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Shrimp Maturation Tanks—Size Matters

September 14, 2007

 

 

Guillermo Jaramillo (guillojara@yahoo.com): For hatchery folks: What is the minimum space required for natural matings of Penaeus vannamei?  I’ve seen many tank sizes and shapes, most of them large to optimize the number of broodstock, but small enough so that the mated females can be hand-captured from the sides of the tank.  Has anyone built small maturation tanks for natural mating?  What was their size and shape?

 

Josh Wilkenfeld (josh.wilkenfeld@gmail.com): “In the beginning”, I think that many of us tended to work with circular tanks.  The smallest size I used was about 12 feet (3.7 meters) in diameter, with a surface area of about 10.5 square meters.  Over time, as commercial hatcheries became larger and larger, construction costs and a demand for more efficient use of space moved us to bigger tanks, usually rectangular in shape, often without rounded ends, keeping at least the width limited so that workers looking for mated (open thelycum) or fully mature (closed thelycum) females could reasonably reach about halfway across the tank, working from either side.  The largest maturation tanks I’ve worked with comfortably were about 3 meters by 40 meters (120 square meters), but I think a more reasonable compromise to the goddess of cost and function would be about 3 meters by 15 meters (45 square meters).

 

I have not directly addressed your original question: “Has anyone built small maturation tanks for natural mating?”  It would probably help us if you would indicate the actual reason for your interest.  Knowing that one of your [Shrimp Improvement Systems, a P. vannamei broodstock company in Florida, USA, that’s owned by Indonesia’s PT Central Proteinaprima] primary objectives is development and maintenance of different species and lines of broodstock shrimp, I would guess that you are at least partly concerned with maintaining groups of animals isolated from each other, to keep them from crossbreeding, to avoid interference in studies where pheromones released in the water might affect other groups of shrimp sharing the same water, or simply for quarantine purposes.  Of course, another reason for thinking about smaller rather than larger tanks may simply be that you are working with space limitations imposed by trying to fit into an existing structure—always a frustrating experience.  In any case, here are a few things to consider when designing maturation tanks:

 

1. Broodstock size animals (say females of about 43 grams and males of about 37 grams) need some space to go through their chasing and mating ritual.  This can easily be disrupted if they run into right-angle corners, which they are more likely to do in smaller, but more space-efficient square or rectangular tanks.  Hence, the smaller the tank, the more important it becomes for it to be circular, or to have well rounded corners, which direct shrimp traffic around the curves.

 

2. Another consideration that is important in deciding tank size is the best density to work with for mating purposes.  My own experience is that the best results in terms of the percentage of females mated per night seems to be at a density of about 4-5 shrimp per square meter (using same average sizes for females and males as in #1 above), and you actually start reducing mating efficiency at densities above 8 shrimp per square meter.  For that reason, I prefer to work at about 6-7 shrimp per square meter as a compromise in use of space and mating efficiency.  Even in a situation where you prefer to work with smaller size tanks, the size of the tank may be dictated by the minimum number of spawns you need per day per tank.  A tank 12 feet in diameter with a surface area of 10.5 square meters can hold about 73 shrimp.  I usually work with a ratio of about 47.5% males and 52.5% females, so that would mean about 38 females and 35 males in 10.5-square-meter tank.  With a mating rate of about 9% females/day (a reasonable assumption for vannamei in a commercial operation), this would give about 3-4 spawns per tank per day.

 

3. I’m just extrapolating here, but using some of the reference numbers I’ve given above, I think you could probably work with a tank as small as 8 feet (2.44 square meters) in diameter and still get about 1.4 matings per night.

 

If you are not concerned about whether different groups of shrimp actually share the same water, you might consider constructing the less expensive, space-efficient, larger, raceway-type tanks and using simple but very effective dividers constructed with PVC pipe and plastic screening material.  I’ve done this on several occasions and it works very well.  Depending on the size of your compartments, you may still have to contend with the question of square corners, but this also can be dealt with through design of the dividers.

 

Dallas Weaver (deweaver@scientifichatcheries.com): I did some work with tanks that were five feet in diameter (about two square meters) and I think they were too small.  We got maturation, but had difficulty in getting natural mating.  We were just game playing so the information is not scientifically valid and should be viewed as one nonbiologists amateur experience.

 

Durwood Dugger (dugwood@gmail.com): I can add a few minor comments from a cost-efficiency perspective.

 

If I remember correctly, the Bureau of Commercial Fisheries, which later became the National Marine Fisheries Service, in Galveston, Texas, USA, did a number of studies looking at maturation tanks in the 1960s.  As I remember, they experimentally determined that tanks 12 feet in diameter (4 square meters) were the point of diminishing returns regarding effective matings.  That became the basis for an industry norm.  Based on this work and our own, we still don’t recommend maturation tanks smaller than 4 square meters and prefer them to be at least 5 square meters.

 

We never recommend anything other than round maturation tanks from both a capital and operating cost point of view.  Regarding tank shape, on the basis of simple physics we believe that anything that isn’t round requires more energy and more cleaning, which disturbs and stresses the broodstock.  Consequently, as you deviate from round tank designs, you have more water quality problems, more health issues with your broodstock, and an increased risk of contaminating resulting spawns.  Circulation and self-cleaning become more hydraulically problematic when the tank isn’t round.

 

The increased energy inputs, labor, and maintenance costs of rectangular tanks easily offset the 15-20% loss of floor space associated with round tanks.  Not to mention that round tanks are almost always cheaper to build than straight-sided tanks because they require less material to contain equal amounts of water.  We also try to avoid concrete maturation tanks because of the sterilization difficulties associated with their typical micro-porosity and cracks, though, in some parts of the world, they are often cheaper to build.  We believe that the adaptability of portable, nonporous round tanks offer a significant premium that in the long run offsets any initial savings on round concrete tanks.  How many concrete tanks do you see moved from one area to another, or site to site?

 

Alfredo Medina R. (amedinar1961@yahoo.com): If my memory serves me right, the tanks constructed at a commercial installation in Mexico were 3.5 x 20, that is to say 70 square meters, and used 8 to 10 shrimp per square meter.  The mating percentage was 15%, averaging 160 thousand nauplii per female.

 

I really haven’t observed significant differences in the amount of sludge on the bottoms of circular and rectangular tanks,  Sludge control depends on the number of valves for incoming water and drainage.  It’s manageable.

 

In the case of “individual” maturation tanks or for “a single pair”, as a matter of personal opinion, I lean toward rectangular tanks of 3.5 x 7.0 meters.  As you know, there are many useful materials for constructing maturation tanks; it’s only a question of cost.

 

Josh Wilkenfeld (josh.wilkenfeld@gmail.com): I agree that round fiberglass tanks are ideal in many cases.

 

Following are some additional thoughts:

 

1. In some of the places that I’ve worked, obtaining fiberglass tanks of reasonable quality has turned out to be a bigger problem than one might think, and they can be considerably more expensive.  Unless what you want is an off-the-shelf product, obtaining what you want can get to be a very time consuming process including fabrication of molds, inevitable errors and shipping.  For example, when I was in Indonesia in the 1980s, we had to find a willing fabricator for elliptical/V bottom larval rearing tanks in Jakarta.  There were no capable fiberglass fabricators in the provinces where we were contracted to build five hatcheries for the government.  It took months to get the first prototypes of the various types of tanks we needed, and many more months to get them constructed and then shipped by truck/ship/truck in pieces to the hatchery sites for final assembly.  It may be easier now, but it was very difficult then.

 

2. As Alfredo indicated, both he and I have worked with rectangular tanks at a number of different facilities and have been satisfied with them in terms of management and productivity.  I agree that even in these non-round tanks that were designed with slopes, inlets and drains that were supposed to help keep the tank bottoms clean, we did have to siphon on an almost daily basis to keep them truly clean.

 

3. Portable fiberglass tanks with a one-time slick finish are indeed the ideal, but sometimes (or even many times), they don’t turn out to be a practical option.  Concrete can also be very expensive in some places, and plastered, epoxy-painted, or not, they usually turn out to be problematic in terms of a sterilizable finish.  For that reason, I almost always line tanks with HDPE liners.  When working with HDPE liners, construction with concrete hollow blocks (CHB) is often less expensive than solid concrete, and there are even less expensive solutions (banded plywood, partial sub-floor level tanks), none of which are quite as elegant as fiberglass, but sometimes, it’s necessary to make due with what is readily available.

 

Keep in mind that if the basic tank frame is constructed of CHB, the blocks may still need to be plaster coated before forming the HDPE liner.  Cheap blocks tend to crumble with time and/or exposure to water, and the disintegrating material can puncture the liners.  These are concerns that are also eliminated with fiberglass.  However, properly constructed lined ponds can be remarkably durable.  I visited the two SyAqua hatcheries in Mazatlán (originally the Super Shrimp hatcheries constructed in 1996 and 1997) and was pleased to find that the original HDPE liners we installed were still in use and in excellent condition.

 

4. With good water treatment, tank management and regular prophylactic treatments of the broodstock animals with 5 parts per million (ppm) of Copper Control (a static treatment, once every two weeks for two hours, staggering treatments so that only 25% of tanks are affected at a time), we’ve been able to keep our animals in great condition, although spawns might be reduced for about three days in the treated tanks.  We also do a quick dip of spawners in 100 ppm Argentine (followed by an immediate dip in clean seawater to avoid gill damage) before placing the shrimp in spawning tanks, and we thoroughly wash all the harvested eggs, then treat them with a 30 second dip in 50 ppm Argentine before transfer to hatching tanks, to avoid potential disease or bacterial contamination.  I don’t know if it would be possible to avoid some of this labor and handling by using round, fiberglass systems with better hydrodynamics, but I would probably take the same precautions with spawner and egg treatments in any case.

 

5. No doubt that siphoning is a time-consuming pain, and it does disturb the broodstock to some degree.  It’s been a long time since I’ve worked with round maturation tanks, but it seems to me that even in those tanks, it was still necessary to siphon almost daily to keep the bottoms really clean.  I agree it was probably less work (and thus less disturbing) than what has to be done in the larger rectangular tanks.

 

I want to make one other point regarding the percentage of mated females per night, which comes to mind after reading Alfredo’s comments.  The only time I’ve seen spawning rates of 15% a night was when we were working with a Venezuelan strain of P. stylirostris, which were truly rabbits in their reproductive capacity, especially starting with the F1 generation that we reared on site.  However, we always worked with ablated females.  During the late 1990s, Alfredo also got to work with locally pond-reared vannamei in one of the Super Shrimp hatcheries located in Mazatlán (I restricted the El-Golfo hatchery to stylirostris at that time, to maintain the virus-free status of that line of shrimp).  During my most recent work with Genitech in Mexico using virus-free, locally available strains of vannamei, I focused on production without ablation and found that at least with these shrimp (mixture of imported strains from Venezuela, Columbia, local Mexican stocks, and probably others, dating back to the late 1990s), there was virtually no difference in spawning rates, and we were actually getting more nauplii per spawn from unablated females than from ablated ones.  We averaged about 8-10% spawners per day.

 

Skipping off of the subject of tank size, shape, and construction materials, there are obvious advantages to working with non-ablated females, among them reduced losses from ablation mortalities, self-governing maturation frequency (better recovery of necessary reserves by females, better quality eggs and nauplii) and actual increased productivity in terms of nauplii per spawn.  It also meant that with heated and unheated water systems, we could (at least during the cooler parts of the production season), actually “turn on” some tanks by warming them to 28°C and allow others to rest by holding them at about 21°C, so that not all the females were in a constant reproductive mode.  These are things that I had hoped would be eventually achieved through hormonal inputs in the feed, so I was very happy to see that the animals in Mexico seem to have reached a state of domestication that made it possible to avoid ablation before the long-awaited breakthrough with hormonal control.

 

Having said this, I wouldn’t be surprised to hear that similar results have been obtained with vannamei in other places as well, and probably also with other species that have not had the same level of aquaculture attention.  I wonder if anyone has actually made significant headway in terms of controlling shrimp reproduction through feed-mediated hormonal inputs.

 

Guillermo Jaramillo (guillojara@yahoo.com): Thanks for all your valuable input on my question about the size and shape of vannamei maturation tanks.  The reason for my question: At Shrimp Improvement Systems, we need to maintain many lines of shrimp in our very limited maturation space.  The minimum tank size consideration is to prevent spermatophore deterioration by keeping males sexually active.  Based on your comments and with this space-limited situation, the rectangular tanks—with rounded corners—would be the best option even with all the constraints they have when compared with the “self-cleaning” round tanks.

 

Sources: 1. The Shrimp List (a mailing list for shrimp farmers, “shrimp-subscribe@yahoogroups.com”).  Subject: Mating area for P. vannamei.  September 6-11, 2007. 2. Bob Rosenberry, Shrimp News International, September 14, 2007.

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