Hatchery conditions for developing PL involve a relatively safe, predictable environment with stable and comfortable water temperature and salinity, where feed is constantly available and predation is generally not a problem. Most shrimp farmers spend substantial resources and effort during pond preparation, to stock their PL into a growout environment with the best possible environmental conditions, as free of predators, competitors and stress as possible, and with ample supply of adequate food organisms. Still, the transition from hatchery conditions to those prevailing in open growout systems such as tanks and ponds, where water conditions can continually or unpredictably change (day/night, dry/rainy seasons over the production cycle) can be a traumatic experience for PL unless the transition is gradual and stress is minimized.
Different stress tests have been used to challenge a batch of animals and determine how hardy the PL are, to decide on a suitable acclimation schedule. These tests or challenges usually involve subjecting a PL sample of 100-200 animals to thermal, osmotic and/or chemical shock for 1-4 hours and "counting the survivors". A widely used challenge is the one proposed by Clifford (1992). He suggested a standardized "stress test" method where a sample of animals are placed in a container or tank, and the water salinity and temperature are simultaneously brought down to 20 ppt and 10 C, respectively, for four hours (a test lasting under four hours does not adequately account for lingering PL mortalities). A variation of this test is to use a 100-150 ppm formalin challenge. According to Clifford (1992), a survival of 80-100% of the test animals indicate high quality PL, while 60-79% survival is considered acceptable and survival under 60% justifies either rejecting the batch or holding it in the hatchery for a few more days to try to improve their strength and quality.
Another variation for assessment of PL fitness is the stress test proposed by Brock and Main (1994). In their one- or two-parameter test (temperature and/or salinity) fitness test, 100 randomly collected PL are placed in a bucket containing 10-15 litres of water at 22 C and 5 ppt (two parameter test) or at hatchery ambient temperature and 0-1 ppt. Animals are maintained under these conditions for one hour , and survivors (animals that swim and respond normally) counted. The population is considered to have passed the test if survival rate is 80% or higher.
Published acclimation procedures have been available for several years. For example, the shrimp farming manual published by the Panamanian Ministry of Agriculture and Livestock and published over 14 years ago (MIDA, 1984) provides general acclimation recommendations that are still used today, including the following: a salinity increase/decrease of no more than 3 ppt per hour; density of transportation: 1000-5000 PL per gallon (approximately 260-1,300 PL/liter); avoid sudden temperature changes (more than 3-4 C); maintain dissolved oxygen levels at 6-7 ppm.
At some farms when the differences in water salinity and temperature between the pond or tank to be stocked and the water the postlarvae are shipped in are minimal (>3-4 ppt; 1-2 C), acclimation can be as simple as placing the plastic bags used to transport PL in the pond for 30-60 minutes (after making small holes in the bags) and then releasing the animals, although this method is not recommended. Acclimation can also be started in transit if PL are shipped in bulk in large containers, by slowly adding water with the desired characteristics (pond temperature, salinity, pH) and completed upon arrival to the farm. But in general most farms have either dedicated acclimation stations or use temporary stations set up along side the pond to be stocked, and some farms will have both.
There are several variations but the typical acclimation process basically involves holding the PL for a period of time in tanks and slowly adding water from the pond to be stocked in order to equalize various parameters (mainly salinity and temperature). Special attention must be paid to minimize all sources of stress. Dissolved oxygen levels should be maintained near saturation, and PL must be fed and closely monitored all the time. Live and frozen Artemia, cooked egg yolk and other foods are used, but in general the use of live Artemia is discouraged because of the potential to stimulate cannibalistic behavior among the PL. Packing densities depend on PL age and time in transit, and can range from 1,500-3,000 PL/liter for smaller animals (<PL8-10) to 800-1,000 PL in the case of larger animals (>PL15) and in-transit times of over 12 hours. Reduced shipping water temperatures (to 18-22 C) and the addition of various compounds to shipping water (including ammonia suppressants, buffers and activated carbon) will increase PL survival over extended shipping schedules. Acclimation densities should not exceed 300-500PL/liter depending on animal size and duration of acclimation.
Salinity is probably the most critical parameter to manipulate during PL acclimation. Different authors recommend various salinity acclimation. For example, an increase/decrease of no more than 3 ppt per hour (MIDA, 1984). Drazba (1993) reported successfully using a salinity acclimation schedule which averaged 18-20 hours on commercial famrs in Guatelama and Ecuador, in ponds where rainy season salinities ranged from 0-2 ppt and from 6-10 ppt during the dry season. This schedule involved the following times: 20 minutes (35 ti 15 ppt), 30 minutes (15 to 10 ppt), 60 minutes (10 to 4 ppt), and 120 minutes (4 to 0 ppt). ...
After acclimation is concluded it is very comon to use "survival cages" or "survival buckets" ("jaulas" o "cajones de sobrevivencia" in Spanish) to provide an estimate of PL survival 24-48 hours after stocking. This information is very important because it is the basis for additional PL stocking if initial PL survival is not considered adequate. Villelon (1991) and Clifford (1992) describe the use of these cages in detail. Cages are made of wooden frames and 500 micron screen, or plastic buckets (10-20 gallons) with both ends as well as "windows" covered with screen. Two to four cages are placed on the bottom or the stocked pond (without disturbing the anaerobic layer), together or spaced apart (to better represent spatial microhabitat variability within the pond), making sure the PL will have access to the pond bottom. Between 50-200 PL are placed into each cage or bucket and held undisturbed for 24-48 hours (different farms have different procedures). Then surviving PL are counted, averaged among number of cages used and an estimate of survival after acclimation and stocking is made. In general, survival rates higher than 85% are considered to represent a successful acclimation and stocking with good quality larvae; between 60-85% may indicate less optimal PL quality or stressful conditions during transit, aclimation and/or stocking. Survival rates lower than 60% are a strong indication that poor quality PL were used, or be evidence of stressful handling, acclimation and/or stocking, and managers should consider stocking additional PL to compensate for the possibly high initial mortality.
Nutrition is a key aspect in the ability to tolerate changing environmental conditions. Postlarvae fed a diet with high HUFA (highly unsaturated fatty acid) content are better able to withstand salinity changes during acclimation and transition to growout system after stocking. Rees et al. (1994) studied the HUFA requirements of P. monodon postlarvae (PL10) in an experiment assessing the possible relevance of Artemia enrichment with HUFA. They reported that postlarvae grew well on an Artemia diet with low HUFA content but the animals showed low ability to withstand osmotic stress. These authors further stated that when feeding HUFA-enriched Artemia for 5 days the postlarvae exhibited significantly higher survival and resistance to subsequent osmotic stress. However, when very high dietary HUFA levels (n-3 HUFA, 31.2 mg/g DW) were used no growth-promoting effect was observed, indicating that an excessive HUFA is probably not recommended.
It is also important to consider that different penaeid species have different salinity tolerances and preferences, and that stress occurs if animals are placed outside their area of tolerance. For example, Rodriguez (1981) studied stress resulting from different salt concentrations as detected by measurements of total protein and total osmotic pressure in blood serum of P. vannamei and P. stylirostris. He found that P. vannamei has higher tolerance even though P. stylirostris naturally occurs in coastal areas where conditions are predominantly marine with little freshwater influence. He also reported that the effect of low salinity on survival is similar in both species, and that a significant decrease in total blood protein is observed for P. stylirostris at a salinity of 50 ppt, associated with an extremely high osmotic pressure in an effort to osmoregulate.
Oxygen consumption in penaeid species can differ in response to changing salinities. For example, Gaudy and Sloane (1981) studied the effects of salinity on oxygen consumption in cultured postlarvae of Penaeus monodon (PL32) and P. stylirostris (PL35). Although they reported no significant difference in oxygen consumption or metabolic rates between non-acclimated and fully-acclimated animals, they determined that P. stylirostris displayed a tendency (non-significant at P<0.05) to increase respiration at lower salinities.
(excerpts from article by D.E. Jory in Aquaculture Magazine, March/April 1998)