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The species identification of larval fish is very important for sustainable fishery resource management. However, identification based on morphological characters is very difficult, complex and error-prone. DNA barcoding with the sequence of cytochrome c oxidase I (COI) gene was used to identify larval fish species from 10 stations in the tributaries of the lower Ing River. One hundred and six samples were collected between May 2016 and April 2017. The average length of the COI nucleotide sequences was approximately 640 bp. A total of 99 nucleotide sequences were identified in 35 species, 31 genera, 19 families and 9 orders, with 97-100% identity with entries in both the GenBank and BOLD databases. The genetic distance within species ranged from 0.000 to 0.004. However, seven samples were identified at only the genus level because their sequences had not been reported in any databases. Based on IUCN conservation status, most species were classified as least concern (77.14%). Approximately 69.23% of all species were related to human uses in fisheries, aquaculture or aquariums, whereas 30.77% of species were not assessed. Trichopsis vittata (family Osphronemidae) (90%) had the most frequency of occurrence, followed by Oryzias minutillus (family Adrianichthyidae) (70%) and Trichopodus trichopterus (family Osphronemidae) (70%).

The species diversity of cichlids was investigated in Kwan Phayao from August 2016 to May 2017. Four cichlid species were found, including Oreochromis niloticus, Oreochromis mossambicus, Coptodon rendalli and Coptodon zillii. Due to similar characterizations, it is very difficult to identify each species. Three molecular methods were used to distinguish these four species. DNA barcodes or partial cytochrome c oxidase I (COI) gene sequences were amplified by PCR and sequenced. In Oreochromis sp. and Coptodon sp., 707- and 704-bp fragments were amplified, respectively. Polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) analysis showed clear differences between the four cichlid species after digestion with three restriction enzymes, ScaI, HindIII and PdiI. ScaI and HindIII separated Oreochromis sp. from Coptodon sp. due to different fragment sizes. PdiI distinguished each cichlid species in the same genus. Finally, high resolution melting (HRM) analysis showed the sensitivity of the primers for discriminating these species with small amplicons and melting curves. From the comparison, HRM analysis was the most efficient method because the primer was shown to be sensitive for discriminating the four cichlids. In addition, it was inexpensive and required a short time to detect large samples. However, direct sequencing or DNA barcodes were still necessary in the case of the COI sequences of organisms of interest, which have not been reported in any databases. These four cichlids are alien species in Thailand; thus, species identification is very important for fishery management.