WWW.WAS.ORG • WORLD AQUACULTURE • MARCH 2013 57 understood but may be related to novel mechanisms for host– viral interaction in crustaceans (Flegel 2001). It is urgent that multiple infections in non-diseased shrimp be investigated so that we can clearly understand why viral infections sometimes cause disease and sometimes not (Flegel et al. 2004). Indian prawns infected with WSSV show oxidative stress, as indicated by increased lipid peroxidation in tissues (including the gills), and depressed activity of antioxidant enzymes (Mohankumar and Ramasamy 2006). Lipid peroxidation occurs as a consequence of pro-oxidants liberated when ferrous iron (Fe2+) is transformed to ferric iron (Fe3+). Removing pond sediment increases ferrous iron concentration in the water column (Boyd 1990). It is likely that suspension of pond bottom sediments by aeration increases oxidation of these radicals to ferric oxide and subsequently leads to oxidative stress in shrimp (Ruiz-Velazco et al. 2010). Fallout from aeolian clouds of desert dust are a common phenomena in desert-saline shrimp culture ponds. Fe, Mn, Cu and Cd are principal components in the bulk dust samples (Behairy et al. 1985). The average amount of dust ranges from 1 to more than 10 t / km2 / month. Depending on prevailing climatic conditions, these particles may be blown over great distances, but are ultimately subjected to the fate of returning to the lithosphere as fallout or in precipitation (Forstner and Wittman 1979). Genetic Issues The loss of genetic variability in Pacific white shrimp broodstock in Brazilian hatcheries over succeeding generations and inbreeding depression in small populations contribute to the reduction in capacity of broodstock to adapt to environmental changes (Sbordoni et al. 1986). Despite contributing to the maintenance of phenotypic characteristics of commercial interest, increased inbreeding rates may result in genetic homogenization and a drastic reduction in heterozygosity (Goyard et al. 2003, Freitas and Galetti Jr. 2005). Knowing the severity to which inbreeding affects economically valuable traits is essential when designing an efficient and effective shrimp breeding program or germplasm protection strategy (Moss et al. 2007). Inbreeding can be defined as the mating of individuals that are related by ancestry and results in a reduction of heterozygosity within a population (Falconer and Mackay 1996). Inbreeding depression is the effect of inbreeding measured as the reduction in mean phenotypic performance with increasing levels of inbreeding within a population (Falconer and Mackay 1996). Clearly more research is needed to evaluate the effects of inbreeding on shrimp under diverse stock management strategies and grow-out conditions (Moss et al. 2007). Intraspecific crossbreeding can improve performance of a farm animal by non-additive genetic effects. Different geographic populations represent potentially different genetic backgrounds. The performance of hybrids of different populations generally exceeds that of parents in growth rate, fecundity and adaptability to the environment. Heterosis, or hybrid vigor, results from the crossing of inbred lines or genetically divergent populations (Falconer 1989). Heterosis may be interpreted by the recombination of gametes from crossbred parents and epistatic superiority from the pure breeds. Advantaged dominant genes restrain deleterious recessive genes. Inbred lineages often suffer from low offspring viability and other problems (Falconer 1989). If the crossbreeding of different populations is applied to the establishment of base populations, then a systematic selection program based on additive genetic performance may be a more effective approach to improvement than crossbreeding (Yi et al. 2006). A new reference center for Indian prawn germplasm conservation is imperative to regulate and control gene flow amongst intraspecific Middle East geographical populations of F. indicus. Intellectual tithing on breeding technologies and resharpening the biosecurity razor is a must for the Saudi shrimp industry. Patterns and propensities of exotic salt water aquarium introductions, such as damsel fish and sharks from Sri Lanka into the Kingdom ought to be banned. The laboratory of biosecurity in Saudi science education is a must for an ecofriendly future. Current shrimp disease hits are warning lessons and preparation for future shocks must be on the table. Future Shrimp Farming For shrimp aquaculture to overcome disease threats and to resume growth in production, a new blueprint for shrimp farming industries throughout the world is required. Commercial shrimp farming in the future will be based on: 1) specific pathogen-free and genetically improved shrimp stocks; 2) biosecure systems including enclosed, reduced water-exchange and increased water-reuse culture systems; 3) biosecure management practices; and 4) co-operative industrywide disease control strategies (Lotz 1997). Commercially acceptable biosecure culture systems that are under cover and use recirculated sea water will need to be developed for shrimp production (Lotz 1997). It is likely that no single compound or strategy will provide a solution to the problem of disease within aquaculture and that, in reality, a suite of techniques will be required, including manipulation of the rearing environment, addition of probionts as a matter of routine during culture, and the use of immunostimulants and other compounds during vulnerable growth phases (Smith et al. 2003). Protection of Indian prawn from white spot disease by oral administration of inactivated white spot syndrome virus has been tested (Singh et al. 2005). It seems unlikely, however, that putative vaccines will totally control potential disease outbreaks without the assistance of other measures such as improved sanitation, diets formulated with pre- and probiotics, and heightened biosecurity (Rowley and Pope 2012). Biosecure approaches today involve reduced water exchange and the use of high-health or specific pathogen free (SPF) seed. Although these efforts have had a positive impact on commercial shrimp farming, the risk of crop loss still exists. Survival of shrimp aquaculture in the long term depends on appropriate techniques that are tailor-made to ease negative environmental impacts and monitor the introduction and spread of pathogens. These technologies must be integrated in a holistic approach to shrimp cultivation, incorporating major advances used in terrestrial meat production with similar novel approaches in aquatic animal husbandry. With these technologies in (CONTINUED ON PAGE 58)
RkJQdWJsaXNoZXIy MjExNDY=