Introduction
1.1 Aquaculture development and its contribution to food security worldwide
Aquaculture, a practice with origins dating back 2000 years in China and during the Roman Empire, has seen significant global expansion in recent decades, with a global output of 51.7 million tonnes in 2006, accounting for 47% of world fisheries production valued at US$ 78.8 billion China led as the top aquaculture producer, followed by India and Vietnam To meet the projected global food fish demand by 2030, aquaculture production must increase by 30-40 million tonnes, as wild capture fisheries have reached a plateau Given the limited space for aquaculture development, enhancing production efficiency within existing areas is crucial Unlike terrestrial livestock, where domestication and stock improvement have proven effective in boosting productivity, similar advancements in aquaculture species remain challenging.
1.2 Roles of aquaculture in Vietnam
Vietnam's aquaculture sector has experienced remarkable growth over the past decade, expanding from 614,900 hectares in 2000 to 1.008 million hectares by 2007 Total fisheries production, which includes both wild catch and aquaculture, surged from 1.782 million tonnes in 1998 to 4.149 million tonnes in 2007, with aquaculture growing at an impressive average rate of 19.4% per year, significantly outpacing the 4.82% annual growth of wild capture fisheries This sector is vital to Vietnam's economy, contributing significantly to foreign currency earnings through exports, ranking fourth in total export value Additionally, Vietnam holds the eighth position globally for fisheries product exports, achieving an average growth rate of 20.9% from 1996 to 2006 Consequently, fish and fisheries are essential commodities that are crucial for Vietnam's economic development.
Figure 1 Fisheries production in Vietnam, 1998-2007 period (General Statistics Office, 2008)
Total Marine and brackish w aterFreshw ater
1.3 Lifecycle, natural contribution and translocation of giant freshwater prawn ( GFP )
The giant freshwater prawn, Macrobrachium rosenbergii, is the largest species in its genus, with males reaching up to 320 mm and females up to 250 mm in length While adult prawns inhabit freshwater environments, females migrate to estuarine areas for their larvae, which require brackish water for early survival and development Mating occurs year-round in the wild, with peaks during the rainy season in tropical regions and summer in temperate climates The larvae hatch as planktonic organisms and undergo 11 metamorphic stages in brackish waters before becoming post-larvae Although the dispersal ability of this species in marine environments is not fully understood, larvae can survive in 100% artificial seawater until the post-larval stage, suggesting limited potential for marine dispersal.
2003) Following metamorphosis, post-larvae migrate upstream towards freshwater Post-larvae are omnivorous and can also be cannibalistic (New, 2002)
The giant freshwater prawn (GFP) is native to South and Southeast Asia, ranging from Pakistan to southern Vietnam, and is also found in northern Oceania and some western Pacific islands Recently, GFP has been introduced for research and aquaculture in various non-native regions, including countries in the Americas, Africa, Europe, and parts of Asia like China and Taiwan Currently, GFP is being farmed in over 43 countries worldwide.
1.4 Giant freshwater prawn farming (worldwide and in Vietnam)
The global status of GFP was comprehensively reviewed by New in 2000 and updated in
2005 Global aquaculture production of GFP has increased dramatically from 5,246 tonnes in
From 1984 to 1998, global aquaculture saw a remarkable growth in the GFP culture industry, expanding at an annual rate of over 27% between 1992 and 2001, significantly outpacing marine shrimp culture's growth of just 3.9% per year By 2010, the farmed output of major freshwater prawn species, such as M rosenbergii and M nipponense, was projected to reach between 700,000 and 1 million tonnes In 2001, the leading producers of farmed GFP were China, with 128,300 tonnes, followed by Vietnam at 28,000 tonnes, and India at 24,200 tonnes, although Vietnam's actual production figures were somewhat uncertain due to classification issues with FAO data.
In 2005, China emerged as the leading global producer of GFP, with India, Thailand, Bangladesh, and Taiwan following closely behind; however, Vietnam also plays a significant role in production, despite its output not being included in FAO statistics By 2007, the worldwide farmed GFP volume surpassed 213,000 tonnes, boasting a market value of over US$ 943 million, according to FAO reports from 2009.
The Mekong Delta of Vietnam produces a significant amount of Giant Freshwater Prawn (GFP), with 300 million post-larvae (PLs) imported for aquaculture While GFP production primarily targets the domestic market, only 311 tonnes, valued at approximately US$ 2.3 million, are exported (VASEP, 2008) Key production systems include rotational rice-prawn and pen culture models, noted for their productivity and economic efficiency, though they require substantial investment and technical expertise, making them suitable mainly for large-scale farmers (Son et al., 2005; Phuong et al., 2006) Conversely, the integrated rice-prawn culture model, while less productive, is more accessible for small-scale and poorer farmers due to lower financial requirements (Lan et al., 2006) With over 320,000 hectares of rice fields available, the Mekong Delta holds significant potential for expanding rice-prawn culture GFP is recognized as a priority species for development in Vietnam, with an aquaculture production target of 60,000 tonnes by 2010 across 32,000 hectares (Government of Vietnam, 1999).
Figure 5 Rotational rice-prawn culture model
Despite the global proliferation of GFP for commercial aquaculture, the genetic quality of cultured stocks has received insufficient attention Most stocks outside Asia originated from a limited broodstock, with only 12 prawns initially imported to Hawaii in the 1960s to establish the first culture stock These progeny were later exported to various non-indigenous regions, leading to widespread GFP cultivation However, after years of relying on unimproved stocks, many countries have reported declining productivity, raising concerns within the industry For instance, Taiwan's commercial culture stocks saw a drop from 16,000 tonnes to 8,000 tonnes in the early 1990s, primarily due to inbreeding depression Moreover, sourcing broodstock from grow-out ponds instead of the wild has led to increased inbreeding, which is believed to be a significant factor in the decline of growth performance in Thai culture stocks.
Selecting broodstock based on their readiness to spawn can negatively impact harvest weight and may lead to the use of smaller females as breeders in hatcheries, which could unintentionally reduce marketable size across generations (Mather and de Bruyn, 2003) The authors also note that while the genetic attributes of cultured stocks globally remain largely unknown, various factors indicate that genetic diversity is at risk.
M rosenbergii populations are experiencing a decline in genetic variation within culture stocks, largely attributed to hatchery management practices and the escape of cultured stocks into the wild, which negatively impacts wild diversity This issue has been documented in species such as Atlantic salmon and tilapia Additionally, deformities like white tail and appendage diseases are prevalent in global GFP industries Addressing these challenges is crucial for the aquaculture sector, emphasizing the need for systematic genetic improvement of GFP culture stocks.
1.6 Strategies of genetic improvement in aquaculture species
Systematic artificial selection has proven to be an effective method for enhancing the productivity of farmed terrestrial animals, with significant advancements seen since the 1940s Selective breeding programs have led to remarkable increases in growth rates, with pigs experiencing doubled growth, and dairy cows and hens producing up to 2.5 times more milk and eggs, respectively (Gjedrem, 2005a) For instance, the average weight of broiler chickens has increased to 3.5 times that of 1957 levels by 1991 (Havenstein et al., 1994) Today, the high yields in land animal production are largely attributed to the development and cultivation of genetically improved breeds (Gjedrem, 1997).
Despite the significant potential for genetic improvement, breeding programs for aquatic species have only recently gained traction, even though fish and shellfish exhibit a higher selection response compared to terrestrial farm animals (Olesen et al., 2003) This is largely due to the extensive loss of natural genetic diversity in terrestrial livestock over centuries of domestication, while aquatic species remain largely underutilized Historically, most artificial selection efforts have focused on finfish, with recent large-scale breeding initiatives targeting salmonids, such as Atlantic salmon (Gjedrem, 2000; Quinton et al., 2005) and Coho salmon (Hershberger et al., 1990; Neira et al., 2005).
Genetic improvement programs for species such as channel catfish and Nile tilapia have demonstrated significant advancements, with growth rate enhancements ranging from 10% to 20% per generation—far exceeding the gains seen in traditional livestock like cattle, pigs, and sheep Specifically, the economic benefits of selective breeding for Nile tilapia can reach between US$ 4 million and US$ 32 million, with impressive benefit-to-cost ratios of 8.5 to 60 This cost-effectiveness is largely attributed to the heritability of genetic gains, ensuring that improvements in one generation are passed down to the next.
Despite some progress in fish stock improvement, breeding programs for crustaceans have been limited due to the unique characteristics of these species One significant challenge is the complexity of their lifecycles, which involve multiple larval stages and often lack adequate rearing conditions (Jerry et al., 2001) A key advancement for green-fingered producers was the discovery that larvae need brackish water to survive past five days, enabling successful rearing through all stages (New, 2000) Additionally, the simultaneous production of progeny from numerous parents and the control of environmental variables to reduce growth variation between sexes pose further difficulties The absence of suitable external marking tags for most species, due to their moulting process, complicates breeding programs that require recognizable individual or family lines to accurately estimate breeding values.
Genetic parameter estimates in crustaceans can be biased by maternal or environmental effects, impacting selective breeding programs initiated for commercially important species such as penaeid shrimp and freshwater crayfish Selective breeding trials have demonstrated a significant response to selection for growth rate, achieving approximately 10% gains per generation While hybridization has been widely successful in crops like maize, it has also been applied in aquaculture to enhance production traits, exemplified by commercial hybrids of channel and blue catfish, as well as hybrid common carp Although interspecific hybrids may not always exhibit heterosis for desired traits, they can be valuable if they inherit advantageous characteristics from both parent species For instance, the hybrid catfish in Thailand combines the rapid growth of the African catfish with the desirable flesh quality of the Thai species Similarly, the hybrid between rohu and catla carp showcases the small head of rohu while growing as quickly as catla However, maintaining pure species for hybridization and the unpredictability of crossbreeding outcomes pose significant challenges in this breeding strategy.
1.6.3 Integration of molecular markers into a breeding program
Evaluation of growth performance in a diallel cross of three strains of giant
three strains of giant freshwater prawn ( Macrobrachium rosenbergii ) in Vietnam
- Presented at Australasian Aquaculture conference ‘ Innovation in a global market’, 03-06 August 2008, Brisbane, Australia
The current study utilized a crossbreeding approach among diverse GFP strains to enhance culture productivity, leveraging hybrid vigor for immediate performance improvements A complete 3x3 diallel cross was conducted, involving two wild Vietnamese strains from the Dong Nai and Mekong rivers and a domesticated Hawaiian strain originally from Malaysia Chapter 2 assessed the growth performance of three purebred strains and six reciprocal crosses, highlighting the typical sexual dimorphism in GFP, where males generally exhibit faster growth than females The analysis compared the growth performance of males and females across different crosses to evaluate the influence of genetic background on growth traits, revealing evidence of improved growth rates in crossbred strains compared to their purebred counterparts.
Evaluation of growth performance in a diallel cross of three strains of giant freshwater prawn ( Macrobrachium rosenbergii ) in Vietnam
Nguyen Minh Thanh a,c,* , Raul W Ponzoni b , Nguyen Hong Nguyen b , Nguyen Thanh Vu c ,
Andrew Barnes and Peter B Mather are affiliated with various esteemed institutions, including the Centre for Marine Studies at the University of Queensland in Brisbane, Australia, and the WorldFish Center in Penang, Malaysia Additionally, they are connected to the Research Institute for Aquaculture No 2 in Ho Chi Minh City, Vietnam, and the School of Natural Resource Sciences at Queensland University of Technology in Brisbane, Australia.
The giant freshwater prawn (Macrobrachium rosenbergii) is a key species in inland aquaculture across tropical and subtropical regions This study aimed to assess the growth performance of three strains from distinct geographic origins to enhance stock improvement in Vietnam Crosses were created over two generations using two domesticated Vietnamese river populations (Dong Nai and Mekong) and a long-cultured Hawaiian strain After 15 weeks of growth in hapas, results indicated that the Hawaiian strain outperformed the purebred strains, with specific cross combinations showing significantly greater growth Notably, certain crosses involving Dong Nai or Mekong as dams and the Hawaiian strain as sires achieved higher mean body weights than their purebred counterparts Males exhibited heavier weights than females, though social factors influenced male morphotype variations The findings highlight the potential for heterosis and the importance of cross direction, suggesting opportunities for crossbreeding or the development of a composite population for future selection The study also addresses potential challenges encountered during the trials.
Keywords: Macrobrachium rosenbergii, stock improvement, diallel cross, heterosis.
The giant freshwater prawn, a crucial crustacean species in inland aquaculture, is widely produced in tropical and subtropical countries, with a global farmed volume of over 194,000 tonnes in 2004, valued at over US$ 810 million, predominantly from Asia However, this figure is likely an underestimate, excluding Vietnam's production, which significantly increased from 3,000 tonnes per year in the 90s to over 10,000 tonnes by 2002 As a result, the giant freshwater prawn has become a high-priority species for development in Vietnam, with an ambitious target of 60,000 tonnes reared in 32,000 ha by 2010.
M rosenbergii has been widely distributed across at least 43 countries for research and commercial aquaculture Despite its global presence, the genetic characteristics of cultured stocks have received little attention Most broodstock originate from the ‘western’ form collected in Malaysia and later moved to Hawaii, leading to concerns over declining productivity due to inbreeding and the selection of early-maturing, smaller females for breeding This practice negatively impacts the weight at final harvest While global aquaculture of M rosenbergii has grown rapidly, genetic stock improvement strategies remain largely unutilized Therefore, systematic breeding programs are essential to enhance economically important traits in this species.
Significant productivity advances have been achieved via selective breeding programs in
Selective breeding programs have been extensively developed for fish species such as Nile tilapia (Oreochromis niloticus) and Rohu carp (Labeo rohita), while only a limited number of commercially important crustacean species, like certain marine penaeid prawns and freshwater crayfish, have undergone similar initiatives Despite the scarcity of selective breeding in crustaceans, notable progress has been made, achieving approximately 10% genetic gains per generation in fast growth rates.
An effective method to enhance the productivity of cultured stocks is through crossbreeding, which includes both intraspecific crossbreeding and interspecific hybridization, to harness the benefits of heterosis or hybrid vigor in the offspring This technique is widely adopted in the horticulture and grains sectors to boost the yields of essential crops (Lamb, 2000) Although its application in aquaculture has been limited, there are notable instances of hybrid crosses demonstrating superior performance compared to pure lines, such as the common carp, Cyprinus carpio (Bakos and Gorda, 1995; Hulata).
Crossbreeding, particularly through diallel crossing, has been effectively utilized to transfer beneficial traits among strains and to create a genetically diverse synthetic base population before starting breeding programs, as seen in the GIFT project (Bentsen et al., 1998) This method is relatively simple and cost-effective for enhancing local strains compared to selective breeding However, while selective breeding programs may require significant investment—such as the annual US$ 3 million for Atlantic salmon breeding in Norway, which yields an estimated US$ 45 million in profit (Gjedrem, 1997)—they can offer substantial long-term economic benefits The genetic improvement initiatives for Nile tilapia have also played a crucial role in advancing individual aquaculture industries and tailoring breeding objectives for specific cultured species.
The giant freshwater prawn (M rosenbergii) is naturally found across South and Southeast Asia, from Pakistan to southern Vietnam, including parts of northern Oceania and some western Pacific islands While adult prawns inhabit freshwater environments, females migrate to estuarine areas for their larvae, which require brackish water for early survival Research indicates that GFP larvae have limited capacity for marine dispersal Studies have revealed significant genetic diversity in wild M rosenbergii populations, suggesting high spatial variation among major river drainages This genetic diversity presents an opportunity to enhance breeding programs, potentially leading to the development of improved lines for the aquaculture industry.
This study explores the potential for hybrid vigor among different strains of the green freshwater prawn, M rosenbergii, collected from distinct drainage basins in southern Vietnam Two new culture strains were developed from wild populations, and a diallel cross was established with a third strain, ‘Anuenue,’ sourced from Hawaii and originally from Malaysia The main objectives were to assess the performance of individuals from various genetic backgrounds and evaluate the outcomes of all possible reciprocal crosses.
2.1 Collection of wild GFP populations and broodstock conditioning
Wild M rosenbergii populations were collected from two discrete drainage systems in
In November 2005, Vietnam initiated the establishment of founder stocks for developing new culture strains of prawns The first population was sourced from the Dong Nai River, while the second was collected from the Mekong River, which features a vast geographical basin To ensure genetic diversity, juvenile prawns were gathered from three widely dispersed sites across Vietnam The collection process was conducted at night, from 9 PM to 6 AM, to maintain optimal dissolved oxygen levels After two months, healthy adult prawns were selected and transferred for conditioning as broodstock.
For two months, male and female specimens from each strain were kept in separate hapas for conditioning, receiving a diet of 40% protein commercial prawn pellets twice daily, supplemented every other day with chopped beef liver or squid (Daniels et al., 2000) The Hawaiian strain, which has been cultivated outside Southeast Asia for over 30 years and originated from Malaysia, was introduced to Vietnam in early 2000 (New, 2000).
In 2005, juveniles were used for on-farm trials in the Mekong Delta, with broodstock collected at the conclusion of the trials from a local hatchery in An Giang province The initial collection numbers for each strain are detailed in Table 1.
Table 1 Collection sites, date and number of individuals collected to established Vietnamese foundation strains.
D Dong Nai River 10º56’N 106º49’E 4 Nov 2005 437 juveniles
Note: (i) Juvenile: sub-reproductive individual
(ii) Adult: sexually mature individual
(iii) Breeder: individuals selected as broodstock for breeding experiments
2.2 Mating design and family production
The study was carried out at the National Breeding Center for Southern Freshwater
Aquaculture at the Research Institute for Aquaculture No 2 in Vietnam focuses on breeding practices using wild Dong with orange-colored ovaries Healthy males with long, dark blue claws are selected for mating with receptive females, which are isolated in submerged baskets Successful mating typically results in egg-laying within 1-3 days Males are removed after females become berried, and these females are monitored every three days for egg development If fertilization fails, new pairs are introduced Fertilized females are kept in brood baskets for 15-18 days until the eggs turn grey, at which point they are transferred to hatchery containers for hatching Larvae from each strain are reared in containers with a stocking density of 60 larvae/l, utilizing an open water system with daily water exchanges of 20-40% The larvae are initially fed newly hatched brine shrimp nauplii for the first 10 days, followed by a mix of brine shrimp nauplii and a specially formulated feed.
In the G1 generation, a total of fifty-eight full-sib families were generated, ranging from one family for the Hawaiian x Dong Nai strain to thirteen families for the Hawaiian x Hawaiian strain During development, some females experienced egg clutch abortions The first larval family was recorded on April 29, 2006, with the last observed on June 4, 2006, indicating that all G1 larvae were produced within a 37-day period.
Table 2 Number of dams and sires for each strain combination in G1
Note: (i) D = Dong Nai, M = Mekong, H = Hawaiian
(ii) 1 st figure = number of pairs set up; 2 nd figure in brackets = number of successfully spawned females
2.2.2 Generation 2 (G2) families were produced in the G2 (Table 3) The age difference between the first and last larval family was 40 days
Table 3 Number of dams and sires for each strain combination in G2.
Note: (i) D1 = Dong Nai x Dong Nai G1, M1 = Mekong x Mekong G1, H1 = Hawaiian x Hawaiian G1
(ii) 1 st figure = number of pairs set up; 2 nd figure in brackets = number of successfully spawned females
Estimates of strain additive and non-additive genetic effects for growth traits in a
- Presented at The 10 th International Symposium on Genetics in Aquaculture ‘Roles of aquaculture genetics in addressing global food crisis’, 22-26 June 2009, Bangkok, Thailand
- Submitted to Aquaculture (in review)
Chapter 2 revealed potential heterosis in specific crosses, indicating that the direction of crossing influences outcomes Building on these findings, Chapter 3 conducted a thorough analysis of growth data to confirm heterotic effects on growth traits in experimental GFP The observed growth variation stemmed from both strain additive and non-additive genetic effects, which were evaluated using a mixed model These results provided a foundation for recommending an effective breeding strategy for GFP in Vietnam.
Estimates of strain additive and non-additive genetic effects for growth traits in a diallel cross of three strains of giant freshwater prawn ( Macrobrachium rosenbergii ) in Vietnam
Nguyen Minh Thanh and colleagues from various institutions, including The University of Queensland and The WorldFish Center, conducted research on marine studies and aquaculture Their collaborative efforts involved experts from Australia and Vietnam, focusing on sustainable practices in aquatic resource management This multidisciplinary team aims to enhance knowledge and strategies for improving fisheries and aquaculture systems in the region.
A study on the giant freshwater prawn (Macrobrachium rosenbergii) in Vietnam assessed additive genetic, heterotic, and strain reciprocal effects through a diallel cross involving local wild strains from Dong Nai and Mekong, alongside a newly introduced Hawaiian strain The findings revealed that strain additive genetic and reciprocal effects were more significant than heterotic outcomes for growth traits The Hawaiian strain exhibited the highest strain additive genetic effect for harvest body weight at +10.2%, while the Mekong strain showed the lowest at -11.6% Heterotic effects were minimal and statistically insignificant, ranging from +0.7% for carapace length to +1.5% for body weight Notably, reciprocal crosses involving the Hawaiian strain as sire resulted in faster growth rates compared to their counterparts The strong correlation between strain additive performance and overall performance (r=0.71 to 0.77) indicates the potential for enhancing M rosenbergii culture stock through direct selection The article also discusses breeding strategies for the genetic improvement of giant freshwater prawns in Vietnam.
Keywords: Macrobrachium rosenbergii, diallel cross, additive genetic effect, heterosis, reciprocal effect
The giant freshwater prawn (Macrobrachium rosenbergii) is a highly valued decapod crustacean in inland aquaculture, yet most global stocks remain wild and undomesticated The sustainability of GFP farming is threatened by low production efficiency and disease susceptibility, as evidenced by recent productivity declines in Thailand and Taiwan Notably, white tail and muscle diseases have been reported in various regions, including Taiwan, China, and Thailand This situation underscores the urgent need for systematic stock improvement programs to enhance economically important traits in M rosenbergii aquaculture.
Crossbreeding in aquaculture, encompassing both intraspecific and interspecific hybridization, leverages hybrid vigor to enhance the performance and fitness of offspring compared to their purebred counterparts This method serves as an effective strategy for boosting the productivity of farmed finfish and shellfish In the United States, the catfish industry generates over 300,000 tonnes annually, with 25-30% of this production derived from a hybrid of channel and blue catfish Similarly, in Thailand, over 80% of catfish farmers utilize hybrid catfish (Clarias macrocephalus x C gariepinus) due to their rapid growth rates and high resistance to diseases.
In addition, crossbreeding combined with selective breeding of common carp in Hungary has produced three hybrids that show high positive heterosis compared with pure lines (Bakos et al.,
Several studies have indicated a lack of observed heterotic effects in crossbred strains, with some only reporting results up to the larval stage (Sarver et al., 1979) and others noting that heterotic outcomes were not evident (Dobkin and Bailey, 1979) Additionally, experiments aimed at creating interspecific crosses between Macrobrachium species, such as M rosenbergii x M malcolmsonii, have been conducted (Soundarapandian and Kannupandi, 2000).
Research by Graziani et al (2003) on M carcinus revealed significant reproductive barriers, leading to low survival rates and unsuccessful hybridization Earlier studies by Malecha et al (1984) and Meewap et al (1994) explored the heritability of growth rates in GFP In India, a genetic improvement program for GFP was launched (Jahageerdar, 2007), although the results of this initiative have yet to be published.
This study utilized genetic resources from various geographical strains of GFP from Vietnam and Hawaii in a complete diallel cross to explore the potential of crossbreeding for enhancing growth rates in this species Building on the findings of Thanh et al (2009a), which reported the relative growth performances of three strains and their reciprocal crosses, we aimed to estimate the magnitude of both additive and non-additive genetic effects on growth performance in the three M rosenbergii strains and their reciprocal crosses.
The diallel cross study involved two local wild populations of M rosenbergii from Vietnam, specifically the Dong Nai (D) and Mekong (M) strains, alongside a domesticated Hawaiian strain (H) sourced from on-farm trials in the Mekong Delta Wild GFP juveniles and adults were collected from the Dong Nai and Mekong Rivers in November 2005 and were maintained separately in hapas for two months to select mature individuals as broodstock for conditioning.
In this study, larvae and juvenile production for generations 1 (G1) and 2 (G2) was conducted through single-pair matings of parental strains, with G2 parents sourced from G1 progeny of three purebred strains Larvae from each strain combination were reared in 70-liter circular containers at a density of 60 larvae per liter, utilizing an open water system Over two generations, a total of 58 full-sib families were produced in G1, while G2 yielded 77 families, as detailed in Table 1 The larvae production spanned 37 days for G1 and 40 days for G2.
Table 1 Number of full-sib families for each strain combination in G1 and G2
Note: (i) Left figure = number of families for each strain combination in G1; right figure = number of families for each strain combination in G2; (ii) G1 and G2 = generation 1 and 2
Post larvae (PLs) were initially raised in 1 m³ fiberglass tanks for 15 days before being transferred to fine mesh hapas (2.5 x 4 x 1.5 m) in a 2,000 m² earthen pond for an additional 2.5 months PLs from multiple families within a single strain combination were pooled, with each strain combination reared separately To accommodate the age differences among families, those with similar ages (maximum 5 days apart) were combined into single nursing batches, resulting in several batches across strain combinations The age variation between the first and last batch of juveniles was 37 days for the G1 strain and 53 days for the G2 strain.
Mean body weight at stocking for the G1 and G2 was 5.2 g (range 2.5-8.0 g) and 5.4 g (range 3.4-7.1 g), respectively Prawns were fed twice daily with a 25% protein commercial pellet (Tay
Do CX4, Trung Nhan Co.) at a feeding rate of 4-5% of their body weight
During the harvest, a total of 50 female and 50 male prawns were randomly sampled from each hapa, measuring carapace length, standard length, and individual body weight However, in some hapas, the number of sampled males was below 50 due to varying mortality rates Ultimately, the total number of prawns measured for Generation 1 (G1) was 2,700, while for Generation 2 (G2), it was 2,674.
A mixed model developed by Thanh et al (2009a) was utilized to estimate both additive and non-additive effects for growth traits This model partitions the strain combination (cross) effect into strain additive genetic effects, total heterosis, and general reciprocal effects The specific formulation of the model is expressed as: y bdfijmn = P + G b + S d + (GS) bd + (SA) df + Tk i a i + Tk ij h ij + Tt i r i + p m (g b ) + e bdfijmn.
The observed response of the nth individual, denoted as y bdfijmn, is influenced by several factors including the overall mean (P), fixed effects from different generations (G b) and sexes (S d), as well as their interactions (GS) bd and (SA) df with age at harvest Additionally, the model incorporates additive genetic effects represented by coefficients (k i) for each strain, which can vary between 0.0, 0.5, or 1.0, alongside the total heterosis effects (h ij) between strains The general reciprocal effects (t i and r i) are also accounted for, with specific values assigned based on whether the strains are purebreds or crosses Lastly, random effects from the mth hapa (p m (g b)) are included, reflecting the nested structure of the analysis across multiple hapas.
Due to the statistically significant impact of age on growth traits (Thanh et al., 2009a), growth data, including carapace length, standard length, and body weight, were adjusted for harvest age based on generation and sex Notable variances in growth traits were observed between males and females, prompting four distinct analyses The first utilized the original untransformed data, while the second involved transforming the data by applying weights based on the inverse of the standard deviation for each sex (Hill, 1984) The third analysis similarly weighted records by the inverse of the variance within each sex Lastly, the fourth approach created a weighting factor based on the reciprocal of the variance within sex, employing the WEIGHT option in PROC MIXED of SAS (SAS Institute, 2000), a method also utilized by Gjerde et al.