The Benefits of Shrimp Nurseries
Over the last six years, nursery systems have evolved rapidly in Latin America. Now, the culture period in nurseries is longer, the biomass is larger and the transfer systems and water control systems (zero exchange) are better. Feeds and probiotics have also been improved. The target is to produce stronger postlarvae that have been acclimated to the pond environment. This results in better survival during the first days of culture and minimizes exposure to disease vectors.
Since the causative agent of EMS/AHPND is a Vibrio, nursery systems allow farmers to focus on controlling the Vibrio bacteria in a smaller area and delaying exposure to the pathogenic agent. In Mexico and Asia (more on both below), this is one of the strategies used to mitigate early mortality syndrome (EMS).
The biggest cost in shrimp farming is during growout. Stocking larger postlarvae (PLs), say PL-45, instead of PL-12, reduces the duration of the growout period by 20-30 days. A shorter cycle means lower feed and energy costs. Feed conversion ratios (FCR) can be reduced by 10-30%.
Evolution of Nursery Systems in Mexico
The whitespot virus, together with local weather limitations (Mexico has extreme weather conditions, hot summers and cool winters) forced farmers to modify their nursery systems.
The circular, 50-metric-ton tanks of 2008 evolved into 100-ton raceways in 2009 and then to larger 500-ton raceways in 2011. Currently, even larger rectangular tanks with capacities of 800 to 1,000 tons, enclosed in greenhouses, are used. Probiotics are used to control water quality. The protocols for the different probiotics are adjusted depending on the biomass, water quality and animal health conditions:
• For organics, 3 parts per million (ppm) every 72 hours
• For toxins, 2-5 ppm every 48 hours
• For pathogens in the water, 2 ppm every 72 hours
High-quality hatchery feeds with fewer fines are the most desirable feeds. Nursery managers do not follow a fixed feeding table. Usually, they feed high-energy diets with at least 45% crude protein at 2-hour intervals. Feed amounts are adjusted depending on feed trays, water quality and animal growth.
Since 2012, pumps have been used to transfer PLs from nursery tanks to growout ponds, reducing mortality during transfer to only 3-5%. The transfer process has been refined so that the animals are not out of the water for more than 30-60 seconds. In 2013, using pumps, the transfer distance was increased up to 3 kilometers and the volume up to 15 kilograms of PLs a minute. It is important that transfers are carried out in the morning, late afternoon or at night. The condition of the PLs should be checked before every transfer.
A recent development has been the reduction of the stocking density in the raceways to achieve larger postlarvae and higher survival rates. Biomasses of 6.9 kilograms per cubic meter have been achieved.
The Asian Experience with Nurseries
In 2013–2014, the nursery techniques used in Mexico were successfully transferred to a Malaysian shrimp farm. Postlarvae were stocked in raceways for 25-30 days to produce 1-to-1.5-gram PLs, which were then stocked in growout ponds at a density of 70-80 PLs per square meter. The feed conversion ratio (FCR) in the raceways was 1.0-1.5 depending on the duration of culture. In the growout ponds, shrimp grew to 17-18 grams, and the FCR was 1.3. Survival in the growout ponds was 90-95%.
In Thailand, there were mixed results. There was success at the nursery stage, but due to the condition of the animals during transfer, results were poor in the growout stage. Alternative ideas were put in place such as stocking in cages that had no contact with the bottom for the first 30 days of growout. Reportedly, this avoided an EMS outbreak. Another option was separating 20 to 30% of the pond with a net, stocking this area with PLs, and then releasing them into the whole pond after 20 to 30 days.
In Vietnam, the implementation of nursery systems has been hampered by a lack of investment capital, technology and poor biosecurity. One success was reported in central Vietnam, but poor animal condition during transfer let to limited growth in the ponds. The poor condition was attributed to sub-optimal feeds and poor water quality in the nursery.
Farmers in Asia should invest in the new technology developed in Latin America and refine it for Asia.
Source: Aquaculture Magazine. Editor Greg Lutz (email firstname.lastname@example.org). Nursery Phase in Shrimp Farming: Benefits Beyond EMS. Fernando Garcia Abad (email email@example.com). Volume 41, Number 4, Page 6, August/September 2015.