Nguyen Huu Ninh, Ngo Phu Thoa, Wayne Knibb, Nguyen Hong Nguyen

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1 Accepted Manuscript Selection for enhanced growth performance of Nile tilapia (Oreochromis niloticus) in brackish water (15-0 ppt) in Vietnam Nguyen Huu Ninh, Ngo Phu Thoa, Wayne Knibb, Nguyen Hong Nguyen PII: S (14) DOI: doi: /j.aquaculture Reference: AQUA To appear in: Aquaculture Received date: 30 October 013 Revised date: 19 February 014 Accepted date: 0 February 014 Please cite this article as: Ninh, Nguyen Huu, Thoa, Ngo Phu, Knibb, Wayne, Nguyen, Nguyen Hong, Selection for enhanced growth performance of Nile tilapia (Oreochromis niloticus) in brackish water (15-0 ppt) in Vietnam, Aquaculture (014), doi: /j.aquaculture This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

2 Selection for enhanced growth performance of Nile tilapia (Oreochromis niloticus) in brackish water (15-0 ppt) in Vietnam Nguyen Huu Ninh a, Ngo Phu Thoa a,b, Wayne Knibb b, and Nguyen Hong Nguyen b a Research Institute for Aquaculture No.1, Dinh Bang, Tu Son, Bac Ninh, Vietnam. b Faculty of Science, Health, Education and Engineering, University of the Sunshine Coast, Maroochydore QLD 4558, Australia. *Corresponding author: Telephone ; Fax ; nhninh@ria1.org 1

3 Abstract The main aim of this paper was to report genetic parameters and selection response from a synthetic population of Nile tilapia selected for improved growth performance in brackish water systems in Vietnam. The synthetic base population was formed in 007 from the best performing individuals for growth produced from a complete diallel cross involving three strains of Nile tilapia, namely GIFT (Genetically Improved Farmed Tilapia), Taiwan and NOVIT4 (GIFTderived) strains. Selection was practised for increased harvest weight in brackish water (15-0 ppt) over four generations from 008 to 011. A total of 1,006 individuals had performance data records. They were offspring of 341 sires and 450 dams (averaging 3,000 offspring and 70 sires and 90 dams in each generation). Mixed models fitted to the data included the fixed effects of generation, sex, their two way interaction and a linear covariate of age within sex and generation. The random terms in the model were sire within generation and dam within sire and generation. The estimates of heritability for body traits and survival were moderate to high (0.7 to 0.53). Genetic correlations between harvest weight and body length were high and positive (0.97), whereas those between body traits and survival were low and not significantly different from zero. Genetic gain per generation was measured as estimated breeding values and expressed in actual units (original scale of measurement) and genetic standard deviation unit ( G ). The improvement achieved for harvest weight ranged from 1.1 to 1.6 G after four generations of selection (one year per generation). Selection for increased harvest weight was however accompanied by a non-significant decrease in survival by -0.4%-units or G. The large genetic variation in both harvest weight and survival, however, suggest that there is scope for

4 simultaneous improvement of both traits in this population of Nile tilapia. It is concluded that our selective breeding program has succeeded in developing a productive strain of Nile tilapia under brackish water systems, but the future work should include survival rate in the recording system, selection index and breeding objective. Keywords: genetic parameters, selection response, growth and brackish water. 3

5 1. Introduction Tilapia is one of the important culture species in Vietnam and world wide (FAO, 01). Vietnamese production in 010 was 10,000 tonnes, making up 5% of total aquaculture production in the country (Vietnamese Directorate of Fisheries, MARD 011). Production of tilapia in Vietnam has been mainly practised in freshwater systems, ranging from small scale backyard farming, rice-fish integrated systems to large scale semi-extensive and intensive culture in ponds or in cages installed in rivers and reservoirs. Vietnam is characterised by a long coastal area where a large body of brackish water is available for aquaculture. The presently available breeds of Nile tilapia generally have limited capacity to reproduce, survive and grow in brackish water systems. Only a few other species of tilapia of commercial importance are known to tolerate salinity over 15 ppt (Hopkins et al., 1988; Legendre et al., 1989). The Florida red tilapia hybrid, considered to be one of the high performing tilapia breeds cultured under high salinity (15-30 ppt) in tropical conditions, was derived from crossbreeding a mutant red O. mossambicus with O. aureus, O. niloticus and O. urolepis hornorum. However, this breed suffers from lack of cold tolerance (Watanabe et al., 006) and red tilapia strains generally shows slower growth than Nile tilapia (Santos et al., 01). Salinity tolerance differences were reported among tilapia species and between species of Oreochromis genus. In the Oreochromis genus, O. spilurus, O. mossambicuss and O. aureus show greater degree of salinity tolerance than Nile tilapia (O. niloticus) (Stickney, 1986; Suresh 4

6 and Lin, 199). Significant differences in the growth of five strains of Asian red tilapia were found (Romana-Eguia and Eguia, 1999) when grown in fresh, brackish and salt water. The overall growth of the five strains was more rapid in brackish water (17 ppt) than in either freshwater or salt water (34 ppt). Basiao et al. (005) also reported a significant difference in growth between three strains O. niloticus in both fresh and saline water (3 ppt). A large body of the literature generally indicates that highly euryhaline tilapia species exhibit low growth performance. Attempts have been made to increase the salinity tolerance of Nile tilapia (O. niloticus) by crossing it with high euryhaline species to produce hybrids for aquaculture. Significant variation in growth and survival was observed among 7 tilapia crosses (5 purebred and crossbred), involving euryhaline species (O. spilurus, O. aureus, O. mossambicus) and O. niloticus namely Genetically Improved Farmed Tilapia (GIFT) strain (6 th generation), Freshwater Selected Tilapia (FaST) and super male (YY male) tested in 10 different varying salt culture environments (Tayamen et al., 00). Progenies of O. aureus O. spilurus showed highest growth, whereas survival rate was greatest for O. mossambicus O. spilurus. Lutz et al. (010) also reported that six lines of tilapia of various origins and their cross combinations exhibited highly significant genetic and maternal effects on salinity tolerance. A synthesised result from the literature shows that while between species differences highly suggestive of genetic component to salinity tolerance, there is actually no published data to show if salinity tolerance could be increased by selection. 5

7 There is also a paucity of scientific knowledge regarding genetic variation in salinity tolerance within Nile tilapia strains. Initial experiments were conducted at the Research Institute for Aquaculture No.1 (RIA1), Northern Vietnam, to evaluate the growth and survival of the GIFT and Vietnamese strains of Nile tilapia in fresh and brackish water earthen ponds. The heritability estimates for harvest weight were moderate in both brackish (0.19) and fresh water (0.). The genetic correlations of harvest body weight and survival were relatively low (0.45 and 0.4, respectively) between the two test environments. These results suggest a substantial additive genetic variance for the traits that can be further exploited through a selective breeding program. However, in view of the strong genotype by environment interaction for harvest weight and survival traits observed, separate breeding programs should be considered for Nile tilapia in fresh and brackish water farming (Luan et al., 008). Therefore, we, at RIA1, conducted a selective breeding program to develop a saline tolerant strain of Nile tilapia for brackish water environment. In the present paper, we report genetic parameters and selection response for body weight, standard length and survival recorded in the synthetic population of Nile tilapia (O. niloticus) which has undergone four generations of selection for high growth in 15-0 ppt brackish water from 007 to Materials and methods.1. Origin and establishment of the base population The base population was formed from three strains of Nile tilapia (O. niloticus), namely GIFT (Genetically Improved Farmed Tilapia), Taiwanese and NOVIT4 strains. The GIFT strain 6

8 originated from 106 families selected for high growth rate over six generations in Philippines (Eknath et al., 007; Bentsen et al., 01). The fish were imported to RIA1 in 1997 from the GIFT International foundation Inc., Philippines. The Taiwanese strain of Nile tilapia was introduced from Taiwan to Southern part of Vietnam in 1973 and was then transferred for culture in the North. The Taiwanese stock has been kept in the National Life Gene Bank since The NOVIT4 strain is the GIFT- derivative selected over seven generations for high growth under freshwater environment at RIA1 from 1998 to 006 (Luan et al., 008). In 007, a complete diallel cross involving three strains (GIFT, Taiwanese and NOVIT4 strains) was carried out using parental brooders randomly sampled from each strain. Single pair mating conducted in freshwater hapas resulted in a total of 87 full-sib families, nine to ten families per cross. The progeny were nursed in freshwater environment until fingerling size about 5 to 18 g/fish. They were tagged using PIT (Passive Integrated Transponders) tag. Fingerlings were then acclimatized and gradually transferred from freshwater to brackish water, initially at 9-10 ppt and increased to ppt and 0 - ppt in five days. Fish were closely monitored in hapas during the first three weeks in brackish water pond. The survival was generally moderate (47.4%) to high (96.8%) during this period. Growth testing was conducted in brackish water ponds varying between 15 and ppt over a period of 8 months. At harvest, body traits data were recorded and genetic evaluation was then performed to estimate breeding values for all individuals in the pedigree. The model included the fixed effects of sex, cross combinations and their two way interaction, and the random effect of the additive genetics of individual fish. The best performing (highest EBV) individuals were selected to form the base population (G0) regardless of their genetic make-up. 7

9 .3. Family production of subsequent generations (G1 to G4) In subsequent generations, single pair mating was conducted in separate hapa ( m) to produce paternal full- and half-sibs (one male mated to two females). Before mating, the female and male breeders were conditioned in separate hapas of 00 m (5 0 m) suspended in brackish water ponds of 8 to 10 ppt. The female was transferred to the breeding hapa before the male was introduced. A total number of 50 breeding hapas were installed in a brackish water pond. Fertilized egg or larvae were collected weekly and immediately transferred to hatching trays ( cm) for artificial incubation in brackish water of 14 to 15 ppt. The water temperature during incubation varied between C and 30 C. The collection date of eggs or larvae was recorded for each mating pair. Once the pair mating was successful, a given spawned female was removed to allow the given male to mate with a second female so that production of any given two paternal families was completed usually within two weeks. When yolk-sac was absorbed, the full-sib larvae were transferred to nurse separately in hapas ( m) installed in the same brackish water (14-15 ppt) earthen pond. The same stocking density of 0 fish per m was applied to all families. During the first 3 to 4 weeks of rearing in hapas, the fry were fed five times daily by a commercial powder feed with a dietary protein level of 30%, at the rate of 10% of their body weight. The next nursing period until physically tagging (average body weight of 5 to 18 g/fish) was applied by feeding 30% of protein pellet feed twice daily at 7 am and 4 pm at the rate of 5% of total body weight. When fingerlings reached about 5-18 g, a random sample of fingerlings per family was tagged using PIT tags. Tag identity, standard length and live body weight were recorded for each fish. Data on 8

10 time of mating, spawning, egg or larvae collection, family nursing, survival rate, number of tagged fish were also recorded. After tagging, the fingerlings from different families were pooled together and conditioned - 3 days in hapas of 00 m suspended in earthen ponds. During this period, fingerlings that lost their tags or died were replaced by other individuals from the same family. The difference in survival rate among family after tagging resulted in a large variation in number of individuals tagged per family..4. Performance testing Communal grow-out of all families was conducted in a brackish water pond (3000 m ). The salinity level in brackish water pond fluctuated between 15 and 0 ppt due to variation in ambient temperature during the grow-out period. In each generation, one grow-out pond was prepared according to standard procedures before stocking. Stocking density of fish in the pond was from one to two fish per m of surface water. The fish were fed twice daily a commercial pellet containing 8% protein content at a rate of 5% body weight. Water parameters such as dissolved oxygen, ph, temperature, saline level in the pond were monitored every day. After harvesting, male and female brooders were evaluated as candidates for selection and then conditioned separately for maturation in hapa of 00 m..5. Harvest and measurements Following a grow-out period of about 150 to 10 days, fish were harvested initially using three drags of a seine net, then to complete the harvest, the ponds were drained out the following day, early in the morning. The harvest fish were immediately transferred to large hapas of 00 m (5 9

11 0 m) for one to two days of conditioning without feeding before the individual tag number, body measurements and sex were recorded. They were weighed using a digital scale (nearest to 0.1 g). Standard length was also measured with a ruler. The sex ratio was close to 1:1 (50.8% male and 49.% female). Over the growth test period, the percentage of fish that lost their tags was 1. to.6% across generations. There were no observed differences in body weight between the animals that lost or retained their tags. Survival was recorded as a binary trait and coded as 1 if the fish were still alive or 0 if the fish were absent at harvest. When this trait was expressed as percentage, survival rate was high and averaged from 75.3 to 91.9% across the selection generations. Some mortality occurred due to cold weather or due to extreme draught weather in some years during the course of the selection program..6. Selection and mating The harvest data was entered into a database and genetic evaluation was then conducted to determine estimated breeding values (EBV) for all individuals in the pedigree. The animals with highest EBV for body weight were selected to become parents to produce the next generation. The model used to estimate breeding values included the fixed effects of generation, sex, their two-way interaction and a linear covariate of age by generation and sex subclass. A combined between and within family selection was applied in the selection program. A restricted number of individuals (6-8 females and -4 males) were selected per family to constrain inbreeding. The number of selected females and males were two or three times greater than the actual needs to prevent breeding failure from mortalities during breeding. Inbreeding was restricted by avoiding mating of full-sibs, half-sibs or cousins. Across generations, the average proportions of 10

12 females and males selected were 4.43 and 3.48 % respectively which corresponded to the selection intensities of.38 and.33, respectively. This selection procedure and experimental size were adhered to, as much as possible, in all generations (G0 - G4 corresponding to years 008 to 011) from which the data used for this study were collected. The pedigree structure (number of sires and dams, number of family and progeny in each generation) are presented in Table Quantitative genetic analysis.6.1. Genetic parameters Estimation of genetic parameters for performance data and survival were carried out on a total of 1,006 individual fish produced from 341 sires and 450 dams over four generations of selection from 008 to 011. A preliminary analysis using a general linear model (GLM) was firstly performed to determine significance of fixed effects, using the GLM procedure in SAS (SAS Institute Inc., 005) and also verified by Wald statistics in ASReml (Gilmour et al., 009). The GLM model included the effects of generation, sex and their two way interactions. Age from birth to harvest within generation and sex was fitted as a linear covariate for body traits. A mixed model was fitted to analyse the whole data set to estimate the fixed effects and variance components. The mixed model was written as follows: y iknpq = μ + G i + S k + (G*S) + AGE(G, S) + S n + D p + e iknpq (Equation 1) 11

13 where, y iknpq is an observation for body weight, length and survival of the individual q; μ is the overall mean; G i is the fixed effect of generation (i = 1,, 3, 4); S k is the fixed effects of sex (k = 1, ); AGE is a linear covariate; S n is the random effect of the n th sire; D p is the random effects of maternal and common environment to full-sibs; e iknpq is the random residual effect associated with individual iknpq. A complete pedigree of the experimental fish from G0 onwards ( ) was available to account for genetic relationships among individuals and was used in the analysis to avoid possible bias in the estimation of phenotypic and genetic parameters and to improve their accuracy (Kennedy, 1990). All computations were carried out using the ASREML software package (Gilmour et al., 009). Variance and covariance components were estimated using residual maximum likelihood. Convergence for log-likelihood of variance component estimation was considered satisfactory when two successive rounds of iteration changed by less than 0.1%. All known pedigree information was included in the analyses through a relationship matrix. Sire and dam (the non-genetic component including maternal and common environmental effects) were fitted as random effects. This model used pedigree information to partition the observed phenotypic variance of a trait into various genetic and environmental components hence it enabled the estimation of variance components with minimum bias. Heritabilities for body traits and survival were estimated from a univariate model (Equation 1). Survival was recorded and analysed on the observed (0, 1) scale, using linear mixed model. Threshold models with both logit and probit link functions to analyse survival were not converged. The genetic variance ( A ) was calculated as 4 S where S is sire variance. The 1

14 dam variance component ( ), in this case, was a combination of the maternal, dominant and D common environmental effects, also named as common full-sib effects ( ). The D C "and(dam)" option used in ASReml assumed equal sire and dam variances ( ). The S D phenotypic variance ( ) was calculated as the sum of the additive genetic sire variance ( ), P the dam variance ( ), the common full-sib ( ) and the residual variance ( ), as or h P S A P D C D e P S C e C. Then the heritability was calculated as C. The common environmental effect was calculated as c. Genetic and phenotypic correlations were estimated from a two-trait sire and dam model with the same fixed effects as described above (Equation 1). The correlations were calculated as the covariance 1 divided by the product of the standard deviations of traits: r where was the 1 estimated additive genetic or phenotypic covariance between the two traits, and and 1 are the additive genetic or phenotypic variances of traits 1 and, respectively. Standard errors of all the estimates were obtained directly from ASReml (page 18, release 3.0). As square root transformation of body trait data did not improve genetic parameter estimates, only the results on 1 P e S original scale of measurements are presented..6.. Selection response 13

15 Genetic gains in body traits and survival were measured as changes in estimated breeding values (EBV) using the model that included both animal and fullsib family as the random effects (Equation ). The animal and dam model was used to obtain EBV for all individuals in the pedigree, and expected to result in minimum bias in breeding values estimates. The EBVs calculated for body weight in both original scale of measurements and square root transformation as well as in standardized scale by generation and sex were in good agreement. Hence, the EBV estimates on the observed scale were used and were expressed in both actual unit (original scale of measurement) and in genetic standard deviation unit. The mathematical notation of the model is written as follows: y iknpq = μ + G i + S k + (G*S) + AGE(G, S) + I n + F p + e iknpq (Equation ) where G i, S k and AGE are the same fixed effects as described in equation 1, whereas I n is the additive genetic effect of individual fish and F p is the common full-sib effect of dam. 3. Results 3.1. Descriptive statistics Basic statistics for body traits and survival by generations (spawning years) are given in Table 3. Survival rate recorded over an average grow-out period of 71 days from stocking to harvest varied with generations, being lower in 010 and 011 than in 008 and 009. The coefficient of variation (CV, %) was greater for body weight than for total length (Table 3). 3.. Significance of fixed effects 14

16 Wald F statistics resulted from mixed model in ASReml showed that the main effects of generation, sex and their interaction were all statistically significant (P < 0.001). A linear covariate of age within sex and generation fitted in the model also had significant effects on body traits and survival. As observed in several tilapia populations, males had significantly larger body weight at harvest than that of females (93 ± 8.1 vs. 48 ± 7.8 g). The difference in body weight between the two sexes was 4.8, 16.0, 8.8 and 3.4% in generations 1,, 3 and 4, respectively Heritability estimates Estimated heritabilities for traits studied were moderate to high (Table 4). The fraction of variance due to maternal and common environmental effects was from 3 to 8%. The heritability for survival using sire and dam linear mixed model was high Genetic and phenotypic correlations Genetic and phenotypic correlations between body weight and standard length were positive and high, close to unity (Table 5). The genetic correlation of body weight and length with survival was not different from zero. The sign of the phenotypic correlations was consistent with that of the genetic correlation estimates (Table 5) Selection responses Genetic gain was measured as estimated breeding values (EBVs) for body traits (weight and length) and survival from bivariate and uni-variate animal models including also a fullsib family 15

17 effect, respectively. The EBVs were expressed in actual units of measurements and genetic standard deviation units. The predicted yearly genetic trend for body weight increased steadily from 5.7 g in 008 to 31 g in 011 corresponding to 0.30 to 1.61 genetic standard deviation units, respectively (Table 6). In contrast to body weight, survival rate had a tendency to decline during the course of the selection program, although magnitude of the reduction was small (only - 0.4%-units or genetic standard deviation units after four generations of selection) (Table 6). 4. Discussion Our study demonstrated that genetic selection effectively improved performance of Nile tilapia in brackish water of moderate salinity (15-0 ppt). The improved strain of Nile tilapia developed in the present project for aquaculture in brackish water systems is of practical significance in the context of Vietnam with long coastal line where a large area in Deltas is projected to be affected by salinity intrusion due to sea water rise and changing climate (Allison et al., 009). Survival of the animal from stocking to harvest was relatively high ( %). The gain achieved for harvest weight in the present population is comparable to those reported in the same species (Nile tilapia) from selection programs conducted under favourable freshwater pond environments. The currently selected population still show large genetic variation in characters studied, with the estimates of heritability for body traits ranging from 7 to 53%. The heritabilities obtained in our study were generally higher than those reported for Nile tilapia of GIFT origin in the literature (Eknath et al., 007; Nguyen et al., 007; Nguyen et al., 010; Ponzoni et al., 011; Bentsen et al., 01; Trọng et al., 013). The estimates reported for Nile 16

18 tilapia in the literature ranged from 0.10 to 0.65 (Nguyen, unpublished review). Survival is also an economically important trait for Nile tilapia especially under brackish water systems because it affects the number of fish harvested and marketed, and hence production yield per unit of culture and farmers income. The high heritability for survival in our study also suggests possibilities for improvement of this trait in future breeding programs for this population. The estimate of heritability for survival reported previously by Luan et al. (008) was 0.0 for Nile tilapia also reared in brackish water (8-0 ppt). A number of studies also report high heritability for field survival in Atlantic salmon (Ødegård et al., 006; Lillehammer et al., 013). However, the heritability for this trait reported in other species is low, being around 10% (Rye et al., 1990; Gjerde et al., 004). Consistent with other reports in fish, genetic correlations between body weight and standard length in the present population are high and positive which suggests that body trait measurements were closely genetically correlated and are likely to be controlled by similar sets of genes. Hence, any one of these traits tested could be used upon which to select, on its own or simultaneously. The genetic correlations between body traits and survival were low and not significant in this population. Several studies report low but positive genetic correlations between body weight and survival rate (i.e. favourable) (Gitterle et al., 005; Rezk et al., 009). There are also exceptions of high and positive genetic correlations between growth and survival as reported by Luan et al. (008) and Nielsen et al. (010). Survival may be effectively improved through improved husbandry, management and feeding practices. Santos et al. (01) show that high protein diet remarkedly improved survival rate from stocking to harvest in both Nile and red tilapia. 17

19 Our breeding program for Nile tilapia in salinity water (15-0 ppt) yielded a good response to selection for increased harvest body weight. The yearly improvement from 0.30 to 1.6 genetic standard deviation units ( G ) is comparable with other selection programs for the GIFT strain tested in freshwater pond environments that are more conducive to tilapia culture (Ponzoni et al., 005; Nguyen et al., 010; Hamzah et al., Submitted). The gain obtained in our study was also somewhat higher than that reported for the Egyptian Nile tilapia of 0.48 G by Rezk et al. (009). When the genetic gain was expressed in percentage of the base population mean in 007, it was in the range from 3 to 7.5% across generation. The precent genetic gain in Nile tilapia ranges from 10 to 13.3% in pedigreed population estimated by fitting animal models (Bolivar and Newkirk, 00; Gall and Bakar, 00; Ponzoni et al., 005; Hamzah et al., Submitted). The genetic gain per generation achieved in our current breeding program under brackish water was in line with those reported in aquatic animals generally ranging from 5 to 0% (Gjedrem, 000). Despite the significant improvement in harvest body weight achieved, survival during grow-out had a tendency to decline. However, magnitude of the reduction was small (only -0.9% in actual unit or genetic standard deviation units) and not significant based on the prediction errors. A reduction in survival by -0.07% or genetic standard deviation units was also associated with a selection program for high growth after three generations in Egyptian Nile tilapia (Rezk et al., 009). The long term selection for increased harvest weight in the GIFT strain also resulted in a decline in survival rate by -0.0 to -0.1 G after 10 generations in Malaysia (Hamzah et al., unpublished). These results suggest that there is a need to conduct a routine data recording to incorporate this trait in selection indices and breeding objectives in the 18

20 future breeding program for populations of Nile tilapia, undergoing through selection for high performance. Because determination of economic values for traits of economic importance especially for fitness and functional traits such as survival is difficult, a desired selection index approach can be used to restrict undesired changes in survival rate during grow-out for this species. Our results reported here demonstrate that selection to improve tilapia performance in brackish water environments is effective. Although the genotype by environment (G E) effect was not examined in the present study, the earlier results reported by Luan et al. (008) showed that the magnitude of this effect was large, with a low genetic correlation for homologous body traits between freshwater and brackish water (r G = 0.45). In practical terms, only 45% gain can be captured in production when the animals are selected in one or another environment. Culture of this improved strain of Nile tilapia under freshwater is not the sole objective of our present study. It is, however, likely that the saline-tolerant strain of Nile tilapia developed from our program can also perform well in freshwater systems. In a review of 4 studies across species, Falconer (1990) argued that antagonistic selection (i.e. selection under less favourable environments) may produce genotypes that can perform well across testing environments, whereas synergistic selection (i.e. selection under ideal environments) may result in sensitive genotype. This hypothesis deserves further studies across farmed aquaculture species when resources permit. The selection line originated from this study also constitute a valuable source of genetic materials for subsequent studies to have a better understanding of physiology and genomic aspects of this improved strain as well as mechanisms of osmoregulatory adaptation in Nile tilapia. 19

21 5. Conclusion The genetic gain estimated from the present population of Nile tilapia indicate that significant and sustained genetic progress in the desired direction has been achieved in harvest body weight after four generations of selection under brackish water. The high heritability obtained for body weight also suggests that the population will continue responding to future selection. Due to the antagonistic genetic correlation between weight and survival, a desired gain selection index should be applied to avoid unfavourable changes in survival in the future breeding program for this species. Acknowledgements This project was funded by Vietnamese Ministry of Agriculture and Rural Development. We would like to thank Dr Pham Anh Tuan, Deputy Director of the Vietnamese Directorate of Fisheries for his support and technical advice during the course of the selection program. We also thank Dr Bjarne Gjerde and two anonymous referees for constructive comments that helped improve the manuscript. 0

22 References Allison, E. H., A. L. Perry, M. C. Badjeck, W. Neil Adger, K. Brown, D. Conway, A. S. Halls, G. M. Pilling, J. D. Reynolds, and N. L. Andrew Vulnerability of national economies to the impacts of climate change on fisheries. Fish and Fisheries 10: Basiao, Z. U., R. V. Eguia, and R. W. Doyle Growth response of Nile tilapia fry to salinity stress in the presence of an internal reference fish. Aquaculture Research 36: Bentsen, H. B., B. Gjerde, N. H. Nguyen, M. Rye, R. W. Ponzoni, M. S. Palada de Vera, H. L. Bolivar, R. R. Velasco, J. C. Danting, E. E. Dionisio, F. M. Longalong, R. A. Reyes, T. A. Abella, M. M. Tayamen, and A. E. Eknath. 01. Genetic improvement of farmed tilapias: Genetic parameters for body weight at harvest in Nile tilapia (Oreochromis niloticus) during five generations of testing in multiple environments. Aquaculture : Bolivar, R. B., and G. F. Newkirk. 00. Response to within family selection for body weight in Nile tilapia (Oreochromis niloticus) using a single-trait animal model. Aquaculture 04: Eknath, A. E., H. B. Bentsen, R. W. Ponzoni, M. Rye, N. H. Nguyen, J. Thodesen, and B. Gjerde Genetic improvement of farmed tilapias: Composition and genetic parameters of a synthetic base population of Oreochromis niloticus for selective breeding. Aquaculture 73: Falconer, D Selection in different environments: effects on environmental sensitivity (reaction norm) and on mean performance. Genetical Research 56: FAO. 01. Fisheries Statistics. Gall, G. A., and Y. Bakar. 00. Application of mixed-model techniques to fish breed improvement: analysis of breeding-value selection to increase 98-day body weight in tilapia. Aquaculture 1: Gilmour, A. R., B. Gogel, B. Cullis, R. Thompson, and D. Butler ASReml user guide release 3.0. VSN International Ltd, Hemel Hempstead, UK. Gitterle, T., M. Rye, R. Salte, J. Cock, H. Johansen, C. Lozano, J. Arturo Suárez, and B. Gjerde Genetic (co)variation in harvest body weight and survival in Penaeus (Litopenaeus) vannamei under standard commercial conditions. Aquaculture 43: Gjedrem, T Genetic improvement of cold-water fish species. Aquaculture Research 31: Gjerde, B., B. Terjesen, Y. Barr, I. Lein, and I. Thorland Genetic variation for juvenile growth and survival in Atlantic cod (Gadus morhua). Aquaculture 36: Hamzah, A., R. W. Ponzoni, N. H. Nguyen, H. L. Khaw, H. Y. Yee, and S. A. M. Nor. Submitted. Genetic evaluation of the Genetically Improved Farmed Tilapia (GIFT) strain over ten generations of selection in Malaysia. Journal of Tropical Agricultural Science. Hopkins, K., M. Hopkins, and D. Pauly A multivariate model of tilapia growth, applied to seawater tilapia culture in Kuwait. In: The Second International Symposium on Tilapia in Aquaculture. ICLARM Conference Proceedings:

23 Kennedy, B Use of mixed model methodology in analysis of designed experiments Advances in statistical methods for genetic improvement of livestock. Springer. Pp Legendre, M., S. Hem, and A. Cisse Suitability of brackish water tilapia species from the Ivory Coast for lagoon aquaculture. II-Growth and rearing methods. Aquatic living resources : Lillehammer, M., J. Ødegård, P. Madsen, B. Gjerde, T. Refstie, and M. Rye Survival, growth and sexual maturation in Atlantic salmon exposed to infectious pancreatic necrosis: a multi-variate mixture model approach. Genetics, selection, evolution: GSE 45: 8. Luan, T. D., I. Olesen, J. Ødegård, K. Kolstad, and N. C. Dan Genotype by environment interaction for harvest body weight and survival of Nile tilapia (Oreochromis niloticus) in brackish and fresh water ponds. The proceedings of the 8th International Symposium on Tilapia in Aquaculture. Egypt. Pp 31-38, Lutz, C. G., A. M. Armas Rosales, and A. M. Saxton Genetic effects influencing salinity tolerance in six varieties of tilapia (Oreochromis) and their reciprocal crosses. Aquaculture Research 41: e770-e780. MARD Fisheries statistics. Vietnamese Directorate of Fisheries. Nguyen, N. H., H. L. Khaw, R. W. Ponzoni, A. Hamzah, and N. Kamaruzzaman Can sexual dimorphism and body shape be altered in Nile tilapia (Oreochromis niloticus) by genetic means? Aquaculture 7: S38-S46. Nguyen, N. H., R. W. Ponzoni, K. R. Abu-Bakar, A. Hamzah, H. L. Khaw, and H. Y. Yee Correlated response in fillet weight and yield to selection for increased harvest weight in genetically improved farmed tilapia (GIFT strain), Oreochromis niloticus. Aquaculture 305: 1-5. Nielsen, H. M., J. Ødegård, I. Olesen, B. Gjerde, L. Ardo, G. Jeney, and Z. Jeney Genetic analysis of common carp (Cyprinus carpio) strains: I: Genetic parameters and heterosis for growth traits and survival. Aquaculture 304: Ødegård, J., I. Olesen, B. Gjerde, and G. Klemetsdal Evaluation of statistical models for genetic analysis of challenge test data on furunculosis resistance in Atlantic salmon (Salmo salar): Prediction of field survival. Aquaculture 59: Ponzoni, R. W., A. Hamzah, S. Tan, and N. Kamaruzzaman Genetic parameters and response to selection for live weight in the GIFT strain of Nile Tilapia (Oreochromis niloticus). Aquaculture 47: Ponzoni, R. W., N. H. Nguyen, H. L. Khaw, A. Hamzah, K. R. A. Bakar, and H. Y. Yee Genetic improvement of Nile tilapia (Oreochromis niloticus) with special reference to the work conducted by the WorldFish Center with the GIFT strain. Reviews in Aquaculture 3: Rezk, M. A., R. W. Ponzoni, H. L. Khaw, E. Kamel, T. Dawood, and G. John Selective breeding for increased body weight in a synthetic breed of Egyptian Nile tilapia, Oreochromis niloticus: Response to selection and genetic parameters. Aquaculture 93:

24 Romana-Eguia, M. R. R., and R. V. Eguia Growth of five Asian red tilapia strains in saline environments. Aquaculture 173: Rye, M., K. M. Lillevik, and B. Gjerde Survival in early life of Atlantic salmon and rainbow trout: estimates of heritabilities and genetic correlations. Aquaculture 89: Santos, A. I., N. H. Nguyen, R. W. Ponzoni, H. Y. Yee, A. Hamzah, and R. P. Ribeiro. 01. Growth and survival rate of three genetic groups fed 8% and 34% protein diets. Aquaculture Research: doi: /j x. SASInstituteInc SAS/STAT User s Guide, Version SAS Institute Inc., Cary, NC, USA. Stickney, R. R Tilapia tolerance of saline waters: a review. The Progressive Fish-Culturist 48: Suresh, A. V., and C. K. Lin Tilapia culture in saline waters: a review. Aquaculture 106: Tayamen, M., R. Reyes, M. J. Danting, A. Mendoza, E. Marquez, A. Salguet, R. Gonzales, T. Abella, and E. Vera-Cruz. 00. Tilapia broodstock development for saline waters in the Philippines. Naga, The ICLARM Quarterly 5: Trọng, T. Q., H. A. Mulder, J. A. M. van Arendonk, and H. Komen Heritability and genotype by environment interaction estimates for harvest weight, growth rate, and shape of Nile tilapia (Oreochromis niloticus) grown in river cage and VAC in Vietnam. Aquaculture : Watanabe, W. O., K. Fitzsimmons, Y. Yang, C. Lim, and C. Webster Farming tilapia in saline waters. Tilapia: biology, culture, and nutrition:

25 Table 1: Number of sires, dams and progeny with performance records at harvest by generation and line Generation Population Sire Dam Progeny 007 Base Selection Selection Selection Selection Total

26 Table : Reproduction and management timeline (day-month-year) Generation April May Activities Mating Stocking Grow-out Harvest 06September April 16 June April 10 June March July 007 July 007 Feb 008 0September September November 008 Sept 007 Feb August 009 August 009 Jan August 010 August 010 March August 011 August 011 Feb 01 10February February January 010 0March February 01 5

27 Table 3: Number of observations (N), simple mean, minimum and maximum, standard deviation and coefficient of variation (%) Trait Weight at harvest (g) Length (cm) Survival (%) Generation N Mean Minimum Maximum Standard Deviation Coefficient variation Survival was recorded as a binary trait (0 and 1) and expressed as percentage in Table 3. For survival, minimum value = the lowest survival family and maximun value = highest survival family). (%) Measurements of standard length were not available in generations 1 (008) and 3 (010). 6

28 Table 4: Heritability (± standard errors) and common environmental effects (± s.e.) for body traits and survival Traits h c Weight 0.53 ± ± 0.0 Length 0.7 ± ± 0.0 Survival 0.53 ± ±

29 Table 5: Phenotypic (above) and genetic (below the diagonal) correlations among traits estimated using bivariate animal model Weight Length Survival Weight 0.89 ± ± 0.0 Length 0.97 ± ± 0.0 Survival 0.30 ± 0.16 ns 0.06 ± 0.04 ns ns = non-significant different from zero 8

30 Table 6: Response to selection measured as estimated breeding value in actual unit (±s.e.) and genetic standard deviation ( A ) unit. Year Direct response Correlated responses Weight Length Survival Actual unit A unit Actual unit A unit Actual unit A unit ± ± ± ± ± ± ± ± ± ± ± ± A unit = Actual unit/ A where A is the genetic standard deviation (the square root of the additive genetic variance for corresponding traits given in Table 4)

31 Highlights We study and report genetic parameters and selection response for improved growth performance of tilapia in brackish water. The high heritability obtained for body weight. Significant and sustained genetic progress in the desired direction has been achieved in harvest body weight after four generations of selection under brackish water.