Management of Ponds Stocked With Blue Shrimp Litopenaeus Stylirostirs
The international shrimp farming industry is under increasing pressure to reduce its dependence on wild stocks of shrimp, and emulate the models developed by other animal husbandry agribusinesses, promoting an increased reliance on genetically improved, domesticated lines of shrimp. Some shrimp farming countries have already embarked on this transformation. In a period of less than 18 months, Mexico has converted more than half of its 24,000 ha. of shrimp ponds to a proprietary, domesticated form of the blue shrimp Litopenaeus stylirostris known as "SS" (Super Shrimp), and many other countries are considering similar measures.
The SS line of blue shrimp originated as wild larvae from Panama, and is currently in its 22nd generation of domestication, with no introduction of extraneous genetic material occurring since the conception of the line in the mid-1980's. SS blue shrimp have undergone a systematic process of selection for enhanced growth, fecundity, and in particular, for increased resistance to the IHHN virus.
Widespread acceptance of monoculture L. stylirostris is compelling farm managers to adapt their standard pond management protocols used traditionally for L. vannamei to accommodate the culture requirements of blue shrimp. The present paper describes the distinctive culture characteristics of this unique strain of domesticated blue shrimp (SS) as they apply to management of semi-intensive ponds. The performance characteristics and field observations presented in this paper were derived from production cycles and experimental trials of SS-shrimp in a large number of commercial shrimp farms in Mexico during the period 1997-1998. Although the information described in this paper is in general applicable to other strains of L. stylirostris, the technical recommendations are intended for use in SS-shrimp culture systems.
SS-shrimp are significantly more tolerant of low water temperatures than P. vannamei, especially in the juvenile and adult stages. In a laboratory bioassay studying the effects of water temperature on survival, 5-7 g SS juveniles exposed to a constant temperature of 12oC for 72 hours experienced no mortalities. A similar group of experimental animals experienced 100% mortality following 24 hour exposure to 8oC; however exposure to 8oC for six hours followed by 18 hours at 12oC, repeated over a 72 hour period, elicited only a 20% mortality rate. The results of this temperature bioassay coincide with data from production trials with nursery ponds in Mexico, which demonstrated that 0.5 g SS can withstand pond temperatures as low as 12oC, with final survivals in the nursery ponds of 70-85%.
SS post-larvae are more sensitive to low water temperatures than juveniles or adults. In order to define lethal limits for low temperatures in newly stocked post-larvae, a series of temperature bioassays were conducted with PL-16 SS. The bioassays consisted of three treatments designed to simulate normal diurnal temperature fluctuations in a pond:
The experimental treatments were run in triplicate over a period of 72 hours. The results are presented in Figure 1. A constant temperature of 18o C (control) resulted in 100% survival during most of the experiment. The larvae maintained at temperatures that fluctuated from 17 to 14oC began to experience mortalities after 24 hours, finishing with a final survival of 77%. Note that survival in treatment No. 3 remained high while the larvae were maintained at 17oC, and that the mortalities increased once the experimental temperature was reduced to 14oC. The other two treatments produced significant mortalities after 12 hours of exposure to the initial experimental temperatures, and concluded with low final survivals of 13% (No. 1) and 43% (No. 2). These results suggest that newly stocked SS post-larvae can safely tolerate minimum water temperatures as low as 17-18oC, but that brief
exposure to lower temperatures may induce mortality.
Treatment No. Temperature profile
1 16 hours @ 13oC, followed by 8 hours at 10oC
2 16 hours @ 15oC, followed by 8 hours at 12oC
3 16 hours @ 17oC, followed by 8 hours at 14oC
4 (control) Constant temperature of 18oC
The temperature related limitations of stocking post-larval SS during periods of very low water temperatures can be resolved by stocking juveniles, which can resist temperatures as low as 12oC. The use of juvenile SS is recommended for stocking ponds that will experience water temperatures below 17-18oC during the first 30 days of the grow-out cycle. In temperate and sub-tropical regions such as Mexico, it has been demonstrated that stocking juveniles during the winter months not only protects against low temperature-related mortalities, but also allows the producer to circumvent the initial 4-6 week period of minimal growth that typically occurs in ponds direct stocked during cold months.
The ideal water temperature range for maximum growth of SS-shrimp is 24-30oC. At temperatures of 20-24oC SS will continue to grow at 50-75% of normal growth rates, and in well managed ponds, satisfactory growth rates can be attained at temperatures between 18-20oC. Figure 2 illustrates winter growth rates of SS, and pond water temperatures in two ponds of a shrimp farm located in central Mexico. Even when morning temperatures dropped below 20oC, the shrimp continued to grow at a rate of almost 1.0 g/week. The growth performance depicted in Fig. 2 clearly confirms that SS blue shrimp are tolerant of relatively low water temperatures, and that satisfactory growth rates can be obtained at temperatures that would normally stress or incapacitate L. vannamei.
The ideal maximum recommended temperature for SS is 30-31oC, although the shrimp can withstand higher temperatures for brief periods of exposure. Water temperatures (PM, bottom) as high as 36oC have been recorded in SS ponds that yielded high survival, although such high temperatures, when combined with other stressors (e.g. high salinities, low D.O.), may induce mortality. Ideally, to optimize survival, water temperatures in excess of 33-34oC should be avoided. Lower temperatures (<32oC) are preferable, because even though high temperatures may not necessarily affect survival, the shrimp's feeding activity may be inhibited, resulting in diminished growth .
To escape high temperatures, blue shrimp will generally seek refuge in the internal channels and deeper areas of the pond. For this reason, ponds constructed with average depths > 1.0 meters, or with deep internal canals, offer superior habitats for culture of SS-shrimp. This recommendation takes on increased importance in areas that typically experience very high afternoon air temperatures.
Survival and growth of SS-shrimp are generally not affected by salinity in the range of15-45 ppt. SS blue shrimp are sensitive to very low salinities (i.e. 0-5 ppt), especially in post-larval stages. Although it is not recommended to stock SS ponds at salinities below 10 ppt, a slow rate of acclimation (e.g. 0.5-1.0 ppt/hr) should be utilized during the terminal phase of the acclimation process at very low salinities. Satisfactory survivals (>50%) have been obtained in ponds that experienced salinities of 1-2 ppt during the final 6-8 weeks of their production cycle. SS can tolerate short periods of zero salinity without negative consequences to growth, but in general, prolonged exposure to salinities less than 5 ppt may evoke mortalities.
Blue shrimp generally outperform L. vannamei under hyper-saline conditions (i.e. >40 ppt). Good growth and survival of SS-shrimp has been recorded at salinities as high as 50-55 ppt. SS can tolerate salinities as high as 55 ppt for brief periods without serious consequences to survival, however, prolonged exposure to salinities above 55 ppt is stressful and may result in mortalities, especially when combined with other environmental stresses (e.g. high temperatures). Salinities in excess of 60 ppt will generally elicit mortalities. As was the case with very low salinity exposure, post-larval SS-shrimp are more sensitive to hypersaline conditions than juvenile shrimp. Increased mortalities have been observed when acclimating SS larvae to salinities in excess of 50 ppt. Prior to stocking hypersaline ponds, moderate water exchange is recommended in order to prevent salinities from increasing.
Like most strains of L. stylirostris, SS is slightly more sensitive to low dissolved oxygen (D.O.) conditions that L. vannamei. Tolerance limits for low D.O. depend on the health status of the shrimp (e.g. presence of epicommensal organisms fouling the gills), and other coinciding environmental stresses, for example high temperature (Table 1).
Low D.O. may also temporarily depress feeding activity, which should prompt corrective measures from the pond managers. Short term exposure of SS larvae to D.O. stress has been linked to subsequent increases in disease susceptibility.
Secchi Disk Turbidity
SS-shrimp are notably sensitive to direct exposure to sunlight, and possess a conspicuous aversion to high transparencies. Not only will SS aggressively avoid areas of the pond where light impinges on the bottom, but it is suspected that the animal may become stressed by exposure to high submarine light levels. Precautions should be taken to avoid stocking SS larvae in ponds with secchi disk turbidity > 60-70 cm, unless the ponds are very deep. A rigorous program of inorganic fertilization should be implemented in order to create adequate phytoplankton blooms prior to stocking. Secchi turbidity of 35-45 cm is ideal for all stages of SS grow-out.
Daily water exchange requirements for SS-shrimp are comparable to those used to sustain similar biomasses of white shrimp. Maintenance of stable phytoplankton populations and D.O. levels above 4.0 ppm are considered
more important priorities than heavy water exchange for successful culture of SS-shrimp. A typical, fixed, water
exchange program for semi-intensive ponds raising SS would be: 2-3 cm/day during the first 3 weeks of the culture cycle, followed by 4-7%/day up to day-50, and 8-10% until harvest. Water exchange should be managed in accordance with pond water chemistry parameters, fertilization programs, and the health of the shrimp.
Table 2 summarizes ideal, minimum, and maximum environmental tolerances for SS-shrimp.
NUTRITION AND FEEDING
Role of Natural Productivity
Whereas L. vannamei possesses an omnivorous feeding behavior, L. stylirostris is notably more carnivorous. Mixed populations of phytoplankton and zooplankton stimulate growth of newly stocked white shrimp, whereas a predominance of zooplankton and benthic fauna is preferred for maximum growth of newly stocked blue shrimp larvae. (The carnivorous predisposition of stylirostris is corroborated by the fact that consumption of Artemia nauplii by blue shrimp larvae in larviculture tanks is higher than for vannamei larvae.) Zooplankton densities in the ponds of 3-5/ml (excluding protozoans) favor fast initial growth of juvenile SS. In the absence of naturally abundant populations of zooplankton, controlled applications of organic and inorganic fertilizers will generally stimulate natural productivity. For example, 500-1000 kg/ha of pesticide-free chicken manure applied to the pond bottom before filling, followed by 3-4 applications at 48 hour intervals of 10-15 kg/ha of Nutrilake (plus optional monoammonium phosphate, if phosphate levels are < 0.2 ppm) during the initial pond filling process, has produced positive results for the author.
Due to the carnivorous nature of L. stylirostris, higher dietary protein requirements are generally required in grow-out diets for blue shrimp than are typically administered to white shrimp. A culture biomass of blue shrimp greater than 1000 kg/ha usually requires a high quality 35% protein feed to maintain maximum growth rates. Most producers initiate the production cycle with a 40% starter diet in "crumble" presentation, shifting to a pelleted 35% formulation when the juveniles attain 3-5 g in size.
In 1997, a replicated dietary trial with SS was conducted in a commercial shrimp farm testing three commercial feeds: a 35% protein "standard" (ST) feed, a 35% "high performance" (HP) feed, and a 40% protein "standard" feed. The ponds were stocked with SS-shrimp at densities of 10-13/m2 during the winter production cycle in Mexico. The results were as follows:
(*): Economic feed conversion = feed conversion x feed price (US$/kg)
Means assigned the same letter are not statistically different
Average production was 1630 kg/ha (13.0 kg/1000 PL) of 17 g shrimp with a final survival of 81%. Dietary formulation did not influence survivals. Upgrading the 35% protein feed to the high performance diet (HP), which contains squid meal and a higher percentage of high grade fish meal, improved growth, feed conversion, and economic feed conversion in comparison to the "standard" 35% protein diet. Increasing the protein content to 40% did not significantly improve growth or feed conversion. A 40% protein feed is recommended for high salinity (>45 ppt) culture systems.
The higher dietary protein requirement of SS-shrimp, in comparison to white shrimp, is offset by the faster growth rates and the fact that SS can reach the larger, more lucrative commercial sizes (31-35 to 26-30) that are difficult to attain with vannamei. Under ideal culture conditions and with a properly managed feeding program, cumulative growth rates for SS-shrimp typically range from 1.0-1.3 g/week, with weekly growth increments of 1.5-2.0 g/week during the last two months of the grow-out cycle. (The relatively slow growth rates recorded in the feeding trial were a product of low (winter) water temperatures experienced during the first two months of the experiment.) Even at intensive culture biomasses (> 5,000 kg/ha), the growth curve of SS-shrimp is generally linear, and does not level off at the end of the production cycle, a phenomenon commonly experienced with vannamei. Using commercial feeds similar to the "HP" diet described above, SS-shrimp has performed well in intensive culture systems stocking ponds as high as 60/m≤, and harvesting 7300 kg/ha of 21 g shrimp.
Juvenile SS-shrimp typically exhibit a much more aggressive feeding behavior than white shrimp. They will migrate considerable distances within a pond in search for food. The aggressive feeding behavior of SS-shrimp is conducive to the use of mechanical feed applicators, for example, tractor powered blowers. Even in very large ponds (e.g. 10 ha.), 100% of the daily feed ration can be applied around the perimeter of the pond by feed blowers without any apparent detrimental effect on feed consumption or growth. When feeding exclusively around the perimeter it is advisable to periodically inspect the pond bottom along the feed path to assure that no excessive accumulations of uneaten feed are present.
SS also possess a voracious appetite. In an experimental feeding trial in which the total feed ration was offered on feeding trays (six 0.42 m2 trays per ha.), the SS-shrimp consumed as much as 7 kg of pellets from
each tray in less than 12 hours. This voracious feeding behavior lends itself to the practice of applying 100% of the daily ration on feed trays, and requires fewer trays per ha. to dispense the feed to the shrimp. Administering 100% of the feed on trays not only reduces feed conversion and mitigates pond bottom deterioration, but it also provides a mechanism for instantly adjusting feeding rates in response to sudden changes in the physiological state of the shrimp. For example, it is not uncommon to observe 50-75% fluctuations in feed demand during a 12 hour period due to increased molting activity. Shrimp growers that do not employ feed trays will not be able to respond promptly to these fluctuations, and risk a wasteful provision of feed at a time of diminished appetite. Similarly, the ability to rapidly adjust feeding rates based on feed tray observations is an important consideration when designing your feed tray management program. Management protocols in which feed tray results are used to adjust feed rations applied 24 hours after the inspection may not provide a timely reaction to the shrimpís changing appetite.
A practical feed tray methodology that can be used to manage feeding in SS ponds consists of feeding three times per day with the total amount of feed in each ration adjusted at each feeding based on the results of an examination of the trays during the previous feeding, or at a fixed interval (e.g. 2 hours) after filling the feed trays. The grower has the option of utilizing three distinct feeding tactics: 1) Use a limited number of trays (e.g. 1-2/ha) strictly as indicators, 2) Apply 100% of the total daily ration on many trays (15-30/ha), or 3) A hybrid approach in which the majority of the daily ration is administered via trays, with a portion (20-30%) applied directing to the pond by boat or mechanical feeder. The authorís preference is methods 2 or 3.
Feed trays should be installed in the pond the day after stocking, and replenished daily with a fresh aliquot of 100 g (or ml) of granular feed. During the first 14 days of the culture cycle, the feed trays are used only to observe the health and feeding behavior of the shrimp, and to provide a very qualitative estimate of survival. During this period, it is not necessary to record feed amounts in the trays, nor adjust feeding rates. On day-15 after stocking, begin recording the amount of excess, uneaten feed on each tray, and initiate regular adjustments to the feed rations as soon as the trays begin to display no uneaten feed.
A minimum of three daily feed applications is recommended to maximize the growth potential of SS-shrimp, with the largest feed allocation devoted to a night-time feeding. Based on observations of diurnal feed tray activity in experimental studies at select shrimp farms that fed at 06:00, 12:00, 18:00 and 00:00, it was noted that SS display an active nocturnal feeding behavior, with night-time feeding preferences increasing with the age of the shrimp. If possible, at least 50% of the total daily feed ration should be provided at night.
As with most species of shrimp, feed consumption usually reflects the health of the animals. Graphing daily feed consumption is an excellent tool for the early detection of disease outbreaks, and for corroborating survival estimates generated from population sampling.
Another unique behavioral idiosyncrasy of blue shrimp is their tendency to distribute themselves in non-homogeneous patterns in the ponds. Blue shrimp will frequently congregate in clusters, especially in shallow ponds. The non-uniform distribution of the shrimp produces large variations in shrimp capture during population sampling, which complicates accurate estimates of survival. In order to reduce the variations in shrimp capture during population sampling, the following measures are recommended: 1) Sample weekly all ponds > 30 days in age, at 3-4 fixed sampling stations per ha., 2) Use a nylon cast net with a minimum of 3 kg of lead weights, and as much as 4-6 kg of lead if your ponds are shallow or transparent, 3) Sample in the early hours of dawn, or if necessary at night, 4) When using a motorized boat, shut off the motor as you approach the sampling station, 5) Do not feed or exchange water immediately before or during the population sample, 6) Avoid altering the sampling crew, equipment and routine. Sampling weekly allows you to generate a "moving average" of the last 3 or 4 weekly samples, which should lessen the variability between samples, and permit a more accurate estimate of survival. A 4-week moving average also automatically includes all lunar phases in the sampling period.
Domesticated L. stylirostris of the SS strain are more resistant to low temperatures and high salinities, than the white shrimp L. vannamei, especially in the juvenile and adult stages. Juvenile SS can survive at water temperatures and salinities as low as 12oC and 55 ppt, respectively, and will grow at temperatures of 18-20o C and salinities of 50-55 ppt.
Post-larval SS are more sensitive to environmental extremes (low temperatures, high salinities, low oxygen) than their juvenile counterparts. Hence, stocking of juveniles is recommended for periods of harsh environmental conditions.
Adequate phytoplankton densities should be maintained at all times in the ponds, since blue shrimp possess an aversion to high transparencies and light penetration.
Blue shrimp possess a more carnivorous feeding behavior than white shrimp. Natural food organisms in the form of zooplankton are essential for the nutrition of newly stocked SS post-larvae. A high quality 35% protein feed favors maximum growth potential of juvenile SS. Faster growth rates leading to larger more lucrative commercial sizes of harvested shrimp aid in offsetting the additional feed costs.
SS-shrimp display an aggressive feeding behavior, which favors the use of mechanical devices that apply feed around the pondís periphery. The aggressive feeding behavior also facilitates the application of 100% of the total feed ration on a limited number of feed trays, thereby reducing manpower and material requirements for such a feed management system.
A minimum of three feed rations per day is recommended, with a majority of the total daily ration allocated to nocturnal feeding(s).
SS-shrimp tend to congregate in the ponds in a non-uniform distribution, which increases variability in the population sampling program. Weekly sampling at dawn of fixed stations in each pond using a heavily weighted cast net, and generating moving averages of the last 3 to 4 weekly samples generally will reduce inter-sample variability enough to be able to estimate survival.
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