Thailand—CPF’s Prawn Improvement Program
The January/February 2017 issue of AQUA Culture AsiaPacific magazine contains a valuable article by Dr. Donghuo Jiang on Charoen Pokphand Foods’ (Thailand) five-year program to breed a bigger, faster-growing, heartier freshwater prawn (Macrobrachium rosenbergii).
Dr. Jiang says: “To our knowledge, this is the first prawn selective breeding program in the world based on specific pathogen free (SPF) populations that have maintained SPF status consistently for five years. Cumulatively, 38% genetic gains have been achieved through a between-and-within-family selection scheme. More initiatives still need to be taken to increase the accuracy of selection...and to generate greater yield and profitability for the prawn farming industry.”
The giant freshwater prawn, Macrobrachium rosenbergii, is an economically important freshwater species farmed on a large scale in many countries. The major producers are China, India, Bangladesh, Thailand, Vietnam and Malaysia in Asia, and Brazil and Ecuador in South America. World aquaculture production of prawns reached a peak of 226,800 metric tons in 2007. Since then, production has slipped slightly, but remains above 200,000 tons a year.
Among the reasons for the decrease in production were outbreaks of diseases, especially the white tail disease (WTD), which resulted in low numbers and poor quality postlarvae (PLs). Others reasons were genetic deterioration, possibly due to unintentional inbreeding from improper broodstock management and the shift to farming the Pacific white shrimp, Penaeus vannamei, in low salinity or freshwater areas.
In Thailand, giant freshwater prawns are a favored seafood and have a high market value. They are the main ingredient in a famous Thai dish, Tom Yum Goong (najshffo), hot and sour prawn soup. Aquaculture production of prawns in Thailand increased steadily until WTD struck the industry in 2003. The causative agents of WTD have been identified as two viruses, M. rosenbergii nodavirus (MrNV) and its associated extra small virus (XSV). The viruses cause a milky whitish appearance in larvae, postlarvae and early juveniles, and result in mass mortality at prawn hatcheries, leading to huge economic losses. The hosts of these viruses include marine shrimp, Artemia and aquatic insects. Studies showed that WTD can be transmitted both vertically from infected broodstock to their offspring and horizontally in culture systems.
To systematically address the common issues of deteriorating genetics and high risks of diseases to the prawn farming industry, a genetic improvement program based on specific-pathogen-free (SPF) selection was initiated by Charoen Pokphand Foods Public Co., Ltd., in Thailand (CPF) in late 2009.
Development of SPF Based Populations
To establish a broad genetic base for a selective breeding program, eight founder populations were collected from various geographic locations in India, Myanmar, Vietnam and Thailand. Adult prawns of each founder population were selected, based on their size and healthy appearance, and held in individual containers at a designated quarantine station. Pleopod samples were taken from each prawn and checked for MrNV and XSV using reverse-transcription PCR. Males and females with negative PCR results were used for mating under strict biosecurity conditions. Nauplii were reared in individual tanks to PL-10s, when they were tested again to verify their health status. During 2009-2010, roughly 1,000 adult prawns were screened, yielding a total of 294 broodstock with negative PCR results from two rounds of screening. These broodstock were then used to produce 159 full-sib and half-sib families during 2010-2011. Upon confirming their health status, the juveniles were transferred to a biosecure nucleus breeding center as SPF base populations to initiate the genetic selection program.
Throughout the breeding program, routine health surveillance was implemented in the breeding center, with monthly sampling of different stages for PCR tests at the Charoen Pokphand Foods’ Central Research Laboratory. In addition to MrNV and XSV, other known shrimp viruses, including infectious hypodermal and hematopoietic necrosis virus (IHHNV), Taura syndrome virus (TSV), whitespot syndrome virus (WSSV), yellowhead virus (YHV), infectious myonecrosis virus (IMNV) and monodon baculovirus (MBV) were monitored periodically. New viruses discovered in recent years, such as covert mortality nodavirus (CMNV) and M. rosenbergii Taihu virus (MrTV), were also added to surveillance list in 2015. All stocks in the breeding center have remained free of these viral pathogens for the past five years.
Selective Breeding Program
The breeding goals of the SPF prawn genetic improvement program were to improve harvest size, growth rate, survival and, to a lesser degree, density tolerance. A within-and-between-family selection scheme was employed. Individual families were produced via pair mating. Hatched nauplii from each family were reared in individual tanks to 3-5 grams. For each family, the twenty biggest individuals of each sex were implanted with visible elastomer tags for familial identification.
Communal tests of tagged prawns were carried out in recirculating concrete tanks, with 20 families per batch. At the termination of the performance test, prawns were individually weighed and recorded by gender and number of claws (zero, one or two). Male prawns were further classified into four categories according to their morphological appearance, blue claw, orange claw, small claw and no claw. Female prawns were classified into two categories: egg-carrying females and non-egg-carrying females. All data were stored and used for further analyses.
The estimated breeding values of individuals were calculated for harvest weight (HWT, grams), average daily gain (ADG, grams) and survival rate (SR%), using different statistical models. Within each family, best males and females were visually selected for breeding based on their body size, morphotype, reproductive status and health appearance.
Currently, the selective breeding program is in its 5th generation. The percentage of families selected in each generation decreased from nearly 60% in the 2nd generation to 40% in the 5th generation. On average, the top 20% males and top 30% females were kept from the selected families as breeding candidates to produce the next generation. Genetic analyses showed consistent gains were made for HWT and ADG, while improvement for SR% was not significant. The cumulative genetic gains were estimated to be 38% in five generations.
Many growth trials have been conducted to evaluate production performance of the genetically improved SPF strain under commercial prawn farming conditions in Thailand. In growout ponds, stocked with one-to-two-gram juveniles at a density of 15 juveniles per square meter, average weight of males and females can reach 40 and 35 grams, respectively, in 90 days with survival rates of 80-85%.
Considerations for Further Genetic Improvement SPF and SPR
Compared to penaeid shrimp, prawns are less susceptible to disease problems; however, in recent years, as prawn farming moves towards greater intensification and polyculture with fish, shrimp, crabs and bivalves, disease problems have become an increasing concern. Evidence showed that polyculture with P. vannamei might have led to the infection of Enterocytozoon hepatopenaei (EHP) in prawns, resulting in a significant reduction in growth rate. Spirolasma eriochieris, which was originally found to be the cause of tremor disease in mitten crabs in China, might also infect prawns, resulting in chronic mortality during the growout stage.
Selection for Mature Body Size Versus Fast Early Growth
To meet the growing demands of target markets in Thailand and China, prawn farming is moving in two directions. In Thailand, large prawns (greater than 50 grams) are preferred because their high market value. The culture period is typically divided into two phases, nursery and growout. In the nursery phase, postlarvae are stocked directly into small ponds at a density 30-50 PLs per square meter and grown for 2-3 months to 5-8 grams. The juveniles are then transferred into large growout ponds at a density of 3-5 juveniles per square meter, in polyculture with 15-20 PLs per meter of P. vannamei for another 2-3 months prior to harvest.
In China, postlarvae are directly stocked into culture ponds at moderate to high densities (50-100 PLs per square meter) and partially harvested as soon as the large animals in the ponds reach 10 grams. A typical crop involves three or more partial harvests before the final harvest and dry out.
The two modes of commercial prawn production require different breeding goals: large mature body size at low density and the fast early growth rate at very high density. Given the social structure and competition effect among prawns, it is reasonable to expect that the genetic correlation between these two traits is very low or even negative, thus two different breeding strategies would have to be implemented to achieve these two breeding goals tailored for different target markets.
Addressing the Unintended Competition Effect
Prawns have a complex social hierarchy. Blue claw morphotypes are unfavorable because of their dominance, territorial and aggressive behaviors that suppress growth of other males. Orange-claw morphotypes are more desired than blue-claw prawns because they grow quickly, possess higher abdominal carcass weight and are not territorial. Consequently, they can be farmed at higher densities. In our tests, a large number of individuals from many families were stocked into one tank. We found that large variations in body size at stocking could inflate the competition for space and feed, which in turn could affect feeding behavior and cannibalism in tanks and influence growth rate and survivability of the suppressed individuals. Therefore, in addition to the direct genetic effect of individual performance, social genetic effects would have to be properly determined in order to increase the accuracy of selection and to avoid the undesired changes in male morphotypes, which lead to unintended increase of competition, after generations of selection for growth from a competitive environment. Ideally, we will be able to increase the proportion of desired male morphotypes through genetic means and improve both growth rate and density tolerance.
Shrimp News: I contacted Dr. Jiang for information on the availability of CPF’s genetically improved prawn seedstock and broodstock. He responded: “We decided to open prawn broodstock sales to other companies and countries in 2016. After preliminary testing in China in 2016, we shipped more than 10,000 broodstock to China at $30 each in the last month. Now we are actively working with people from Myanmar, Bangladesh and Vietnam. I also got an inquiry from an enthusiastic entrepreneur in California, USA, who was looking for SPF prawn broodstock. I wanted to support that customer, but the cost of shipment and amount paper work on a small order made the transaction impractical.”
Information: Donghuo Jiang, PhD, Vice President, Aquaculture Research and Development, Charoen Pokphand Foods Public, Co., Ltd., C.P. Tower, 313 Silom Road, Bangrak, Bangkok 10500 Thailand (Email firstname.lastname@example.org, Webpage http://www.cpfworldwide.com/en/contact). Previously, Dr. Jiang was Technical Director of SyAqua Siam and shrimp breeding Project Manager of the aquaculture division of SyAqua, the aquaculture division of Sygen International, USA.
Sources: 1. AQUA Culture AsiaPacific (Editor/Publisher, Zuridah Merican, email email@example.com). Genetic Improvement of the Giant Freshwater Prawn Based on SPF Populations. Donghuo Jiang. Volume 13, Number 1, Page 42, January/February 2017. 2. Email to Shrimp News International from Donghuo Jiang on February 5, 2017. 3. Bob Rosenberry, Shrimp News International, February 6, 2017.