IHHN VIRUS AND INTENSIVE CULTURE OF PENAEUS-VANNAMEI - EFFECTS OF STOCKING DENSITY AND WATER EXCHANGE-RATES

Producers and researchers have long suspected correlations between ''runt deformity syndrome'' (RDS), morphological anomalies, poor production. and viral infection for various penacid shrimp species. Unfortunately. evidence has been circumstantial and little research has been focused on defining harvest size distributions as it relates to viral histopathology. The present study highlights these elements, in a factorial experimental design, using increased stocking density (60/m2 and 100/m2) and decreased water exchange (100%, 50%, and 10% per day) as factors. Twelve 29.2-m2 tanks were stocked with suspected IHHNV-infected Penaeus vannamei postlarvae from a Central American hatchery, and were harvested after 164 days. Low dissolved oxygen levels resulted in poor survival (43.5% and 53.6%) in tanks which were stocked at 100/m2 and limited to 10% water exchange per day. Density and water exchange regimes had little effect on water quality, survival, or growth in other experimental groups. A ''runt deformity syndrome'' and other physical anomalies are described and the former is correlated with increased incidence of IHHNV associated Cowdry type A inclusion bodies in 4 organ systems. A trend toward decreasing incidence of inclusion bodies was found as exchange rates were increased from 50% to 100% per day. Distributions of size at harvest indicate less skewness and kurtosis as well as lower incidence of deformities in low survival tanks, suggesting increased mortality of more severely infected RDS shrimp.