Tilapias (family Cichlidae) are of importance in aquaculture and fisheries. Hybridisation and introgression are common within tilapia genera but are difficult to analyse due to limited numbers of species-specific genetic markers. We tested the potential of double digested restriction-site associated DNA (ddRAD) sequencing for discovering single nucleotide polymorphism (SNP) markers to distinguish between 10 tilapia species. Analysis of ddRAD data revealed 1,371 shared SNPs in the de novo-based analysis and 1,204 SNPs in the reference-based analysis. Phylogenetic trees based on these two analyses were very similar. A total of 57 species-specific SNP markers were found among the samples analysed of the 10 tilapia species. Another set of 62 species-specific SNP markers was identified from a subset of four species which have often been involved in hybridisation in aquaculture: 13 for Oreochromis niloticus, 23 for O. aureus, 12 for O. mossambicus and 14 for O. u. hornorum. A panel of 24 SNPs was selected to distinguish among these four species and validated using 91 individuals. Larger numbers of SNP markers were found that could distinguish between the pairs of species within this subset. This technique offers potential for the investigation of hybridisation and introgression among tilapia species in aquaculture and in wild populations.

Masculinization is one method of sex reversal technique that can direct the sex of fish. The aim of this research is to know the effect of different concentration ofcoconut waterto percentage of male guppy fish through immersion method.This research was conducted in August - October 2018 at Fisheries Basic Laboratory, Program Study of Aquaculture, Sriwijaya University. This research used completely randomized design (CRD) with four treatments and three replications i.e without coconut water (P0), coconut water 20%(P1), coconut water 30% (P2) and coconut water 40% (P3). The observed parameters were the percentage of male guppy, survival rate and water quality (temperature, pH, ammonia and dissolved oxigen). The result showed the percentage of male guppy was 46.66 % (P0), 70.00 % (P1), 80.00 % (P2) and 83.33 % (P3). Water  quality during maintenance were temperature 20.1 – 28.9 0C, pH 6.5 – 7.2, DO 3 – 6.8 mg/L, and ammonia 0.01 – 0.09 mg/L.  Masculinization is one method of sex reversal technique that can direct the sex of fish. The aim of this research is to know the effect of different concentration ofcoconut waterto percentage of male guppy fish through immersion method.This research was conducted in August - October 2018 at Fisheries Basic Laboratory, Program Study of Aquaculture, Sriwijaya University. This research used completely randomized design (CRD) with four treatments and three replications i.e without coconut water (P0), coconut water 20%(P1), coconut water 30% (P2) and coconut water 40% (P3). The observed parameters were the percentage of male guppy, survival rate and water quality (temperature, pH, ammonia and dissolved oxigen). The result showed the percentage of male guppy was 46.66 % (P0), 70.00 % (P1), 80.00 % (P2) and 83.33 % (P3). Water  quality during maintenance were temperature 20.1 – 28.9 0C, pH 6.5 – 7.2, DO 3 – 6.8 mg/L, and ammonia 0.01 – 0.09 mg/L.

Novel SNP markers revealed species contribution within a hybrid tilapia line (Molobicus) undergoing selection for growth performance in brackish water.• The backcross base population, developed from feral Oreochromis mossambicus and GIFT, matched the predicted 3:1 ratio for O. mossambicus and O. niloticus respectively.• The selected lines showed a significant increase in O. niloticus-specific alleles.