Biotechnology-Aquaculture Interface: The Site of Maximum Impact Workshop | |
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Contents
Appendix
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Workshop Report
-Preface
-Final Report
Biosecure Zero-Exchange System:
Maturation and Growout of Marine Animals
Gary D. Pruder
The Oceanic Institute
41-202 Kalanianaole Hwy,
Waimanalo, Hawaii 96795
gpruder@oceanicinstitute.org
© 2001 The Oceanic Institute
Abstract
As progress continues in the development of aquatic production systems, once sacred assumptions are being challenged. In particular, the maintenance of clear clean water through filtration removal of solids and dissolved materials, efforts to minimize bacterial loads and the suppression of algae, once thought "key" to reliable and economical production, are clearly and unequivocally shown to be unnecessary and even counter productive. Compelling evidence has been established for certain marine shrimp, bivalve molluscs and finfish. These findings, besides opening the door for improved growth rates and yields, lead to a drastic reduction in required water usage. Which, in turn, makes possible practical biosecure production systems and inland siting of marine aquaculture.
This paper describes the fundamental issues of biosecure and zero-exchange systems and a companion paper presented by Dr. Shaun Moss, discusses performance to date, economic promise and challenges with managing biocomplexity. An essential parallel message is that aquaculture systems with inherent opportunities to control all inputs and environmental conditions, provide a unique platform for improving understanding of and eventual exploitation of biocomplexity, its positive impacts on both performance, effectiveness and its stability.
Keywords: Biocomplexity, Biosecurity, Zero-Water Exchange, Environmental Compatibility, Reliable and Profitable Production
I. Introduction
It is especially important to note that approaching complex ecosystems taxes conventional scientific and experimental capability. It is envisioned that working with leaders in the biotechnology fields will bring together emerging aquaculture needs and possibilities with new approaches, methodologies, sensors and advanced understanding of such complex systems. The biosecure zero-exchange system described here is leading a technology revolution in aquaculture worldwide. Importantly, The Oceanic Institute (OI) has filed patent applications in the U.S. and selected foreign countries, which describe the system and its operation in detail.
II. Background
The earliest shrimp farms consisted of near shore impoundments to which small numbers (1-3 shrimp/m2) of wild caught shrimp postlarvae (wild seed) were stocked. Several hundred kilograms/hectare of adult shrimp were harvested periodically. These farms in essence practiced natural balanced ecosystem growout; no feed was added to the growth medium, no water was exchanged, no aeration or mixing was performed, and no water treatment was provided.
Today, most shrimp farm operations have retained dependence upon wild seed, but have raised stocking densities. Yields of 1,000 - 3,000 kg/ha became the standard target, with some farm operations reporting yields of 10,000 kg/ha. Prerequisite to these higher yields were substantial modifications in farm management practices including, but not limited to, use and dependence upon supplemental feeds such as trash fish, manufactured feed or both; water exchange; and aeration/mixing. Generally the higher the stocking density, the more the farming system depended upon quality high protein and high-energy feeds, water exchange, and aeration/mixing.
However, concurrent with these increased yields, the impact and spread of shrimp diseases and mortality rates of shrimp have emerged as the dominant concerns of shrimp farmers around the world. Pathogenic viruses and other disease agents for shrimp are now commonly found in wild shrimp populations and in river and coastal waters where shrimp or other crustaceans are or may be farmed. These infectious pathogens are carried by workers, birds, and wind, and may contaminate the shrimp population with devastating results on crop growth and yield in confined systems. Thus, there is a need for an economical system or method for intensive production of high quality disease free marine animals, which also minimizes environmental impacts or side effects.
Development in the area of zero-exchange systems has led to significant changes in the art of aquaculture. For example, Fahs et al., U.S. Patent No. 5,353,745, disclosed an apparatus and method for maintaining aquatic organisms in an essentially closed system, wherein the aqueous medium is removed from the tank, sterilized, and returned following purification and removal of solids produced in the system. Further, Lee et al., U.S. Patent No. 5,961,831, disclosed a similar closed aquaculture system with automated water purification. Both of these inventions provide systems for the growth of commercially desired marine animals with near zero water replenishment. However, these systems utilize only "clean water" or "clean conditions" within the system. Therefore, these systems are not true zero-exchange systems as defined below because in the above noted systems, solids introduced into the system (typically feed) and produced within the system (typically fecal matter and metabolites) are continuously removed from the system along with recycled aqueous medium.
It is because solids are removed from the above systems that the systems illustrate they recognize the importance of maintaining solid residues, fecal matter, particulate matter, metabolites, and uneaten feed in an intensive zero-exchange growout system to provide an environment favorable for high growth rates and yields of shrimp and certain marine animals. "Clean water" systems do not establish these favorable conditions for intensive growth and harvesting of marine animals, specifically at levels of up to 10,000 kilograms per hectare. Adapting both the nutrition source and the mixing/aeration ratio compensates for any unfavorable changes in the system and maintains the favorable conditions throughout the growth cycle, whereas the industry has heretofore utilized filtering, flushing, or removing contaminants from the aqueous medium as the primary mechanism for controlling aqueous medium quality.
III. Detailed Description of a Biosecure Zero-Exchange System
The biosecure system accomplishes the intensive culture of marine animals such as shrimp with zero aqueous medium exchange ("zero-exchange"). More specifically, the system comprises a synergistic interaction between 1) an algae and bacteria population in an aqueous medium for in situ waste treatment and the production of a live nutrition source, 2) marine animals, preferably shrimp, selected for freedom from pathogens, and 3) an adaptable nutrition source to support water quality and prevent the buildup of toxins. The system is sufficiently isolated to prevent the introduction of pathogens to the microbial population and marine animals in the presence of a zero-exchange during the entire growout cycle. When combined, these individual components form a balanced system, wherein the marine animal growth rate is increased by 100% to 500%, relative to "clean water" systems, and provides low cost production of high quality, disease free, commercially desirable marine animals for the marketplace.
Although described in terms of shrimp, one skilled in the biological arts can appreciate that the system is equally applicable to the growth of other similar marine crustaceans and bivalves, marine mollusks, collectively referred to herein as marine animals. For example, the highest growth rates ever reported for oysters have been obtained in the system of the present system. Specifically, oysters grown in accordance with the present system grew to approximately 52 grams (live weight) in 198 days. However, such application will of course involve development and selection of specific pathogen free starter marine animals and adaptation of nutrition source materials to the animals specific requirements.
The system itself is a combination of elements, arranged in such a manner as to produce a beneficial and synergistic effect. Accordingly, for purposes of description, the system has been divided into its component parts, namely: an aqueous medium, the microbial population, the shrimp population, a growout tank and associated equipment, a nutrition source, and a method for using the system to achieve shrimp growout.
A) The Aqueous Medium
The building block of the aqueous growth medium is specific pathogen free water. As used herein, the aqueous medium is based upon water that is disinfected, or derived from a source substantially free of known shrimp pathogens, and optionally further treated to provide a medium suitable for the growth of microorganisms and shrimp. The term "specific pathogen free," refers to elements, microorganisms or animals, which are substantially free from known shrimp pathogens. Such pathogens are well known and outlined in more detail below.
The basic building block of the aqueous medium is usually seawater or an imitation thereof, which has a suitable level of salinity. For applications which are located near the ocean, seawater is the logical choice for the aqueous medium and is preferred, provided the water source is not contaminated with potential pathogens for the shrimp or is disinfected prior to use to remove such pathogens, known or unknown.
Seawater typically has a salinity of about 35 ppt, i.e., it generally contains about 35 parts sodium chloride or equivalents thereof (and other naturally occurring salts) per thousand parts, by weight of water. However, the acceptable water salinity may vary widely. Trial runs with selected shrimp stocks have sustained systems with salinity as low as .5 ppt and as high as 50 ppt Preferably, for this system the salinity is, however, in the range of about 2 to 10 ppt, advantageously at about 5 ppt.
By reducing the salinity of the aqueous medium to levels below that of seawater and by eliminating the need to constantly flush the growout system to remove contaminants, the requirement to use ocean water to grow shrimp is eliminated. This enables the production of shrimp farming facilities inland, away from coastal zones. There, the water used in the aqueous medium may come from a variety of sources. For example, the water may be well water, river or lake water, spring water, brackish water, or even tap water. Additionally, the acceptability of water salinities below sea water further simplifies water disinfection by enabling the use of disinfected fresh water (tap water) with disinfected sea salts, or a synthetic imitation thereof, to provide a suitable level of salinity and mineral content.
Additionally, there must be a limited amount of sulfur present in the aqueous medium to sustain a minimal, yet necessary, amount of anaerobic digestion in the zero-exchange system and create an environment suitable for shrimp growth. However, the sulfur levels should not be so excessive as to cause the production of toxic amounts of sulfur compounds like hydrogen sulfide. The sulfur concentration in the aqueous medium for use in this system, therefore, is typically less than 200 ppt. Advantageously, the sulfur concentration is maintained at levels in the range of 10 to 100 ppt. The sulfur content of the aqueous medium may also be modified and adjusted by adapting the feed supply, altering aeration, or both.
An important aspect is the concept of "zero-exchange." As used herein, zero-exchange refers to a system wherein new aqueous medium is introduced into the system only to replenish water lost for physical reasons, specifically evaporation and experimental sampling, and not for chemical or biological dilution. The concept of zero-exchange also relates to metabolites and solids provided to, or formed in, the system during growout. These solids and metabolites are retained in the system during the growout cycle and are critical for the growth of the shrimp stock and for maintenance of a synergistic microbial population.
B) The Microbial Population (see S. Moss contribution this proceedings)
C) Shrimp Population
The species of shrimp typically thus far used is Litopenaeus vannamei, however, other shrimp species such as Penaeus monodon, P. chinensis, and P. stylirosteris, can also be used. Indeed, essentially any commercially available shrimp species, which has been developed under international standards for freedom from specific pathogens, can be employed in the present system. These standards are provided by the International Council for the Exploration of the Sea (ICES, 1988). All such shrimp are referred to herein as "specific pathogen free (SPF) shrimp" and are well known in shrimp culture. Wild shrimp stocks are to be avoided because of the unknown disease status of the shrimp and the presence of disease pathogens in most wild shrimp populations.
SPF stocks have been used by terrestrial breeders to indicate that their stocks are free of certain specifically listed pathogens including viruses. (Twiehans and Underdahl, 1975). The list of pathogens that fit the definition for L. vannamei has changed over the years. In a publication by Wyban et al. (1993), the working list of certifiable pathogens included protozoans, metazoans and three viruses. The list of pathogens expanded significantly by 1995 and included TSV and YHV. The most current working list includes eight viruses (WSSV, YHV, TSV, IHHNV, HPV, BP, MBV, and BMN), certain classes of parasitic protozoans (microsporidian, haplosporidian and gregarine) and helminth parasite (Lightner and Redman, 1998).
Further, because shrimp are multicellular, complex organisms, their ability to adapt to variations in system conditions is limited. Therefore, shrimp used in the system are preferably specific pathogen free shrimp selected and bred for performance in a zero-exchange environment.
Such shrimp have been developed, maintained, and are commercially available for use by OI, and various cooperating research organizations. Alternatively, such shrimp may be independently developed using standard breeding and selection methods. Standard breeding and selection methods for shrimp are well known and understood (Argue and Alcivar-Warren, 1999).
The shrimp introduced into the system are either postlarvae (about 1/10 of a gram in weight) or juveniles (about 1-5 grams in weight). Typically, postlarvae are used because in the zero-exchange system, the growth rates are sufficient to offset the initial weight advantage of the juveniles in favor of a higher initial stocking density of postlarvae.
The stocking density in actual trials has ranged from 100 to 300 animals per m2. On a per weight basis, the shrimp are typically harvested at about 3 to 6 kg/m2 after roughly 8-12 weeks.
D) The Tank and Associated Equipment/Structure
The shape/configuration of the growout tank in which the aqueous growth medium and shrimp are deposited for the growout cycle is largely a matter of choice. Designs for tanks to contain the aqueous medium, microorganisms, and shrimp in accordance with the present system may be, but are not limited to, raceway, rectangular, and pond growout tanks, as is well understood. Important factors to consider in tank design for the present system are the cover, isolation of the tank, oxygenation, availability of a light source, and circulation.
Where airborne disease is a concern, the tank, and preferably the growout area surrounding the tank can be covered with suitable plastic or other material to prevent the introduction of pathogens. Alternatively, one or more tanks may be housed in a building or other facility to prevent the introduction of pathogens and to isolate the tanks and growth medium from contaminants and pathogens.
If covered or housed in a building, however, the tank cover or building ceiling should be wholly or partially transparent or semi-transparent or have transparent or semitransparent panels in order for light, including photosynthetically active light, to pass through at levels sufficient to insure the health of the microbial and shrimp populations in the growout tank throughout the growth cycle. Alternatively, artificial light, approximating natural light in spectrum, intensity, and cycle duration, may be provided if natural light cannot be made available to the growout medium. Such covers may also be designed to help regulate temperature within the growout facility, to prevent the atmosphere within the facility from getting too hot or too cold, either of which can adversely impact the rate of shrimp and/or microbial growth and health. Such control mechanisms and facilities are well known.
In a small operation or test facility, these covers may be designed to match the shape of the tank to minimize aqueous medium lost to evaporation.
Further, the depth of the tank also affects the growth of the algae population. As noted above, algae are important for the growth of the shrimp. If the tank depth is too shallow, light energy will be wasted. If the tank is too deep, the lower portion of the microbial population will not receive the necessary amount of light, thus shifting the system undesirably toward a predominantly bacterial population. The determination of the correct depth requires light attenuation studies of the specific system conditions, tank configuration, and water column depth, and may be accomplished by using a light meter or similar measuring device.
The bottom of the tank must be lined or otherwise made impervious to exchanges with soil or ground water to exclude possible disease vectors and prevent leakage. Leakage that occurs into the system from untreated water sources or rain should be avoided. Typically, the tank is elevated so that it does not come in contact with the soil or ground, or the tank is an in- or on-ground concrete tank in which the concrete forms a barrier between the growth media and the soil or ground surrounding the tank. Additionally the walls of the tank may be sealed to facilitate cleaning or disinfection or may be lined with a covering which may be cleaned, disinfected, or replaced between growth cycles.
The aqueous medium should be aerated and agitated. Typically, this is accomplished by circulation of the aqueous medium with a sufficient volumetric flow rate to keep solids formed in or introduced into the growth medium in suspension, as described below. The determination of the required rate of circulation is readily calculable.
Such flow generating devices may also contribute to or provide sufficient turbulence to provide adequate aeration of the aqueous medium, particularly if the tank is shallow and has a relatively high circulation rate.
E) Nutrition Source
The unique character of the nutrition source is its design to simultaneously feed the shrimp, foster the growth of microorganisms, influence the character of shrimp excrement which makes the excrement suitable for in situ treatment and recycle, and to prevent harmful substance buildup within the system, such as sulfur compounds, nitrites or nitrates, or ammonia. Comparatively, in "clean water" systems, as those noted above, feeds are typically formulated as nutritionally complete diets, and the uneaten feed and excretory wastes released into the aquatic environment are considered to have a negative or polluting effect on water quality. Therefore, in "clean water" systems the solids are removed from the system to prevent a buildup of toxic nitrogenous and sulfur compounds.
The use of the nutrition source as a control mechanism has been ignored in other systems because external water treatment and sterilization effectively maintains system conditions in a "clean water" state, which until now has been erroneously perceived as desirable. Such external treatment actively filters out toxins, solids, and microorganisms before returning the aqueous medium to the tank. In the zero-exchange system, these steps are not necessary because the feed is be adapted throughout the growout cycle to prevent the accumulation and buildup of toxic levels of the harmful materials while the microorganisms continuously recycle these materials for use in the system.
In order to prevent the system conditions from exceeding acceptable parameters, and possibly result in loss of the shrimp population or retardation of growth, the nutrition source must be adapted to maintain the system conditions at acceptable levels. For example, if the sulfur content of the system begins to elevate unfavorably, a nutrition source of reduced sulfur content should be provided until the sulfur content returns to acceptable levels by action of the microbial population, increased aeration, or both. Likewise, using a lower protein feed will control a buildup of undesirable levels of nitrogen compounds, such as ammonia.
However, because the microbial population recycles the uneaten feed, solid residues, and metabolites, alternative sources of protein remain available to the shrimp in the form of microbial biomass. Specifically, the shrimp consume the microorganisms in concert with the reduced protein and chemical energy in the nutrition source and in so doing, the growth rate of the shrimp is enhanced to rates greater than if the shrimp were to feed off a higher protein and energy nutrition source alone in a "clean water" system. Specifically, shrimp reach harvesting weight in about 8-12 weeks rather than the normal 12-16 weeks.
As with the microbial population, a precise definition of the nutrition source content is not possible because the system is in a constant state of change and self-adjustment, and accordingly the nutrition source content must also constantly change to accommodate these system conditions. Further, the overall amount of the nutrition source introduced varies depending upon the desired stocking density of the shrimp and microbial biomass.
Also, the nutrition source used in the present system must be specific pathogen free. This dictates against using fish byproducts as a source of protein unless appropriately disinfected. Therefore, typical formulations include disinfected feed grains, corn, wheat, soybeans, animal byproducts, synthetic nutrients, or various combinations thereof. A typical starting point of the nutrition source composition is:
The nutrition source is formulated to provide essential nutrients not only to the shrimp but the microbial population as well. Conventional feed formulations metabolized by shrimp, other animals and bacteria may not include all essential nutrients required by the algal population. Specifically, silicate, which is not in the shrimp diet, must be included in the nutrition source for continued algal growth. Further, algae also require Thiamin, Biotin, and vitamin B12, which may not be provided by the feed or bacterial metabolism by-products. By monitoring the system conditions and shrimp growth rates, the nutrition source composition may be adjusted to maintain the system conditions within desired shrimp growth parameters.
The need for continual vitamin and mineral additions conventionally required by other "clean water" systems is discouraged in the zero-exchange system of the present system. This is because such materials accumulate in the microorganisms and therefore remain readily available to the shrimp rather than being continuously flushed out of the system or removed by filtering action. It should also be noted that continuous vitamin and mineral additions are also discouraged because buildup of toxic trace elements such as copper, molybdenum, and selenium can occur.
Also, the feed may be used to adjust other factors such as the particulate organic carbon (POC) and adenosine tri-phosphate (ATP) levels. Preferable POC values in the water column are 7.0 mg/liter or higher. However, it has been shown that reduced POC values also reduce the growth of the shrimp. Specifically, when the POC values were reduced to 1.85 mg/liter, the shrimp growth rate was reduced by 35%. Furthermore, the preferred ATP level is 12.45 micrograms/liter or above. The effect of reducing the ATP level parallels the effect of reducing POC, specifically, as the ATP levels decrease, the growth rate also decreases. For example, when ATP levels were 2.14 micrograms/liter, the shrimp growth rate was reduced by 35%.
As noted above, the shrimp consume the microbial population in the system, as well as the nutrition source. The benefits of using a properly maintained mix of algae and bacteria in concert with an adaptable feed, rather than using only a predominantly bacterial system, is recognized.
Feed conversion is the total weight of nutrition source introduced into the system divided by the total weight of shrimp harvested. When shrimp were fed a diet of 35% protein, the feed conversion ratio in a predominantly bacterial system was 18.9. Therefore, 18.9 grams of nutrition source was needed to produce 1 gram of shrimp at harvest. Comparatively, in the mixed algal/bacterial system only 2.4 grams of nutrition source was needed to produce 1 gram of shrimp. Thus, 7.8 times more nutrition source was needed in the predominantly bacterial system to produce the same weight of shrimp in the mixed system.
IV. Environmental Impact
As Goldberg and Tripleft (1997) of the Environmental Defense Fund charge, the environmental impacts of fish and shrimp aquaculture are primarily wastes (consisting of uneaten feed and animal fecal and other excretory wastes) and the introduction of non-native and/or selectively bred animals with wild populations. Aquaculture wastes are a source of nutrient pollution and can stimulate blooms of toxic algae which can produce large fish and shrimp kills, contaminate shellfish, and potentially pose health hazards to humans. Further, the introduction of non-native and/or selectively bred animals with wild populations can harm local ecosystems by altering species composition or reducing biodiversity. Additionally, the transmission and spread of disease, as discussed earlier, is of great environmental concern. However, it must be recognized that there is no scientific evidence that these conditions are occurring in U.S. fish and shrimp aquaculture. Further, a recent scientific panel has concluded that there is little or no information on the potential impact of viruses on wild shrimp fisheries (JSA Shrimp Virus Work Group 1997).
It is clear that any negative environmental effects of marine shrimp farming must be minimized for the industry to achieve long-term success in the United States. Biosecure zero-exchange systems do not contribute to the deterioration of coastal or estuarine ecosystems and their use will also help to reduce pressures on the shrimp capture fisheries and to alleviate the destructive bycatch (e.g., turtles) problems of the shrimp trawler industry. Developing feeds appropriate for highly controlled production systems and refining methodologies for their use will reduce the likelihood that problems currently affecting global shrimp farming (e.g., disease outbreaks, water pollution, and disruption of the genetic integrity of wild stocks) will appear or worsen in the U.S. Further, the development of all-vegetable diets should reduce the use of fishmeal as a major ingredient in shrimp feeds, conserving fishery resources, eliminating the feed as a potential disease vector, and reducing feed costs overall.
V. Recommendations
(It is envisioned across the board that the following will be undertaken by a team of aquaculture and biotechnology scientists.)
A. Develop testable hypotheses for explaining the drastic impact (doubling growth rate) of the presence and/or involvement of algal/bacterial mixtures on marine shrimp, bivalve molluscs and certain finfish.
B. Develop methodology for testing the growth enhancement hypotheses suggested above.
C. Develop hypotheses to test the apparent requirement for both algal and bacteria to be present simultaneously to accomplish growth rate stimulation. Some evidence indicates that neither algae nor bacteria alone enhance shrimp growth rate.
D. Develop methodology for testing hypotheses above.
E. Determine the effect of selected environmental and nutritional factors on microbial mixtures and relative growth enhancement performance of shrimp, mollusc or finfish.
F. Develop sophisticated control parameters and computer control systems to balance photosynthetic and heterotrophic processes inherent in the emerging high performance aquaculture systems.
G. Explore the possibility of utilizing controlled aquaculture ecosystems to study biodiversity.
H. Consider new and novel approaches to accomplish system biosecurity and to improve animal resistance to disease and overall robustness.
I. Consider new and novel approaches to waste collection, treatment and disposal.
References
Argue, Brad J. and Acacia Alcivar-Warren. 1999. Genetics and Breeding Applied to the Penaeid Shrimp Farming Industry. Proceedings of a Special Session of the World Aquaculture Society Meeting, Sydney Australia April 27-30, 1999. Session Title: Integration of Shrimp and Chicken Models a Controlled and Biosecure Production Systems, pages 29-54. Published by the Oceanic Institute, 106 pages.
Goldburg, R. and T. Triplett. 1997. Murky waters: environmental effects of aquaculture in the United States. Environmental Defense Fund. Available online. www.edf.org
ICES 1988. Code of practice to reduce risks of adverse effects arising from introduction of non-indigenous marine species. In: Proceedings of the 1979 Statutory Meeting of the International Council for exploration of the sea (ICES) C.J. Sinderman and D.V. Lightner, editors, Elsevier Publishing Company, New York.
Joint Subcommittee on Aquaculture (JSA) Shrimp Virus Work Group. 1997. An evaluation of potential shrimp virus impacts on cultured shrimp and wild populations in the Gulf of Mexico and Southeastern U.S. Atlantic coastal waters. Report to the Joint Subcommittee on Aquaculture.
Lightner, D.V. and R.M. Redman, 1998. Strategies for the control of viral disease of shrimp in the Americas, Fish Pathology, 33:165-180.
Twiehans, M.J. and N.R. Underdahl 1975. Control and elimination of swine disease through repopulation with specific pathogen free stock. Disease of Swine, 4th Edition, H.W. Durvae and Lehmen editors, Iowa State University Press, pp 1163-1179.
Wyban, J.A., J.S. Swingle, J. N. Sweeney and G.D. Pruder. 1993. Specific pathogen free Penaeus vannamei. World Aquaculture 24:39-45.