By: Ph.D. Stephen G. Newman*
It is well known that the higher the density of a given population the greater the potential is for pathogens to move between animals. Shrimp are not an exception.
Shrimp farming is perhaps the fastest growing aquaculture sector globally. Total production has been estimated to exceed 5 million MTs in 2022 with no indications that it is slowing down.
As shrimp farming paradigms continue to evolve and adapt to the realities of what appears to be the evergrowing demand for farmed shrimp, the natural tendency has been to increase stocking densities. For many years the standard production model in the Americas was low density culture in large ponds.
Ponds were rarely aerated, organic matter was allowed to accumulate often to toxic levels, feeding was largely inefficient, and consistency of outcomes was highly variable. In Asia land is not as readily available (costs more) and smaller ponds are the only realistic option.
The production model early on was small dirt ponds with some additional aeration. Similar issues with consistency of productivity. As the pond sizes shrink, technology comes into play. Several factors are important for success and to allow stocking densities to increase.
- The use of aerators to ensure adequate oxygen levels reduces stress on the animals and ensures a healthier overall environment.
- Lining of ponds with plastic liners prevents heavy accumulation of sediments and their chemical impact on the pond chemistry and thus the animals.
- Pond engineering designs that allowed organic detritus, a natural part of the process, to be concentrated in the central area of the ponds in sumps. These are drained periodically during production and held in sedimentation (responsible aquaculture practice) or dumped into the environment untreated (irresponsible).
- The use of automatic feeders that avoid waste of feed.
- Properly formulated feeds that contain sufficient digestible protein levels, minerals and vitamins.
- The use of bioremediation with bacteria to clean up accumulated organic matter, limiting the
amount of toxic compound production.
For several decades efforts have been underway to domesticate the mostly widely farmed species, Penaeus vannamei (also called Litopenaeus vannamei). This has been a slow process with incremental progress. At the cutting edge are the animals that CP Thailand has produced. These are fast-growing, pathogen tolerant and high-density (stress) tolerant animals.
They are free of all known pathogens and through methodical testing and following animal histories are free of any other as of yet uncharacterized pathogens. This pristine status is what allows farmers who ensure that the animals are not contaminated with local pathogens to reap the benefits of these animals.
“Most workers in the field will tell you P. vannamei can be considered to be domesticated. Efforts are underway with other species.”
Densities above 300 m2 are not uncommon. Survivals can be high, in the 90% range and growth is nothing short of spectacular with 30 gram plus animals taking 60 days or so to get there under the best of circumstances.
Coefficients of variation (CoV) are measures of overall productivity. There is very little size variation (CoV) between animals at harvest. When they are low it is an indicator of high animal health and consistent performance.
When they are high it is an indicator of the presence of certain pathogens and a population lack of consistent performance. This could be a result of stressors including low DO levels, lack of feed (automatic on-demand feeders solve this), high levels of accumulated organics in the production environment, IHHNV, etc.
“All of these positive results have a critical component. Without this, the risks of disease increase significantly as the densities increase. This is that you must start with clean animals. These are free of known and when properly produced, unknown, pathogens.”
When they become contaminated, the potential is not realized and there can be massive losses. Starting out with clean broodstock is the first step. Feeding them biosecure feeds to stimulate maximum fecundity is next.
Maintaining them in an environment where there is no input of potential sources of pathogens must be assured. In some instances, when a facility is close to the Ocean or to neighbors who have disease issues, this has to include positive air flow to prevent airborne vibrios and other potential pathogens from entering the facility.
They should be spawned under conditions that also ensure that the pathogen free integrity is maintained. This includes adequate disinfection. Once nauplii are stocked, they must be held in biosecure environments.
This means that when they molt into Zoea that they are not being fed algae and or Artemia that have been produced in a manner that is not biosecure. The same thing holds for the hatchery cycle. None of this is complicated.
It is well known that the higher the density of a given population the greater the potential is for pathogens to move between animals. Shrimp are not an exception. Many a farmer who has been trying to grow shrimp at high densities without the aforementioned approaches has seen this. The level at which problems occur will depend on genetics to some extent.
“Stress and pathogen tolerance can be bred for. It also depends on the pathogen. Some pathogens are more virulent than others. This can mean that they spread easier and faster or that they initiate a serious disease process that results in rapid death.”
Farmers who saw the beginnings of the virus that causes WSS saw this even at low densities. The viral loads exploded and many of the “normal” elements of the environment, such as rotifers and copepods, became infected.
This is a real risk that should be considered before increasing environmental biomass loads beyond their carrying capacity. There is not a magic number for this. It will vary depending as above on the animal, the environment, and the pathogen. It is tempting when survivals are good with rapidly growing shrimp stocked at 20 m2 to double it.
“Then the temptation will be there to double it again. At some point when the animals are not free of pathogens (they can be SPF-specific pathogen free-but this does not make them free of all pathogens) the risk of disastrous disease outbreaks increases.”
When ponds are built in close proximity to each other and there is no coordinated proactive disease management strategy in place among the various farm owners the risk increases as well that one’s persons problem becomes another’s.
Their neighbors get what they have and this spreads from there. Countries that historically have grown shrimp at the low-end density wise need to be cautious and work together to limit the potential for increased stocking density related animal health challenges and slowly work towards increasing the carrying capacity of the environment.
“There is not a set formula to follow when it comes to determining the carrying capacity. One knows that there is a problem because of poor animal performance even when things are being done “right”. Most often in today’s world the presence of pathogens in the population is the limiting factor.”
If you follow these guidelines, you will be able to gradually increase your stocking densities without increasing the risks of massive disease outbreaks. If farmers rush these things they may enjoy short term success but invariably the lack of a high level of biosecurity will cause increasing losses.
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.