By: Ph.D. Stephen G. Newman*
The global shrimp farming community has seen significant growth over the last three decades, with more than 5 million MTs being produced annually in 2022-2023. Currently, Ecuadorian shrimp farms are 52 producing around 25% of this total and are doing so at a lower cost than any other countries seem to be able to. Their production model is based on stocking large earthen ponds (average around 10 ha) at low densities, typically around 20 pieces per m2 and three- month cycles to harvest 20 grams plus shrimp.
This contrasts with much of SE Asia where the norm is small, lined ponds, less than 1 ha, stocking at high densities ranging from 50 to 500 or more animals per m2 and 3-6 months of culture (species dependent), with higher costs and density associated risks.
A number of factors have con tributed to this global increase in spite of the never-ending impact of disease on all sectors. The total number of hectares being farmed has in creased in some areas.
The move to wards high density culture, largely in SE Asia, using lined ponds, disinfection, bacterial amendments, aeration, bird nets and crab fences and static systems that are biofloc based has contributed to this increase, although currently this is resulting in what appears to be an overall reduction in market competitiveness due to high costs.
Ecuador has embraced low density production with the use of aerators, automatic feeders to reduce FCRs and control feed waste, and targeted bacterial tools for bioremediation. There have also been improvements in the composition of feeds and feed companies are realizing that the traditional sizing of pellets based on fish is wasteful and environmentally harmful.
Pellet sizes are smaller and geared toward less waste. Genetic improvement programs have become widespread focused on domesticating the most commonly farmed species, Penaeus or Litopenaeus vannamei and others, including P. monodon and the freshwater M. rosenbergii.
Significant progress has been made, with the Thai company Charoen Pokphand (CP) taking the early lead with L. vannamei. They have developed fast growing resilient pathogen free animals. The broodstock come from nucleus breeding centers and have been indoors in perpetual quarantine for many generations, where the selection process has generated lines that grow much quicker than many other stocks available, along with varying levels of disease and stress tolerance. Many companies are working on closing the gap.
CP animals are free of pathogens from the onset. They have been screened for all known pathogens and any new ones that crop up for which DNA sequences are available that allow PCR to be developed. The populations are examined closely by qualified histopathologists for any indications of pathognomonic pathology and the histories of their performances are followed.
“Under the right conditions, they thrive, and their genetic potential is realized. Fast growth to large sizes lessens exposure of the animals to potential threats that can be inherent in production systems, particularly stressors that cannot be controlled, common in outdoor systems.”
However, when these animals are held under less than favorable conditions, subjected to stressors, have poor diets, etc. they are weakened and can be impacted by a variety of obligate pathogens and any number of opportunistic pathogens. If these genetically improved shrimp die under these conditions, then it is hardly logical to expect that others will not as well.
Stress in farmed shrimp is a potential killer. Stress can be defined in terms of the impact that it has on animals. By altering the normal physiologic status of the animals, their homeostatic mechanisms are disturbed. Animals can fail to achieve their genetic potential and become susceptible to problems that non-stressed animals would be able to deal with.
“Three types of stress are recognized: acute stress, chronic stress, and periodic acute stress. Acute stress is a short-term reaction. Chronic stress is prolonged stress that persists. Periodic acute stress is short term but reoccurring. Genetics can influence how animals respond to various stressors and which ones are likely to negatively impact those factors that farmers depend upon for sustainability and profitability.”
In general, chronic and periodic acute stress are the most problematic, although far too many farmers fail to realize that even the short-term nature of acute stress can impact animals ability to weather other stressors and increase their susceptibility to potential pathogens.
Stress is inherent in shrimp and fish farming. The sources can be highly variable. In general, the type of production paradigm can impact the overall stress levels, with low density culture paradigms in large bodies of water with the ability to exchange large amounts of water from pristine sources being typically the least stress ful.
Medium density in smaller ponds with inadequate water exchange and no or inadequate oxygen supplementation aeration, combined with over feeding is more stressful. High density culture paradigms typically can be a high stress environment. Note that these are generalizations and there
are many exceptions.
Many of these can act in concert and there is variability as to what levels are stressful that depends on a myriad of variables, including genetics. Defining what levels of stressors are “normal” and acceptable is not always straightforward. Lab studies used to establish thresholds usually produce results that fail to replicate the real world.
“A level of stressor that is problematic under one set of environmental conditions might not be under another. A given stressor might be relatively benign by itself but pose a far greater threat when it is a component of multiple stressors.”
Commonly, the published limits of tolerance for many of these factors, usually based on controlled lab studies, are levels that are stressful. Determining lethal dosages that kill 50 % of a population establishes LD50s but does not establish the levels at which there is no impact.
Healthy animals are usually much more refractory, but in animals that are carrying a pathogen such as the etiologic agent responsible for White Spot Disease (WSSV) or any other number of pathogens, even small amounts of stress can pose a serious problem.
There are many published observations as to what levels of specific water chemistry parameters are problematic. Most of these studies are laboratory based and do not reflect the complexity of the pond environment. This is further complicated by the fact that there are differences between species, age susceptibility and stock history.
The tools for the improvement of genetics have evolved rapidly over the last few decades and many tools are in common usage that allow for accelerated genetic improvement compared with the older traditional selection approaches such as selecting survivors of disease outbreaks, larger animals, faster growing animals, etc.
Adding genes to an animal is considered a genetic modification and is not likely to be readily acceptable for the production of farmed shrimp, although for farmed salmon, Aquabounty Technologies has pierced this barrier with the introduction of genes that impact the growth rate of farmed Atlantic salmon and the commercialization of these faster growing fish.
This has been a many decades process and still runs the risk of adverse public reaction to the concept of GMO (even though for some widely consumed plants this is the norm). Entire genetic sequences can be sequenced relatively quickly and through the use of SNP (single nucleotide polymorphism) chips, a large number of genes can be screened to determine what genes may be responsible for certain traits, such as increased stress tolerance.
CRISP R (a bacterial antiviral defense mechanism) can modify individual genes to enhance or reduce functionality. Together, these tools offer a great deal of promise to generate strains that are tolerant or even resistant to pathogens (defined here as not being able to be infected by any dose under any conditions of culture) and that are less impacted by stressors.
“There is however complex legal and ethical issues that need to be resolved which are beyond the scope of this article to discuss. As a professional with more than 45 years of experience in aquatic animal health issues, I advocate proactive management of animal health as contrasted with reactive management.”
Prevention is often easier than trying to stop a problem in animals that can not be seen. Antibiotics, while highly effective when used correctly, are widely abused in aquaculture despite strong pressures not to do so. Ignorance is common and desperate farmers will do anything they can to save a crop that they cannot afford to lose.
Unfortunately, there are many all too willing to take advantage of this. Can we expect that in the years to come, these tools will generate lines of animals that are better suited for the current production paradigms?
Evidence to date strongly suggests that for consistent high survivals rates with animals realizing their genetic growth potential, at least three conditions must be met. The PLs must be free of all known obligate pathogens from the onset. This includes those that OIE (renamed WOAH -the world organization for animal health) dictates are of importance and any number of more recently discovered or historically imported pathogens that are not included in their must screen for list.
This pathogen free status must be established via repeat screening, quarantine (one way-animals only out), histopathology and histories. Note that this is for obligate pathogens for the most part. Producing animals that are free of opportunistic bacteria is not the idea. Secondly, they must have diets that contain the micro and macro nutrients that are needed to nourish animals that are growing rapidly (some lines can grow 4 to 7 grams a week).
“Thirdly, stressors that weaken the animals must be kept to a minimum. Nutrition can impact this to some extent, but in general, the nature of the production environment plays a critical role. Genetic programs can produce animals with a wide variety of traits that increase the chances of fast growing, disease tolerant and even disease resistant stocks that can also tolerate stresses to some degree.”
However, shrimp farming is not there yet, and farmers need to understand that they must proactively manage their production systems to maximize the ability of current is not likely that genetic programs will produce an animal that can be abused, fed a poor diet, be exposed to many obligate and opportunistic pathogens, and yet remain refractory to these and to the impact of any number of stressors.
This being said, shrimp have been around in their current form for tens of millions of years. This makes them very successful evolutionary system and there is always the possibility that there is enough genetic variability within them that there could be some strains that are much better suited to the rigors of current shrimp farming paradigms.
Nonetheless in the near future, shrimp farmers must evolve, to be able to benefit maximally from the diverse genetic programs and approaches to production. They should not count on genetics to solve those problems that are inherent in the manner in which they are farming or any magic bullets that are being sold as solutions.
The marketplace is dynamic and the supply and demand market forces that ensure that lowcost producers will continue to gain market share should force greater efficiency and lower costs of production if the industry is to continue to see increases in production to meet increased demand.
Stephen G. Newman has a bachelor’s degree from the University of Maryland in Conservation and Resource Management (ecology) and a Ph.D. from the University of Miami, in Marine Microbiology. He has over 40 years of experience working within a range of topics and approaches on aquaculture such as water quality, animal health, biosecurity with special focus on shrimp and salmonids.
He founded Aquaintech in 1996 and continues to be CEO of this company to the present day.
It is heavily focused on providing consulting services around the world on microbial technologies and biosecurity issues.
sgnewm@aqua-in-tech.com
www.aqua-in-tech.com
www.bioremediationaquaculture.com
www.sustainablegreenaquaculture.com
