Corporación Centro de Investigación de la Acuicultura de Colombia (CENIACUA) and Institute of Aquaculture Research (AKVAFORSK).

The use of domestication and genetic improvement programs in the shrimp industry is rather recent, in spite the fact that selective breeding programs have proven very successful tools for increased production efficiency in all major livestock and several aquaculture species.

During the past decade, viral diseases such as Taura Syndrom Virus (TSV) and White Spot Syndrome Virus (WSSV) have devastated the shrimp industry in several areas of the world. To face these challenges, the Colombian shrimp sector has developed a selective breeding program for Litopenaeus vannamei directed towards development of stock with improved growth and increased resistance to diseases.

National programme
The national breeding programme in Colombia was initiated by CENIACUA (Corporación Centro de Investigación de la Acuicultura de Colombia), in collaboration with AKVAFORSK (Institute of Aquaculture Research) of Norway. The work initially focused on improved resistance to TSV. These efforts were successful and TSV is no longer considered to by a major threat to the shrimp sector in Colombia. However, in 1999 WSSV was first detected in the Americas and wreaked havoc in commercial shrimp farms along the Pacific Coast. Consequently the selective breeding program is currently aiming at improved resistance to WSSV and increased growth, whilst maintaining the level of resistance obtained for TSV.

Selective breeding strategies
The first steps towards the development of an improved stock of L. vannamei in Colombia involved mass selection for increased resistance for TSV, by reproducing animals that survived in severely affected ponds. The results were positive, and demonstrated the feasibility of directional selection for increased disease resistance. However, mass selection for binary traits such as the ability to survive a disease attack is known to be inferior to family based selection schemes. Furthermore, mass selection schemes based on selecting survivors may result in loss of additive genetic variation in the target populations and consequently problems related to the detrimental effects of inbreeding depression.

In order to avoid these pitfalls, in 1997 the Colombian shrimp sector initiated a combined family and within-family selection programme. A nested mating design is applied, in which full- and paternal half-sib families are produced by artificial insemination of two females with milt from the same male.

The base populations for selection (batches 1, 2 and 3) were established in 1998-99 after successful introduction of genetic material originating from nine countries in Latin America. Till date seven batches representing a total of 430 full-sib families have been produced in the program.

Families are hatched and reared separately until they reach approximately 1 gram. At this stage, randomly sampled animals from each family are tagged with Visible Implant Fluorescent Elastomers. Independent samples of tagged individuals from all families are then pooled and evaluated for resistance to specific diseases in experimental challenge tests, and for growth and survival in ponds at commercial farms located on the Atlantic coast of Colombia. Families of batch 3 onwards were challenged with WSSV in replicated tests in tanks at CENIACUA’s laboratory in Tumaco on the Colombian Pacific coast were WSSV is endemic.

Selection strategy
Breeding values are estimated by Best Linear Unbiased Prediction (BLUP) methodology using the data recorded in the experimental challenge tests and during the grow-out trials performed on the commercial farms. The breeding values estimate an individual’s ability to produce high performing offspring. Breeding candidates are finally ranked and selected as parents for the next generation according to an overall selection index that combines the candidate’s breeding values for the various individual traits included in the overall breeding goal.

Prospects for improvement
At present, complete data are available from five batches of families. Substantial additive genetic variation exists for harvest weight, pond survival and resistance to WSSV and TSV, and the prospects for obtaining large selection responses are excellent. The family breeding values for growth and pond survival, in farms not affected by WSSV, are positively correlated. Hence selection for increased harvest weight is expected to provide a concomitant increase in pond survival.

Furthermore, the strong positive correlations between full-sib family means for pond survival and for harvest weight in different test farms demonstrate low genotype by environment interaction, indicating that one breeding population can serve the range of commercial production environments in Colombia.

The negative correlations of intermediate magnitude between harvest weight recorded in commercial farms and WSSV-survival recorded in the controlled challenge test experiments are unfavorable. This result implies that if selection is practiced for only one of these traits, an unwanted correlated response is to be expected for the other trait. A selection program, in the absence of the disease, directed solely at improving growth rate will over time result in a population highly susceptible to WSSV.

Genetic improvement for both growth rate and WSSV resistance can only be obtained if both traits are recorded and selected for simultaneously. This can only be achieved by using a family based selection scheme; the Colombian family selection program is searching for both improved WSSV resistance and growth rate in the breeding nucleus population.

Predicted selection responses
Simultaneous selection for increased growth (recorded as harvest weight) and resistance to WSSV (recorded as survival in experimental challenge test experiments) has provided a mean selection response per generation of approximately 7 % for each of these traits. Growth and resistance to WSSV have been given the same relative weighing in the selection (0.45 and 0.45). The remaining selection pressure (0.10) has been applied to general pond survival, and a positive selection response is also obtained for this trait.

Based on the results obtained so far, the predicted accumulated long-term response to five generation of selection for the breeding nucleus is 37% and 32%-units for harvest weight and survival to WSSV (measured in experimental challenge test experiments as the survival of the family at the time when fifty percent of the overall population dies). The implies that a given WSSV exposure that result in 50% mortality for F0-animals, after a given time, is expected to give only 18% mortality for F5 animals.

Dissemination
A system for supplying improved genetic material throughout the Colombian shrimp sector is in place, with commercial hatcheries serving as multipliers. The dissemination scheme is such that in the final stages of multiplication a very heavy selection pressure for only one trait (growth in the Atlantic coast material, and WSSV resistance in the Pacific Coast material) providing animals for commercial production that are actually superior to the nucleus breeding population for one particular trait. The Colombian shrimp sector has confidence in the breeding stocks and now over 20% of the total production comes from family selected populations.

Biosecurity measures
Since the WSSV epidemic, the biosecurity measures were increased at the maturation and larviculture facilities at Punta Canoa were the breeding nucleus is kept. Candidate broodstock animals are raised in circular 50 tons tanks with low water exchange and permanent aeration. Later they are transferred to the maturation tanks where biofilters are installed, lowering the water exchange levels to 5% per week.

Animals displaying disease symptoms are identified and tested for viral and bacterial pathogens using PCR, bacterial culture and in PCR insitu hybridization techniques. Entry of crustaceans of all kinds is prevented by filtering water supplies and by physical barriers around the breeding nucleus facilities.

Supporting research activities
CENIACUA supports the genetic improvement program by research on techniques and methodologies directed to improving the efficiency of the breeding operation. Work is advancing on cryopreservation of sperm which would facilitate genetic links between batches, document genetic gains and provide a means of safeguarding improved genetic material. Advanced genetic techniques are likely to become increasingly important as supplementary tools in genetic improvements programs for marine shrimp in the future. Recent advances in gene mapping facilitate marker-assisted selection by identifying genetic markers associated with traits of economic interest. Supporting research activities in CENIACUA currently evaluates the potential of using genetic markers to select for resistance to WSSV. The use of genetic marker information may increase the accuracy of selection and lead to even higher selection responses.

Microsatellite analyses are being developed to monitor the level of genetic variation in the breeding populations and as a tool for verifying the mating program.

Acknowledgements
This work has been supported by the Colombian agencies Proexport, Colciencias, the Ministry of Agriculture and Rural Development and the Colombian Shrimp Industry. The constant cooperation of the Colombian shrimp farmers has been a key element of this project.
Corporación Centro de Investigación de la Acuicultura de Colombia (CENIACUA), Punta Canoa, Cartagena de Indias,COLOMBIA
Institute of Aquaculture Research (AKVAFORSK) NORWAY