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In this study we present estimates of phenotypic and genetic parameters for body size measurements, reproductive traits, and gut length for Nile tilapia (Oreochromis niloticus) selected for growth in fertilized earthen ponds for two generations. Throughout the experiment, ponds were fertilized daily with 50 kg dry matter, (dm)/ha chicken manure. No supplementary feeds were added. For the analysis, 6429 fully pedigreed experimental fish from G0, G1 and G2 were used. Generations were discrete and therefore parameters were estimated separately for each year. Heritability estimates for body measurements ranged from 0.4–0.6 for standard length to 0.69–0.79 for head length. Phenotypic correlations between body weight and body measurements ranged from 0.64 to 0.89. Genetic correlations were close to unity. The heritability estimate for maturity at harvest (corrected for sex) was 0.13. Heritabilities for carcass traits were estimated from G1 only and were 0.16 for gutted weight and 0.06 for dressing percentage. Phenotypic correlation between body weight and gutted weight was 0.84 and the genetic correlation was 0.20. Heritability estimate for gut length was 0.22. We also estimated a high genetic correlation between gut length index and standard length (0.78) but a low genetic correlation between gut length index and body weight (0.22 ± 0.28). These results suggest that selection for growth on an herbivorous diet could result in a correlated response in gut length.

In Nile tilapia, breeding programs focus mainly on growth, and information on genetic improvement of fillet yield is scarce. In this study, slaughter data were collected on 1215 tilapia and used to analyze the relationship between body measurements and fillet weight and fillet yield. Fish were obtained from three different origins/strains, and raised in a commercial farm in the Netherlands in closed recirculation systems until a final mean weight of 700 g. Body weight, length, height, width and corrected (= fillet) length were taken prior to slaughter, and used to predict fillet weight and fillet yield using linear regression models. Average fillet yield was 35.7% with large differences between strains (range 34.4-38%). There was a strong almost linear relationship between body measurements and fillet weight, but relationships with fillet yield were weak. R-2 of the regression model for fillet weight was 0.95 and the correlation between observed and predicted values of fillet weight 0.98. The effect of strain/origin was significant for each body measurement. The effect of sex and strain x sex was significant for length and corrected (= fillet) length. The fillet yield model explained 15% of the observed variance; the correlation between observed and predicted fillet yield was 0.38, but there were large differences within strains. We conclude that in Nile tilapia, predicting fillet yield based on body measurements is possible, but correlations can be improved if more accurate methods for measuring body width become available. (C) 2004 Elsevier B.V. All rights reserved.

We have generated a unique resource consisting of nearly 175 000 short contig sequences and 3569 SNP markers from the widely cultured GIFT (Genetically Improved Farmed Tilapia) strain of Nile tilapia (Oreochromis niloticus). In total, 384 SNPs were selected to monitor the wider applicability of the SNPs by genotyping tilapia individuals from different strains and different geographical locations. In all strains and species tested (O. niloticus, O. aureus and O. mossambicus), the genotyping assay was working for a similar number of SNPs (288-305 SNPs). The actual number of polymorphic SNPs was, as expected, highest for individuals from the GIFT population (255 SNPs). In the individuals from an Egyptian strain and in individuals caught in the wild in the basin of the river Volta, 197 and 163 SNPs were polymorphic, respectively. A pairwise calculation of Nei's genetic distance allowed the discrimination of the individual strains and species based on the genotypes determined with the SNP set. We expect that this set will be widely applicable for use in tilapia aquaculture, e.g. for pedigree reconstruction. In addition, this set is currently used for assaying the genetic diversity of native Nile tilapia in areas where tilapia is, or will be, introduced in aquaculture projects. This allows the tracing of escapees from aquaculture and the monitoring of effects of introgression and hybridization.

This study presents results of two generations of selection (G1 and G2) for growth of Nile tilapia. The selection environment consisted of earthen ponds which were fertilized daily with 50 kg dry matter (dm)/ha chicken manure. No supplementary feeds were provided. In total, 6429 fully pedigreed experimental fish were included in the analysis. Survival till harvest was highly variable ranging from 35% to 77% and was affected by initial weight, pond, and age effects. Body weight at harvest (BW) increased from a mean of 67.4 g in the grandparental (unselected) population (G0) to 129.5 g in G2 was affected by initial weight, pond, sex and age effects. Generations were discrete and therefore genetic parameters were estimated separately for each year. Heritability estimates for BW ranged from 0.38 to 0.60, and the heritability for survival ranged from 0.03 to 0.14. The estimated selection response was 23.4 g (34.7%) between G0 and G1 and 13.0 g (14.9%) between G1 and G2. These results demonstrate the feasibility of selection for growth of Nile tilapia in low-input environments.

Fillet weight is an economically important trait in Nile tilapia production for the European market which asks for fish with average body weights of at least 700 g. Genetic parameters to design or optimize breeding programs for these body weights are lacking. In an earlier study we showed that high phenotypic correlations exist between body measurements and fillet weight and low phenotypic correlations exist between body measurements and fillet yield. To evaluate the potential of mass selection for fillet traits, however, genetic parameters are required. The aim of the current study was to estimate genetic parameters for body weight, fillet weight, fillet yield and body measurements. Therefore, slaughter data was collected on 1884 pedigreed Nile tilapias. Measurements of body weight, length, width, corrected (=fillet) length and head length were taken on each fish. Filleting machines were used to fillet fish and measurements of fillet weight, fillet yield and head weight were collected. Subsequently, genetic parameters were estimated. Heritabilities were 0.26 for body weight, 0.24 for fillet weight and 0.12 for fillet yield. The genetic correlation between body weight and fillet weight was 0.99 and between body weight and fillet yield 0.74. The genetic correlation between fillet weight and fillet yield was 0.81. Genetic correlations between fillet weight and body measurements were 0.89 for length, 0.70 for head length, 0.94 for width and 0.91 for corrected length. Genetic correlations between fillet yield and body measurements were 0.62 for length, 0.47 for head length, 0.98 for width and 0.60 for corrected length. The potential of mass selection aiming to improve fillet weight was evaluated by a number of selection indexes. The accuracy of a selection index including only body weight indicated that in this way almost the same amount of the selection response could be achieved compared to what hypothetical direct selection for fillet weight would. The use of only width in the selection index would result in 8.5% lower selection response than the use of body weight. We conclude that body weight is the best predictor for fillet weight compared to body measurements.

Four domesticated strains of Nile tilapia (Oreochromis niloticus L.) were genetically characterized using 14 microsatellite markers and 64 animals per strain. Two strains, Chitralada (AIT) and International Development Research Centers (IDRC) were obtained from the AIT institute, Bangkok, Thailand. The GIFT strain (5th generation) came from NAGRI, Thailand, and the GOTT strain was supplied by the University of Göttingen, Germany. The average numbers of alleles per marker were 5.0 (GOTT), 5.4 (AIT), 5.6 (IDRC) and 7.5 (GIFT). Private alleles were found at all markers with the exception of two. No fixation of alleles was found at any marker. Population differentiation, FST, was 0.178 (great genetic differentiation) and confirmed grouping of the animals in strains. The expected level of heterozygosity ranged from 0.624 to 0.711, but the observed level of heterozygosity significantly deviated from the expected level in three strains. This was probably because of small population size. Moderate to great genetic differentiation was found between strains. A phylogenetic tree reflected the strains known histories. Application of the Weitzman approach showed that all strains have added value for the total genetic diversity and thus should be retained.