UK - A study into the effects of white spot in shrimp farming units has won this year's prestigious Lindsay Laird Award.
The award is given out each year at Aquaculture Europe for the best student poster. This year, Cecilia de Souza Valente (pictured above with Dr Luis Vinatea Arana), a student at the Federal University of Santa Catarina, Brazil, won the award for her study titled: 'Transcription Profile of Target Genes in the Pacific White Shrimp Litopenaeus vannamei Kept in Biofloc and Clear Seawater and Experimentally Infected with White Spot Syndrome Virus'.
The Lindsay Laird Student Award for Innovation in Aquaculture was established by AquaTT to honour the work of Lindsay Laird, who was a Senior Teaching Fellow in the Zoology Department at Aberdeen University and who sadly died from cancer in 2001.
As well as her work at the University of Aberdeen, Dr Laird worked with the expanding Scottish aquaculture industry on different aspects of salmon farming, including defining the standards for organic salmon production and introducing a quality assurance label for farmed salmon.
Ms Valente, the first South American to gain the award, was selected as the winner after 80 posters were judged by Professor Gavin Burnell, University of Cork; Dr Chryssa Doxa, HCMR, Greece; Ms Margaret Eleftheriou, AQUALEX Multimedia Consortium; Dr Claudia Junge, from AQUATT and John Bostock, University of Stirling.
Farming shrimp in biofloc is popular as it can often be an economical and environmentally sustainable alternative to other farming methods. A typical biofloc system features high stocking densities and restricted water exchange. This leads to an accumulation of nutrients from feed which in turn, contributes to the proliferation of a community of microscopic organisms such as bacteria, fungi, algae, protists and zooplankton. Whilst biofloc therefore often offers better biosecurity control, it is not immune to disease.
White spot is a particularly devastating shrimp disease which usually causes mass mortalities of 80-100 per cent.
Ms Valente therefore wanted to compare the survival rates of shrimp farmed in clear sea water to those in biofloc when infected with White Spot Syndrome Virus, and she also looked at the transcription profiles of target genes. Gene transcription profiles are used by scientists to measure activity of different genes. Ms Valente used this method to compare how the shrimps fought back against White Spot Syndrome Virus infection in the biofloc versus sea water environments.
For her study, Ms Valente used 84 shrimp in each system and shrimp were given 15 days to acclimatise before being infected with white spot. The average virus load was lower in the clear sea water unit.
It was not long until the clinical symptoms - lethargy, reddish body and a decrease in appetite - were seen. Shrimp started to die in both systems after 24 hours post infection.
The survival rate was observed to be lower in the biofloc system than the clear sea water, however there was a shrimp survivor in the biofloc system after 72 hours post infection which showed no detectable clinical signs. There was no significant difference in gene transcription profiles of infected shrimp between the two systems.
The Fish Site’s Editor Lucy Towers spoke to Ms Valente about her study.
Q. What inspired you to conduct this study?
Viral diseases are one of the major problems in shrimp farming and although the investigation of the effects of viral infection on gene expression of Litopenaeus vannamei has been addressed recently, nothing is known about the potential effect of biofloc system on shrimp molecular responses.
Q. Did you expect to find that survival rate would be lower in shrimp in the biofloc system?
When we were performing the experiment, three different results could be expected: same or comparable survival rates between shrimp kept in clear seawater and in biofloc, lower survival rate in shrimp kept in clear water, or lower survival rate in shrimp kept in biofloc.
Since the biofloc technology has an appeal as a biosafe system and can also be considered as contributing to stimulate the immune system, we expected to observe higher survival rate in such conditions.
Although animals kept in biofloc may be less vulnerable to pathogenic infection due to a reduced chance of pathogens entering in the system, the WSSv infection can lead to high mortality, as well. It is still necessary to consider the management and biosecurity strategies, in order to avoid the entrance of pathogens.
Q. Why is this likely to have happened?
Due to the rich environment biofloc represents, in terms of presence of a rich microbiota, one can consider that the shrimp immune system is possibly greatly stimulated in a constant manner, with the defence responses at or close to their limit. With the entrance of a very virulent pathogen, such as WSSv, shrimp are not capable to cope with such a challenge, failing to display a suitable defence response.
Maybe facing a less virulent pathogen, shrimp kept in biofloc could respond better, and a higher survival would be observed.
Q. Despite high mortality, why was there a shrimp survivor in the biofloc unit?
Animals have variability in terms of genetic profile; probably the only shrimp survivor at 72 hours post infection in biofloc, and that did not show visible clinical signs, can be considered as less susceptible to WSSv infection.
Q. What do the transcript findings tell you about the shrimp´s response to White Spot?
The transcript findings show that regardless the system shrimps were kept in, there was no difference in gene expression between the systems. It was the viral infection that modulated the gene expression, not the systems. The analysis of a broader range of target genes is under progress but our first results indicate that calcium metabolism and protein turnover are affected by infection.
Q. Would shrimp kept in biofloc usually be more unlikely to have an outbreak of White Spotcompared with other shrimp farming methods?
Our results are based on a study performed in relatively small experimental units. Results in commercial farms could be different. However, based on our previous results we believe that against highly virulent pathogens, such as WSSv, shrimp kept in biofloc may display higher mortality. The producer that chooses the biofloc system should constantly be aware of the conditions regarding farm biosecurity, as well as monitoring animal health.
Q. How will the findings help shrimp farmers?
Identifying WSSv responsive genes in gills and other tissues of L. vannamei after viral infection under different cultivation systems may allow more precise studies about molecular and biochemical processes against viral infection, resulting in better animal health, productivity and evaluation of cultivation systems.