D. E. Jory-1997
Aquaculture magazine, 23(1): 67-75
The increased global demand for seedstock experienced by the shrimp farming industry during the last two decades has prompted extensive improvements and refinements of the basic larval rearing techniques originally developed to raise Penaeus japonicus larvae in Japan in the 1930's by Dr. Motosaku Fujinaga, widely recognized as the "father of shrimp culture." This original large-scale larviculture technique involved the use of indoor rearing tanks, and feeding diatoms to zoeal stages and brine shrimp (Artemia) to mysid and postlarval stages. This technique has been modified and adapted successfully to several shrimp species and widely different culture conditions around the world, but the fundamental principles developed by Dr. Fujinaga are still in use. In this third part of the ongoing discussion on penaeid shrimp hatcheries we will briefly review larviculture methods and its perspectives. Readers interested in more details on applied shrimp larviculture procedures are directed to the excellent manuals by McVey and Fox (1983), Treece and Fox (1993), and Treece and Yates (1993).
Larviculture Infrastructure
Shrimp hatcheries come in three sizes: small, medium and large, but they all share the same basic infrastructure.
Small hatcheries are usually a family operation, with very low setup and operating costs, using simple techniques and untreated water, low culture densities, and with larviculture tank capacity under 50 tons. They usually specialize in the production of either nauplii or postlarvae, and often suffer disease outbreaks and water quality problems. But they can readily shutdown and restart in a relatively short time. Reported survival rates range from 0 to 90%, averaging about 50% and reportedly results in stronger seedstock. These hatcheries have been very successful in southeastern Asia, but are not so common in Latin America, although there are small hatcheries in several countries of the western Hemisphere. Small- and medium-scale hatcheries are usually based on the Taiwanese design, with large larviculture tanks between 5 to 50 tons. Low stocking rates and reduced water exchanges are used, and an "ecosystem" approach is taken by directly fertilizing the tank to promote microalgae blooms "in situ." Large hatcheries use more complex techniques to establish and maintain controlled larviculture environments. They target annual production of 100 million postlarvae or more, and are typically based on the "Galveston" system, with intensive production methods, high stocking densities, and survival rates of 70-80%. When large hatcheries have water quality and disease problems, they typically require several months to get back on line.
Hatcheries must simulate the natural conditions under which shrimp normally undergo larval development. In order to maximize larval survival and growth, and produce strong postlarvae ready for stocking into nursery or growout systems, the hatchery has to provide optimal conditions for the animals. In addition to the larval rearing tanks, there are several functional areas, both indoors and outdoors, that provide support to the larviculture operation and contribute to optimize conditions for the different larval stages. These include the microalgae laboratory and culture area, the Artemia production area, and the seawater treatment and holding area. Larval rearing tanks come in many sizes and shapes, are made of different materials from wood, plastic or concrete and set up for rapid water exchanges and vigorous aeration capability. Water quality in larviculture tanks in western hatcheries is typically maintained through high levels of water exchange, but in Asian hatcheries the water exchange is comparatively much lower. The chelating agent EDTA is routinely added to spawning and hatching tanks to reduce the concentration of heavy metals EDTA may also be involved in reducing bacterial contamination of the shrimp eggs, resulting in improved oxygen transfer and hatching rates. The water supply system normally includes settling tanks to remove large particles, followed by a battery of filters for water treatment and disinfection (including ozone, chlorine and UV). Filtration systems are designed specifically for each site, and are based on how much treatment the water in the area needs. Water must be filtered down to one micron to assure exclusion of protozoans and other naturally occurring but undesirable organisms that can compete with shrimp larvae for space and food in larviculture tanks. Various parameters must be closely and frequently monitored during penaeid larviculture. Recommended water temperature for optimum growth and steady transition from one larval stage to the next is 28 C (+/1 C). Salinity is less critical than temperature, and a range of 26 to 36 ppt is considered optimum. Rapid changes in any water parameter must be avoided as these will stress the larvae. Optimum pH is around 8.0, but a range of 7.8 to 8.4 is adequate. High ammonia and nitrite levels can become a critical, even lethal, problem after Artemia is introduced into the tank in the late zoeal stage and from mysis to postlarvae, but these levels are relatively easy to moderate through water exchanges. It is very important to maintain adequate concentrations of microalgae in the larviculture tanks to help in maintaining proper pH range, and as feed for both shrimp larvae and Artemia; the nutritional value of the latter is much better if it can feed on microalgae than if it is starved.
The function of the microalgae laboratory and the algae production system is to continuously generate starter cultures of selected microalgae species - of known nutritional value to larval shrimp - which are then used to inoculate progressively larger tanks, where microalgae population growth is promoted through the addition of various nutrients until the desired algal cell densities are achieved. The Artemia hatching area includes a number of tanks where brine shrimp cysts are subjected to routine decapsulation to improve hatching rates, and sometime enrichment procedures. These tasks are elaborate and labor-intensive but undoubtedly improve the nutritional quality of Artemia. Brine shrimp are relatively easy to culture and are nutritionally adequate; different strains differ in both size and nutritional quality, but can be readily enriched in the laboratory if deemed necessary. Production of live feeds such as microalgae and Artemia are expensive and elaborate processes that require careful planning, attention, continuous monitoring and timely execution to guarantee high-quality live feed is readily available when needed by the developing shrimp.
Larviculture Methods
Numerous methods are used worldwide to produce penaeid larvae in hatcheries, all modifications of the two basic methods, the Taiwanese and the Galveston methods. The latter is a modification of the former, and the main difference is that in the Galveston method the microalgae are cultured outside the larval rearing tank and are added as required. Over the years many researchers around the world have contributed significantly to the successful modifications and refinements to these techniques. The Taiwanese method uses lower stocking densities and larger larviculture tanks, and produces 20-30 days seedstock. It requires large broodstock numbers, and the microalgae blooms are promoted "in situ" by directly fertilizing tank water. The Taiwanese method performs well in temperate areas, where there is a short production time, but can be comparatively harder to manage because of the large water volumes involved. This method is generally not used in tropical areas because of the management difficulties inherent to tropical conditions. The Galveston or "clear water" method is based on the procedures developed at the Galveston Laboratory (National Marine Fisheries Service) in Texas in the 1970's, and has been successfully adapted to local conditions and implemented throughout the western Hemisphere. It uses high densities of 100 or more larvae/liter, relatively high water exchange rates and elaborate water filtration and conditioning, and produces 6-12 day old postlarvae. Microalgae and Artemia nauplii are cultured separately and added to larviculture tanks as required.
Larval Nutrition Requirements and Feeds
Most hatcheries implement a feeding regime based only on live feeds (microalgae and Artemia), or on live feeds combined with prepared diets. The latter can be produced at the hatchery, or commercial types purchased. Formulated, off-the-shelf, small particulate diets have been very successful as supplements to live feeds, and it is likely that in the near future these diets could totally replace live feeds in hatcheries.
Many algae species are natural food for penaeid shrimp, from larval stages to adult shrimp and clearwater hatcheries must have areas dedicated to the continuous mass production of microalgae. Several species are commonly produced, including species of the genera Chaetoceros, Tetraselmis, Isochrysis, Skeletonema and others. Isochrysis galbana and Chaetoceros gracilis (a solitary marine centric diatom) are considered to be among the best microalgae feeds, and this is due to their relatively small size and content of highly unsaturated fatty acids (HUFAs). Various HUFAs have a key role in the early development of the nervous system in fish and shrimp, and also as precursors for many biologically active compounds - such as prostaglandins - involved in regulating growth and reproduction. A common procedure in the aquaculture industry, particularly in the finfish and shrimp farming sectors, is the enrichment of live feeds such as rotifers and Artemia nauplii with HUFAs before feeding these to larval fish and shrimp. HUFAs are essential for normal shrimp larval development and growth, but the HUFA content in most strains of commercially available rotifers and Artemia is small. According to Barclay and Zeller (1996), there are various enrichment techniques currently used in shrimp larviculture, including microalgae enrichment, the use of microencapsulating oils with a high HUFA content, and emulsified, HUFA-rich marine oils. These authors have recently reviewed the nutritional enhancement of rotifers and Artemia using HUFAs, and suggested that the best strategy for HUFA enrichment of live food organisms for aquaculture is to implement brief feeding of rotifers and Artemia nauplii with HUFA-rich microalgae such as Schizochytrium sp.
From egg to postlarvae the larval development of penaeid shrimp is complex. It involves three stages: nauplii, zoea and mysis. The nauplii stage has 5-6 substages and the zoea and mysis stages have three substages each, with each substage lasting several hours; complete larval development normally takes around 15 days. In the first stage, or nauplii, animals have not developed their mouths and must feed on their yolk sac. In the next stage, or zoea, the animals will feed on unicellular microalgae - starting with smaller species and progressively switching to larger ones (typically starting with a small diatom such as Chaetoceros sp., and then to a larger species such as Tetraselmis sp.) - and commercial microencapsulated diets are routinely used as supplements. It is important to frequently monitor and maintain adequate larval food concentrations and in suspension in the water column through the use of gentle aeration. After undergoing metamorphosis to the mysid stage animals are able to swim and capture live prey, and Artemia nauplii are introduced. After reaching the postlarval stage the animals will look like very small adult shrimp and will readily feed on zooplankton, detritus and commercial feeds.
Probiotics
A major influence on the success or failure of commercial-scale, intensive penaeid hatcheries has been the ability to control pathogenic organisms, particularly bacteria, introduced in the water.
Until a few years ago the common method of controlling populations of pathogenic and undesirable bacteria was through standard practices involving the exclusion or elimination of opportunistic pathogens through various water filtration and disinfection techniques (chlorination, UV, filtration). Other common practices include implementing hygienic procedures (equipment and personnel disinfection), and the use of clean algae cultures and Artemia. In addition, various chemotherapeutants and biocides are also used to indiscriminately eliminate or reduce the numbers of pathogenic bacteria, often in a routine fashion and not necessarily when needed. There are serious problems associated with this practice, including development of resistance, biocide traces in shrimp tissues, and environmental pollution. In recent years the use of probiotics to enhance larval shrimp survival and growth has been widely adopted. The principle is relatively simple: promoting the growth of harmless or beneficial bacteria to the exclusion of harmful species. According to Garriques and Arevalo (1995), probiotics may act through one or more of various potential roles, including the competitive exclusion of pathogenic bacteria, enhanced nutrition by providing essential nutrients, or enhanced digestion by supplying essential enzymes, direct uptake of dissolved organic material mediated by bacteria, or by the production of substances that inhibit growth in opportunistic pathogen species. There is much potential for improvement of probiotics, but work is needed to identify additional suitable species and how to promote their population growth in penaeid larviculture tanks.
Vaccines and Immunostimulants
The immune system of crustacean is relatively primitive and does not produce antibodies, and their immune responses are not specific in nature. This means that crustaceans, including shrimp, can not be "vaccinated" in the traditional definition. However, there are commercial products that claim to stimulate disease resistance. They are being promoted as innovative management tools to promote shrimp health, by preventing and minimizing the impact of outbreaks of pathogenic bacteria both in hatcheries and during growout. In hatcheries these products can be administered to late mysis-postlarvae by immersion or bath, or by microencapsulation using Artemia. Results reported from field testing indicate that these products may have considerable potential to stimulate non-specific immune responses and improve survival and growth of treated animals, thus enhancing production.
Perspectives
Reliable, consistent seedstock pro auction is considered by many as the weakest link in shrimp farming today. Much research work in recent years has resulted in a variety of larviculture methods to produce seedstock, all based on the original work by Dr. Fujinaga in the 1930's. Still, various researchers and authors consider successful penaeid larviculture more art than science, and there is undoubtedly much room for new developments that can improve larval production efficiency. Some of these include a more thorough understanding of the nutritional requirements of the different shrimp larval stages, and how to satisfy these requirements cost-effectively using prepared diets. There is also a need to better understand various processes involved in larviculture tank microecology and management. Research is also needed on promoting the use of probiotics in lieu of chemicals and biocides, and on the development of immunostimulants that can promote health, growth and production.
(University of Miami Marine School, Miami, Florida, USA)
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