Mass selection and inbreeding effects on a cultivated strain of Penaeus (Litopenaeus) vannamei in Venezuela

Artificial selection can significantly improve animal performance in culture, but one of the major concerns in genetic programs is inbreeding, which can affect fitness-related traits, and may have a significant negative impact on production. We present the analysis of production records for 11 generations (1990-2001) of a Venezuelan strain of Penaeus vannamei under mass selection and maintained in a closed reproductive cycle. Symptoms of IHHNV disease were reported during the first years of selection. The reproductive stock of the farm was established from three different populations mixed in successive generations (Mexico first generation, Panama second generation, and Colombia third generation). Reproductive stocks were collected from 1-ha ponds selecting the biggest animals showing no signs of disease nor deformities. Production related parameters exhibited significant improvements through time: yearly averages for survival changed from 59% to 76%, growth rate from 0.76 to 0.87 g/week, feed conversion ratio (FCR) from 1.86 to 1.51:1, production from 1.20 to 2.10 ton/ha, and percentage of deformities from 29% to 1%. The coefficient of variation on final weight changed from 19.7% to 11.6%, during the period analyzed. The high prevalence of deformities during the first years was due to IHHNV disease and probably an effect of accumulated inbreeding of the Mexican population due to a small population size. There were statistically significant relationships between the percentage of deformities and the other variables analyzed, except for growth rate. The effects of an inbreeding depression during the first generations could have decrease until disappearing, probably due to the elimination of deleterious alleles from the genetic pool, as can be inferred from the high degree of relationship between the percentage of deformities and F(t). However, no signs of deterioration on the fitness-related traits have been seen in the last generations. The absence of symptoms for IHHNV disease in the last generations, could suggests that this strain may be tolerant, or even resistant to this pathogen; however, experiments should be carried out to prove this hypothesis. The results in this and other studies on the Venezuelan stocks of P. vannamei, have proven that this is an important resource for shrimp culture in the Americas.