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Does Natural Selection Produce Robust Postlarvae?
Brian Boudreau (firstname.lastname@example.org): Microalgae is known to have a positive effect on water quality and, by displacing Vibrios, a probiotic effect on shrimp health, so we chose algae as a probiotic instead of bacteria at our hatchery and got very positive results.
John Birkett (email@example.com): Brian, we normally use algal blooms through the whole farming cycle—larval rearing, raceway and growout—and we use them as required for pH, carbon/nitrogen and ammonia problems. We also boil molasses and use it in tanks and ponds to stimulate bacteria growth. When boiled, it is a fast carbon source that is safe, and it works with the densities that we use here in Ecuador.
Brian Boudreau (firstname.lastname@example.org): John, there has been a shift away from open, less-intensive hatchery systems that use lots of live microalgae toward closed, hyper-intensive hatcheries that rely heavily on disinfection and the use of biocides. The underlying principle is to kill everything in the incoming water, and then try to grow back only what you want. I disagree with that approach.
Over the years, successful artificial diets have practically replaced algae and other live feeds with the obvious advantage of increased biosecurity, but not all hatcheries sterilize, and as a result, are open to naturally occurring microflora. High-protein, nitrogen-packed artificial diets can have a negative impact on water quality and lead to the use of antibiotics and probiotics to kill or displace harmful bacteria, particularly Vibrios. Microalgae on the other hand is known to have a positive effect on water quality, along with a probiotic effect.
Hatchery managers consider larval survival rates as very important, but growout survivals are much more important. Highly fecund species like Penaeus vannamei produce lots of offspring that are subject to natural selection pressures in the wild, resulting in high rates of mortality during their early life stages. Artificially high survival in the hatchery may not be a good thing for the farm because it implies relaxed selection pressure during the early stages, allowing unfit animals to get stocked on the farm. It makes more sense to take the mortality hit early—when the shrimp are less valuable and the economic losses are smaller. In other words 30% survivals in the hatchery and 70% on the farm are far better than 70% survivals in the hatchery and 30% on the farm.
With this objective in mind, the company that I work for, Grupo Granjas Marinas (GGM), the largest shrimp farming operation in Honduras, has designed a new hatchery system that allows natural selection to take place. We’ve adopted large, two-phase, larval rearing tanks that are stocked at lower densities. The larvae are held in them for a longer period with the aim of producing bigger, stronger, more uniform postlarvae at a lower survival rate. The new system depends strongly on microalgae and encourages other microorganisms in the larval rearing tanks. It’s also more dependent on live feeds, especially large diatoms.
Hank Bauman (email@example.com): Brian, your idea of low survival in the hatchery to screen out weaker animals only has merit if the culture system is basically flawless and the reason for weaker animals is due to genetics. But more times than not, hatchery mortality is due to fouling, over feeding, or beating the animals up with water exchanges. In these cases, the cause of the mortality is random and more due to luck than the genetic strength of the larvae. For this reason, I’ve always had better success in ponds with animals that get high survival and fast growth in the hatchery.
The idea that low larval survivals produce stronger postlarvae was proven false 25 or more years ago when we were producing tiger shrimp in the Philippines. What killed the larvae made the surviving larvae weaker.
Jorge Cordova (www.campacseafoodllc.com): I agree with Brian—increased natural selection at the hatchery level results in stronger animals for stocking the ponds. Even screening out smaller animals at the hatchery level results in better growout economics.
Using raceways/nurseries also allows farmers to stock healthier juveniles. This was proven when whitespot hit Ecuador back in 1999.
Greg Lutz (firstname.lastname@example.org): Hank, anyone selecting for survival in the hatchery without a database to establish a correlated response in the production environment has a very strange interpretation of genetic improvement.
Jorge, data to support your comments would be of great interest.
Hank Bauman (email@example.com): Our hatcheries in the Philippines had higher than average survivals (60-75%) and commanded a premium price because of their higher survival and performance in ponds. Many farms averaged over 90% survivals. Not all weak larvae die and not all strong larvae survive during a bad production cycle. Death is based on bad luck, rather than weak postlarvae. Fungal and bacterial fouling are random. They hit the strong and the weak.
Perhaps there is a stress test you could administer, like high salinity or temperature shock, to weed out the weaker larvae, but those that don’t die may be weakened beyond recovery, which could lead to secondary mortality later.
Brian Boudreau (firstname.lastname@example.org): Hank, I disagree that mortality in a low-density, open-system hatchery is based on bad luck and that fungal and bacterial fouling are random. Random fungal and bacterial fouling will only happen if the hatchery starts with sterile conditions. In our case, we do not sterilize incoming water; the larvae are exposed to the same microflora they will encounter during growout. If survival rates drop, so be it. Our emphasis is not on survival, but on low-cost, strong postlarvae.
Patrick Wood (email@example.com): Producing strong larvae is the way to go. Farmers prefer strong postlarvae, and this can be accomplished by producing strong larvae.
Brian Boudreau (firstname.lastname@example.org): Hank, In 2014, we averaged 62% survivals from 19,025 hectares of ponds, and this year we’re averaging 58% from the 360 hectares that have been harvested so far. The new hatchery plan is to lower stocking densities from 150 nauplii per liter to 100 nauplii per liter in larval rearing tanks that will be 30% larger. In the second phase, from PL-4 to PL-18, the tanks will be 20% larger than those in the first phase. In 2014, our final hatchery survival rate was 50% from stocking 150 nauplii per liter. In our new system, we expect an overall survival rate of 40% after the second phase. We’re hoping for 33 postlarvae per liter. The lower densities will facilitate large-scale use of microalgae, and we will have a much lower feed requirement!
Gruenberg Daniel (email@example.com): Brian, this issue is much more complex than a linear relationship between survival and pond performance. It all depends on what system you are using, what the stocking density is at the hatchery, what specific methods you use for larval rearing, what stage/size PLs you produce, what feeds are available to the larvae, and so on.
It is well documented that you can nurse juveniles to a later/larger stage, and while you lose animals during this nursing phase, you compensate with improved growth and survival in the growout phase, so in this case your statement is true.
However, if your low survival is due to simply poor management, those animals will also likely perform poorly in growout because there may have been subclinical damage to critical organs during the poorly managed larval rearing phase.
In general, I agree with your statements because you’re saying that you’re using low intensity systems with a good mix of natural food and growing your animals to a larger size. Under these conditions, I think you’re right that lower survival in the hatchery system will be compensated by better performance during growout.
Brian Boudreau (firstname.lastname@example.org): There are two kinds of hatcheries. Those that produce for their own farms and are concerned about farm survivals and size uniformity, and those that have no farms and are mainly concerned with hatchery profits.
Greg Lutz (email@example.com): It seems an animal that does well in larval culture might not always do well in the pond environment, where a whole new set of variables affect survival. But I’ve never seen a study structured to answer that question.
Brian Boudreau (firstname.lastname@example.org): No question that selection pressure in the hatchery is different from that in the pond, but this difference is exacerbated when the larval rearing environment is very different than the pond. A lot of hatcheries today have artificial selection pressures because they live in a bubble. We want to turn back the clock to the older Taiwanese-style hatchery.
John Birkett (email@example.com): Here are a few things to consider:
1. A beaten-up larvae due to bad rearing techniques will be a problem during growout. Slow growth and low survivals are very likely to be the result.
2. Two and three-phase larval rearing systems, like those used in Ecuador, produce better results that single-phase systems—and that has been statistically proven in Ecuador. So leaving the weaker or smaller animals behind is a good thing.
3. Natural larval rearing techniques (as natural as possible) meaning no heavy disinfection and no disinfection of water source unless specifically needed, locating your hatchery in an area less prone to biological problems, utilizing algae blooms till the last day of larval rearing, using Artemia or copepods, good tank management, low water exchange with probiotics, and good tank control with good diets is the way to achieve robust larvae.
4. You get the best larvae after a run with fewer problems. Juan Aguirre (firstname.lastname@example.org): As W. Edwards Deming said, “In God we trust, all others bring data.” Hopefully somebody will have the data to correlate hatchery survival with pond survival. Probably some groups have enough ponds and years of accumulated data to get statistical validity.
Patrick Wood (email@example.com): Nursery ponds on farms have always been useful to weed out the runts. We never did direct stockings in Ecuador when the industry was starting up in the early 1980s because we only had access to wild postlarvae, and the availability/seasonality meant we needed to stock up with seed during the good times. It was also a way of checking the quality of the wild seed that you had purchased. We gave our suppliers feedback on the juveniles that came out of the nurseries as a commercial lever and/or to improve their deliveries.
With the development of hatcheries, relatively consistent, year-round supplies of PLs and aquaculturists arriving from Southeast Asia, direct stockings were entertained.
When we were making the move from wild to hatchery produced larvae we did many fun things like stocking mysis stage P. vannamei directly into nursery ponds—which was a total failure. Other people developed pond-side, concrete, acclimation raceways.
Sources: 1. The Shrimp List (a mailing list for shrimp farmers). Subject: Shrimp Disease in the World Bacterial White Patch Disease Caused by Bacillus Cereus. March 26 to 30, 2015. 2. Bob Rosenberry, Shrimp News International, April 6, 2015.
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