By: Giovanni Chasin
The main goals for nursery systems in shrimp farming is: (1) maintain post larvae growing with good health while growing ponds are ready for stocking, and (2) get bigger animals at stocking time in ponds saving 3-4 weeks in the growing cycle. Unfortunately it is not easy to accomplish these two criteria due to problems observed during the time frame animals are in these systems, mainly associated with the presence of the White Spot Disease Virus [WSSV].
At the time of arrival at farm, post larvae coming from the hatcheries show normal behavior and most of the time are negative for WSSV by Lateral Flow Inmuno Assay test [LFIA]. After approximately 10 days in the system, minor mortality is observed and, new screening test result in positive animals for the virus, even considering that system has proper mechanical and UV filtration and bio security measures are adopted.
Sensitivity of LFIA test is considered lower than any PCR technique and the quantity of animals tested is also below the number required to find a prevalence around 2% of the virus in the whole population [expected to be 150 PLs], and our hypothesis is that the viral load when post larvae arrive at farm is under the level of detection by this tool, and after few days in nursery tanks this viral load and prevalence rise to the level of detection.
When mortalities begin, the transfer of animals is imminent to avoid total loss of the batch. This way, nursery facilities composed of two phases remain underused and, stockings with smallest animals at ponds should be done outside the schedule.
It seems that the same is happening in Ecuador 4-5 years ago this days and I would not be surprised if it is for the same cause. In Ecuador, most of the farms are not using the nursery systems anymore, and some producers, are using them to grow up post larvae from nauplii, acting as a larviculture facility. Probably, their APE lines show better performance at growing ponds where densities are lower than in nursery tanks.
In the case of Brazil, more efforts should be made to select wssv-free animals entering into the maturation facilities and tighting biosecurity measures in hatcheries. Let us see what happens next days after the rainy season is completely away.