2.1. Field survey
The field survey was conducted during the 2009–2010 austral summer, in two ponds, named C and D. These ponds have been regularly impacted by vibriosis for several years. The stocking date chosen for this field survey was particularly favourable to the occurrence of the diseases (Lemonnier et al., 2006). The ponds were managed by the farmer according to his usual techniques without advices from scientific team. The ponds (around 8 ha) were dried for more than one month before breeding shrimps and supplied with blue shrimp (L. stylirostris) post-larvae (0.03 g) at 22 individual.m-2 on the 19 October 2009. Shrimps were fed on a daily basis with commercial food containing between 35 and 40% protein. Daily water renewal increased in pond D from 5 to 30% between the beginning and the end of the rearing. To test the effect of water exchange rates (WER) on disease outbreak, WER applied by the farmer were voluntary lower in pond C than in pond D. There was no mechanical aeration. During rearing, shrimp mortality was estimated by counting dead and moribund shrimp at the pond edges or on the filters located on the effluent gates. This estimation should be viewed in this study as a qualitative method. Isolation of the pathogen strains was conducted from hemocultures of 30 moribund shrimp. The different strains isolated were phylogenetically identified using molecular-based tools ( Goarant et al., 2006b and Goarant et al., 2007). The survival rate (%) calculated by the farmer was estimated by the ratio of the number of harvesting shrimp at the end of the rearing to the number of stocking shrimp at the beginning of the rearing.