Tiếng Việt
中文 (中国)
Recently I passed my 40-year mark working with the international aquaculture community. I have heavily focused on shrimp culture over 30 of these years as I have always had a soft spot for shrimp (eating them). The industry has changed considerably since I first became involved. In my early days, I learned from a handful of individuals many of whom were successful. I watched failures as well which were all too common and where I could, learned from these failures as well.
I found myself getting quite cynical at times as what I saw repeatedly was not positive. There were many ideas about certain practices that were simply not true. Some of these have caused immeasurable harm and will continue to do so.
There are many ways to achieve success in shrimp farming. Unfortunately, there are more ways to fail. What I have learned is that there are at least four critical areas that are needed to ensure consistent success. These are:
1.Ensuring adequate levels of oxygen in all aspects of the process.
2.Managing feed in a manner that ensures animals are able to consume as much feed as they want to with little waste and little to no stress.
3.Keeping the environment clean and free of accumulated organic matter that can foul the shrimps’ gills, cause them harm from bacterial metabolites such as hydrogen sulfide and serve as a source of food for a variety of potential and obligate pathogens.
4.Keeping broodstock free of all known pathogens and thereby keeping the PLs at least pathogen free to start.
This article focuses primarily on this last point. There are a number of myths that have become dogma that ensure that holes in biosecurity are common and that they will continue to pose challenges to farmers.
Myth # 1
Low survivals in the hatchery are a good thing. The logic behind this is that the environment is putting pressure on the animals in a manner that kills off the weak animals and leaves only the strongest. Survival of the fittest is what is happening.
The Reality
While this might seem true to many, this is simply not the case. Selection pressures that influence survivals are an important mechanism by which evolution functions. This happens in the wild, not in a hatchery tank on this small of a scale.
What producers need are strong healthy pathogen free PLs able to tolerate the stresses of production. When done properly, animals will be uniform in size and the population as a whole will do well with high survival rates.
Rationalizing that the animals that can survive poor hatchery practices are in some manner superior is a myth. High survival rates tell us that the animals are strong and healthy. When they die off in the tanks it does not mean that the survivors are in any way superior.
What it means that they managed to avoid what was killing others.
Myth # 2
It is OK if the broodstock are carrying low levels of obligate pathogens. Obligate pathogens cause disease in healthy animals. Opportunistic pathogens cause disease in weakened animals. The idea is that low levels of pathogens prepare the animals better to tolerate the real-world conditions.
The Reality
Broodstock should be clean and free of all obligate pathogens. There is really only one way to ensure this. Screening of each individual animal against the array of known pathogens using RT PCR is a critical first step. It is however critical to appreciate that the absence of proof is not the proof of absence.
A negative PCR test means that the test is negative not that the population is free of the pathogen. The path to follow to ensure clean broodstock and thus, by extension, with certain caveats that the PLs will be as well requires at least three things:
1.PCR screening against all OIE pathogens and all other pathogens for which primers are available. See Myth 4 for an elaboration.
2.Holding animals in a nucleus breeding center (NBC). This, when done correctly, is a totally biosecure environment. Once animals are held in this facility no animals from outside can be brought in unless they are from another NBC. The methods for establishing an NBC are well documented although not widely used in shrimp farming.
Some companies claim to have these types of facilities but usually there are holes in biosecurity. The flow of animals from an NBC can only be one way-out. The moment animals are brought in from the outside unless they are from another NBC they should no longer be considered to be pathogen free.
3.Following the history and performance of all animals that are sold. A detailed history must be generated. This is essential for ensuring that if there are animal health issues due to pathogens, whether obligate or opportunistic, that they are not such that there is any reason to believe that they originated in the broodstock.
“This is not always simple however and can require some detective work. In some parts of the world this is almost impossible to do.”
Farmers don’t know what they stocked and the only time that they know they have problems is when they harvest and at that they td onto have the resources to see what might be killing their animals.
If all of these are followed the odds are good that no pathogens will be introduced into the production system as a result of carryover from the broodstock. This assumes that similar practices are being observed in the hatchery and any other system linked to a given group of animals, such as nursery tanks.
Myth # 3
Since ponds can be highly polluted, there is no need to be concerned about the water quality in the hatchery. This is closely aligned with myth 1.
The Reality
Degradation of water quality is a major source of stress and disease. Pathogens that would not kill healthy animals can impact animals that are weakened because of this. High levels of suspended organics can be colonized by a variety of organisms including a number of vibrios both opportunistic and obligate.
Hatchery water needs to be clean. It does not need to be sterile. It does need to be controlled in a manner that ensures that no pathogens or as low of a level that is possible are present.
Myth # 4
Specific pathogen free shrimp (SPF) are resistant to diseases and are stronger in general.
The Reality
SPF does not mean all pathogen free (APF). In fact, there are conditions under which it does not even mean that a population is actually free of a given organism. The way that PCR is used makes it a statistical tool.
“Small subsamples are tested of a population and if they are negative by PCR, it is assumed that all of the rest of the animals are free of these pathogens as well. This is not true.”
The best that one can do is to have a 98% level of assurance. When you are dealing with obligate pathogens this threshold is not enough. If the 2% can carry a pathogen that can wipe out entire farms clearly this is not adequate. The only way to increase the level of testing efficacy is to test every animal.
This can be prohibitive cost wise although recent innovations have allowed one to test animals for an array of potential pathogens for much less than conventional testing (https://www.genics.com.au/).
Each animal can be tested. This refers to broodstock. Once the broodstock are as clean as they can possibly be, routine follow up testing of various life stages can be helpful in ensuring that there are no gaps in biosecurity.
This, along with following the guidelines outlined above in Myth #2 is the only way to be close to absolute certainty that a given pathogen is not present.
Myth #5
Probiotics are the solution to microbial problems. They will prevent animals from getting sick and dying. They do this by impacting animal health and altering the microbial make up to protect them against both obligate and opportunistic pathogens.
The Reality
Probiotics are defined (by FAO-the United Nations) as living microorganisms that are ingested orally and that colonize the gut of animal, impacting the microbiome in a manner that results in the animal resisting disease and being in better health.
“This definition is evolving and today it appears that any microorganism that is used in any manner on any animal, plant, etc. whether it is living, or dead is being called a probiotic. Even if the definition were what it is being described as, these are tools.”
They are not solutions, and they cannot solve problems that are inherent in the manner in which a given paradigm is functioning. For shrimp most products are intended to improve water quality. There is the possibility of other benefits as well although this is not straight forward.
Many lab studies show impacts that do not occur in the field under real world conditions. Using these types of products is not an assurance that the animals will be clean and free of potential pathogens.
Conclusions
In all the years that I have been working with shrimp farmers in a dozen or more countries what I have seen is that far too many of them buy into these myths. If it is not the owner, it might be a technician. If brood stock providers and hatcheries invest in NBC systems and commit to their use over many cycles and follow the guidelines regarding testing and history, animals will be clean.
“If there are problems at least the source can be identified. It is my opinion that if the industry as a whole does not tackle these issues, we are going to continue to see an endless cycle of new pathogens cropping up and the volatility in production will continue.”
Shrimp farming cannot be sustainable without this approach. Recent research has shown that the anti-viral mechanism in shrimp includes what are called EVEs (endogenous viral elements). Pieces of the virus end up incorporated into the shrimp DNA.
This complicates things potentially as some PCR primers will react with these. Their presence does not mean that the shrimp is infected; only that somewhere along the line its progenitors were. Irresponsible third-party testing can result in considerable damage if this is not addressed.
By Stephen G. Newman
“Domesticated Shrimp Postlarvae – The Key To Success”
See more:
- Potential roles of epigenetic regulation and intestinal microbiome in resistance to Vibrio in Pacific white shrimp
- Effect of a microencapsulated probiotic on the intestinal microbiome of Pacific white shrimp
- Robins McIntosh on everything you need to know about EHP and shrimp farming, part 1