish Ictalurus punctatus (Rafinesque), redbreast tilapia Coptodon rendalli (Boulenger), galilaea tilapiaSarotherodon galilaeus (Linnaeus), mosquitofish Gambusia affinis (Baird & Girard), and walking catfish Clarias batrachus(Linnaeus).
The 20 non-native fish species collected represent 5 orders, 10 families, and 16 genera (Table 2); the orders most represented were the Perciformes, Siluriformes and Cypriniformes, with 9, 5 and 4 species, respectively (Fig. 2a). Of the non-native species, 16 are used for aquaculture, 3 in the ornamental fish trade, and 1 for biocontrol (Fig. 2b). Seven of the non-native species originate from Africa, 5 from Southeast Asia, 4 from South America, 2 from Central America, and 1 each from South Asia and North America (Fig. 2c). Of the 20 non-native species, 13 are omnivores and 7 are predators (Fig. 2d).
The non-native fishes collected in the greatest proportions were Nile tilapia (46.31%, n = 5,479), redbelly tilapia (28.52%, n= 3,375), jaguar cichlid (7.38%, n = 873), marble goby (6.61%,n = 782), and mrigal carp (4.09%, n = 484). Based on the values of IRI, Nile tilapia was the dominant fish species in seven of the rivers (not in XJ), and could be recognized as an absolute dominant fish species in five of the rivers (NDJ, WQH, CHJ, JJ and MYJ) (Fig. 3). The redbelly tilapia was the dominant fish species in four of the rivers (BJ, DJ, JJ and MYJ), and could be recognized as an absolute dominant fish species in two of the rivers (BJ and DJ) (Fig. 3). Mrigal carp was the dominant fish species in three of the rivers (BJ, DJ and MYJ); marble goby was the dominant fish species in CHJ and WQH; and jaguar cichlid was the dominant fish species in NDJ and WQH (Fig. 3).