• Biofloc is a clumpy assemblage of microorganisms (bacteria, microalgae, cyanobacteria, fungi, protozoans, micro-zooplanktons, etc.) with dead particulate organic matter suspended in the water column of an intensively aerated or agitated aquaculture system.
• Exopolysaccharides secreted by bacteria, and certain kind of microalgal species helps in the formation of biofloc. These biopolymers act as adhesives to aggregate the dispersed cells of bacteria, microalgae and other particulate organic matter to form a clumpy mass called bioflocs.
• The Indian vannamei shrimp farming industry has been greatly affected by many disease outbreaks wherein in many cases cultures are lost in a matter of 15 days. This causes huge economic loss as a considerable investment is made in preparation of large grow out ponds. To overcome this, onsite nursery rearing of shrimps from its early postlarvae up to an average size of 1 to 2 g is highly recommended and is an emerging trend.
• Nursery ponds in comparison to grow-out ponds are smaller and can be easily managed for operations faster and at a lower cost even in the case of a disease outbreak.
• In biofloc based nursery rearing, these consortia of bacteria and microalgae clean the unwanted nutrients such as ammonia, nitrite, nitrate, sulphide, etc., which are otherwise toxic to the shrimps by utilising them as a source of nutrient and energy in their metabolic pathway.
• In addition to this cleaning process, biofloc themselves act as a fresh feed (in situ feeding) within the system. This is considered to be a low-cost, sustainable feeding tactic and is receiving popularity in farming of white leg shrimp around the world. Biofloc is found to be a nutritionally rich and balanced with a good amount of protein, minerals and other micronutrients.
• Bioflocs not only act as a feed but also manages the water quality with no any additional cost.
• Nursery rearing of shrimp post larvae (PL) in this biofloc based tanks, ponds or raceways before stocking in the grow-out pond has been observed to increase production by 20 – 30 % and lower the cost of production.
• The PL obtained from the hatchery are reared for a period varying from 20 – 45 days until the PL reaches PL 45 or early juvenile stage of size ranging from 0.3 g to 1.2 g. Stocking of large sized post larvae has been observed to reduce growout period from 20 – 30 days and improve the FCR by 10 – 30 %.
• Stocking larger PL which has been acclimated to the pond conditions has been a strategy to mitigate the issue of diseases like early mortality syndrome (EMS) and white spot syndrome virus (WSSV) infections to some extent.
• As the biofloc based nursery rearing system is preferred over the conventional nursery as it is a zero or minimal water exchange based system.
• The biofloc based nursery rearing technology is based on the adjustment of the C: N ratio in a bacteria-based system.
• The addition of carbon-rich sources in well-aerated ponds stimulates the growth of heterotrophic bacteria which in turn make use of inorganic nitrogen in the form of ammonia and nitrite for generating bacterial protein and thereby create a self-nitrifying system with protein rich flax forming an additional feed to shrimp.
• Postlarvae (PL3) reared in biofloc (107 CFU/ml) based rearing system showed significantly higher growth and survival than in conventional non-biofloc system.

White spot disease (WSD) is the most serious threat faced by the shrimp farming industry worldwide. WSD was first reported in farmed P. japonicus from Japan in 1992-93, but was thought to have been imported with live infected post-larvae from mainland China. The disease is transmitted vertically from infected brood stock to larvae and horizontally either by ingestion of infected organisms or through carrier organisms. Most crustaceans including all penaeid shrimps (monodon, vannamei, indicus etc.) and crabs can be affected by WSD. All the life stages of shrimp may get infected by this virus.

Causes

White spot disease is caused by a virus called White Spot Syndrome Virus (WSSV). It is a rod-shaped double-stranded DNA virus of 120-150 x 270-290 nm size, assigned to a new virus family, whispoviridae.

Symptoms

• Affected shrimp exhibit anorexia, lethargy, reduced appetite and reddish discoloration
• Sudden reduction in food consumption, loose cuticle with white spots
• Presence of white spots of 0.5 to 2.0 mm in diameter on the inside surface of the carapace, appendages, and cuticle over the abdominal segments.
• In Pacific white shrimp or vannamei shrimp, white spots may not be clearly visible. Mortality of shrimp may start 2-3 days after infection and reach 80-90 per cent within 5-7 days of onset of first mortalities, necessitating emergency harvest.

Diagnosis

WSD may be diagnosed based on gross signs such as the presence of the characteristic white spots, and rapid mortalities. White spots may not be always seen in the early stages of infection in shrimp. WSSV can be detected using polymerase chain reaction (PCR), or with molecular tools such as dot-blot and in situ hybridisation (ISH) tests.

Prevention

• Vertical transmission through infected brooders is also possible, wherein, the virus is transmitted to larvae. Hence, it is always advisable that PCR tested seeds only are stocked in the ponds. There is no treatment for WSSV. Prevention is the only way to avoid the disease.
• Rapid changes in water conditions have to be prevented.
• Avoiding shrimp stress, fresh feeds(crustacean), frequent water exchange is advisable.
• Once infected, treating the pond with chlorine would be of great help to kill infected shrimps and carriers.