As opposed to what we saw with feed, the previous installment of the series, seeding a tank might seem much less impressive and impactful, but let us remember that is the quality of the postlarvae and the seeding technique the first step that will mark the success or failure of our production.
Seeding is much more than just putting shrimp in a tank; it goes way further before. We can divide the seeding process into five critical steps:
- Postlarvae selection
- Tank or pond preparation
- Production density
- Seed transfer
In this chapter of the series Shrimp Farming Basics, we will discuss each one of these points so that farmers can make better decisions regarding their production and get a better outcome in their farm’s productivity. Let’s get to it.
First things first, what is a postlarva? Well, shrimp, just as the majority of crustaceans, do not have direct development. This means that “baby shrimp” are not exactly like adults; in fact, they go through a series of metamorphoses since they hatch from the egg. These changes are divided into four stages which differ a lot from how an adult shrimp looks like; these stages are: nauplios, zoea, mysis, and finally, postlarvae. Essentially, a postlarva or juvenile is the stage in which the small animals are physically (morphologically) equal to the adults and have similar requirements in terms of temperature, feed, water quality, depth, etc. So, in simple terms, a postlarva is the final stage of the development of shrimp.
Usually, when we buy seed from a laboratory (or PL as an abbreviation for postlarvae) we are asked what kind of PL we would like to acquire PL5, PL10, PL15 etc. Although this might seem confusing to someone who is new to the industry, these numbers are nothing but the days elapsed since the moment of their last transformation, so once the final metamorphosis takes place (that is, when the last mysis stage ends and the first PL stage begins) the organism is known as PL0; so a PL5 means that it’s been 5 days since they became juvenile, PL10 means 10 days and so on. The larger the number, the longer the time they have been kept by the laboratory, meaning they are bigger and sturdier, therefore, more expensive. In this regard, we must select the size that has a better performance in our production conditions for the smallest possible price.
In addition to the PL stage, the farmer must select the genetic line that better suits their management practices. In the case of P. vannamei, two main genetic traits have been selected over the last decades by the PL production laboratories worldwide, resulting in three different strains: one with a high growth rate but with lower resistance to diseases, a second one with lower growth rate but with higher resistance to pathogens, and the third one with increased performance in low salinity.
The selection of the strain depends on the experience, infrastructure, personnel, risk preference, and capital of the farmer. If the farmer is experienced, has high control over the water quality and biosecurity of the farm, counts with qualified personnel, has a risk-neutral or risk seeker personality, and has enough capital for feed, the best way to go is with the high growth strain since a farmer can get either bigger shrimp on the same time-lapse or same size shrimp in a shorter period, increasing the year-long productivity and the profitability of the farm. This strategy has the risk of losing a big share of the farm to disease, but good management should reduce it. Furthermore, it’s important to understand that bigger growth entails higher feed rates. Even if the FCR is the same, since shrimp are growing faster, they would need more feed to achieve their full potential. This is important because if we get a high growth strain and underfeed it, we will have the same result as a high resistance one but without the pathogen resistance boost, meaning lower survival and same growth, resulting in lower productivity; that’s the reason why capital availability (for feed) is key when selecting a high growth strain.
On the other hand, high resistance strains are appropriated when there are low biosecurity levels, control is difficult, there is a history of recurrent pathogens, personnel is less qualified, and the producer is averse to risk. This strain is the most commonly used in Latin America since there are low biosecurity standards and the size of the farms makes it difficult to control. Furthermore, episodes of diseases (such as the White Spot Syndrome Disease or WSSD) have eliminated complete stocks, even disappearing some farms, leaving a scar in LatAm producers, making them highly averse to this kind of risk (hence the low seeding density and the selection of this strain).
Finally, the low salinity strain is self-explanatory. This genetic strain is used in places where sweetwater is abundant or that are far away from the coast. In general, P. vannamei is a euryhaline species, but it performs better in marine environments. With this selection, it is possible to produce in cheaper land away from the coast but close to urban nuclei, with a performance similar to what is observed under optimal salinity conditions. Overall, shrimp perform better in salinities circa 32 ppm, even when compared to this strain, but the benefits of location and land might make this the best choice for some farmers, such as the freshwater producers (like tilapia) looking to dabble in higher-value markets (like shrimp).
Nursery, yay or nay?
Once the PL genetic strain and stage have been selected and bought, there are two things that can be done. First, and it was the original practice, we could seed the PL directly into the prepared ponds for fattening; or second, we could go through an intermediate production stage, between hatchery and fattening, known as nursery.
The nursery stage consists of a series of tanks of different shapes and structures (almost always covered in geomembrane or some kind of sturdy plastic) that have production conditions similar to those observed in the fattening ponds but in a smaller, more controllable, and biosecure size. The objective of the nursery is to provide more time for larvae to adapt to their new conditions with higher biosecurity and more control than that observed in the fattening ponds, increasing survival of the early stages up until they develop more resistance derived from growth. This system also allows to have better control of the larvae quality (which can provide more elements when claiming a guarantee), and it can be easier to identify a disease outbreak before it entails a significant cost to production.
In nurseries, shrimp are maintained at very high densities (up to 10,000 PL per m3) while they adapt to the new system and conditions. This way, the time they stay at the fattening ponds is reduced, reducing risks and increasing profitability. The design and management of a nursery is a topic by itself, but several studies have proven that well managed, these systems are very useful and profitable. By the time the nursery stage ends, shrimp seeded in fattening tanks can reach up to 3 gr (between 1-3 kg per m3 harvested), instead of the 0.1 gr that were used during the early stages of shrimp farming.
Furthermore, during this stage, the sizes and types of feed used are constantly changed. This gives a better chance of increasing growth and reducing feed waste. Overall, the benefits mentioned along these lines result in a series of economic efficiencies, resulting in more profitable and less risky productions.
Several studies (both from industry leaders and researchers) have proven that certain kinds of well-designed and managed nurseries can significantly increase the profitability of the farm. Even though it might be a considerable investment, it sure pays back the benefits, especially for bigger farms that have higher risk and increased management difficulty.
At this point, we have the seed ready to go through the fattening process, but have we considered where they are going to grow? One common mistake made by beginner farmers and small entrepreneurs who want to dabble in the aquaculture business is not to take care of the pond water before seeding.
Depending on the infrastructure and the system, ponds should be filled with water and other additives between 5 days and one month prior to production. Water in the system needs to be balanced, stable and checked before introducing any shrimp in there. In the case of recirculating aquaculture systems, all equipment needs to be checked, calibrated, and tested before starting production; water needs to recirculate, and parameters must be observed to assure that there are no significant changes that might harm our production.
Water preparation or “maturation” is especially important in systems that depend on in-pond bacteria to take care of denitrogenation and water balance, such as biofloc or aquamimicry. In these systems, water needs to be enriched with prebiotics and bacteria at least three weeks before seeding, water needs to be properly aerated, and the water column needs to be in constant movement. This way, water reaches an equilibrium, and it allows floccules to grow (remember that flocs are nothing but bacteria and other organic matter arranged in a floccule). The presence of floccules is key in these systems, not only for water treatment but also as a feed source for the shrimp to be introduced.
In all cases, when we talk about water balance, we mean having control over water quality parameters (such as oxygen, pH, and suspended organic matter) so that they don’t present severe changes during the day. Severe changes in water quality (even if they last a couple of hours) can affect the growth and survival of shrimp. Let’s remember that algae, bacteria, and yeasts’ metabolism have effects on oxygen and pH. In the case of algae, there is an oxygen consumption phase of photosynthesis during the night; if this is not accounted for or algae biomass grows too much, it could have a severe drop in oxygen and pH (due to the increased presence of CO2) that, summed to the presence of shrimp could result in a significant biomass loss if not considered and controlled.
So we now have our ponds and shrimp ready to go but, did I calculate how many shrimp per unit of area the system can hold? Production density is very important. Actually, it’s so significant that it’s the parameter used to classify the kinds of production (from extensive, when there are few shrimp per m2, to super-intensive, where you have a lot of shrimp per m3). The density that the system can manage depends on our ability to control the water quality parameters, the biosecurity of our facilities and management practices, our capital for production, and the infrastructure of the farm.
In general, less experienced farmers are encouraged to produce in lower densities since high concentrations have higher probabilities of presenting diseases and water management problems that, if not treated adequately, could result in a total lot of the crop.
Apart from experience, the capacity of exchanging water quickly and constantly measuring water quality parameters is another significant aspect to determine the carrying capacity of the system. Normally, smaller tanks with liner or plastic surfaces are better suited for intensive production than earth-bottom large ponds due to the fact that there is less area for pathogenic bacteria to grow, and the cleaning and maintenance of the system is easier and more effective between production cycles. Furthermore, if a significant problem is detected in a smaller pond (say a high presence of pathogenic bacteria or a significant hypoxic event), it’s possible to sacrifice that tank’s production without seriously compromising the profitability of the rest of the farm, whereas this loss would be a lot more significant in a large pond; hence, increased density significantly increases risk, and this effect is amplified in large earth-bottom ponds.
Once the production density is determined, we need to count the larvae that we will introduce in each pond (this should also be done when receiving the larvae and before the nursery). For this, there are two main techniques: a manual and an automatic count.
For manual counting, the best technique for shrimp is a volumetric inference. For this, we take a known, smaller volume of the water in which the PL are kept. We take that volume and manually count the number of larvae in that volume. Finally, we extrapolate that number to the full volume in which the larvae are kept. For example, we take 1 liter of water and count 150 larvae; if we have 20 m3 of water, we can infer that we have 3 million PL in the full system. It is important to do this method several times before jumping to a conclusion, so take at least 10 samples of a known volume, count the larvae and then do an average before scaling; this way, we reduce the error associated with chance and increase the statistical meaning of our method.
Automatic counting is somewhat similar, but the counting is not made manually by the farmer or technician, it is done with cameras and AI algorithms. The farmer takes a known volume of water, it is then introduced in a counter, and the system reveals the number of PLs in that volume. When the full volume is entered, the machine gives back the total amount of PLs in our water.
Once we know how many larvae we have per unit of volume, we just need to know the volume or area of our tanks or ponds to know how many liters we need to introduce in the system to achieve the desired density.
Finally, we are ready to start the fattening process. We selected the best genetic strain for our region and production method, we know what density we want to use depending on our system, we know how many PLs we have per liter, we also have the ponds balanced and ready to receive shrimp, and we have decided to go through or not to go through a nursery process, so the final step is to introduce the PLs in our fattening tanks.
If we decided to go through a nursery phase, the transfer would be fairly easy. Since the water used in the nursery has the same parameters as the one in our fattening ponds, we only need to pass the shrimp from nursery ponds to fattening ponds. If the farm is big in terms of area, several nursery systems can be added along the farm to facilitate the transfer, the closer to the production ponds, the better. Transfer can be made manually with buckets or other containers with known volumes (although this could take a lot of time), it can also be semi-automated, by transporting large volumes in a vehicle and pumping the water to the pond using a mobile water pump, or it can be automated with the installation of a pipe system that connects the nursery with the fattening ponds. This way, using a ball valve, we can control the water flow rate and introduce the desired volume in each tank.
On the other hand, if there was no nursery phase, we first need to acclimate the PLs to the water conditions of our ponds. To do this, we can first separate some water in another container or put the bags with the PLs directly in the production ponds; then, we will introduce our water slowly and over a considerable amount of time in the different bags. This is a slow process, but if it’s rushed, it can result in high mortality levels due to temperature, salinity, ammonia, oxygen, or any other water quality parameter shock.
As we discussed over this paper, seeding is a more complex process than just putting shrimp in water if we want to succeed in the shrimp aquaculture business. If done properly, we could reduce seeding mortality by almost 95-100%, reducing our overall production costs and maximizing the profitability of the farm.