Greenhouse-Enclosed Raceway System at Waddell Mariculture Center

Introduction

Several factors have limited the growth and expansion of the pond-based shrimp farming industry in the United States (US).  Extensive and semi-intensive pond production systems are typically managed with high rates of water exchange (Boyd and Clay 1998).  This production method raises environmental concerns regarding effluent discharge into receiving waters (Hopkins et al. 1995; Goldburg and Triplett 1997; Boyd and Clay 2002).  Pond-based culture systems are also typically restricted to coastal regions, where land costs are inherently high (Landesman 1994).  Another major concern is the limited growing season often found in temperate and subtropical regions.  For example, in coastal South Carolina water temperatures conducive for shrimp production only exist six to seven months per year.  This factor restricts overall production and usually allows only one crop per year (Main and Fulks 1990).  Finally several viral disease outbreaks have caused major damage to pond reared shrimp in the US and globally (Brock et al. 1997; Flegel et al. 1997; Lightner et al. 1998; Lightner 1999).  

In response, some recent research efforts in the US have been focusing on the use of greenhouse enclosed raceways for intensive to super-intensive shrimp production (Davis and Arnold 1998; Browdy et al. 2001; Van Wyk 1999, 2000, 2001; Samocha et al. 2002).  These production systems offer several advantages over the typical pond-based systems.  The raceways can be managed with zero to minimal water exchange thus greatly reducing the environmental impact due to effluent discharge (Browdy et al.2001).  Biosecurity protocols can be implemented to manage disease vectors (Moss et al. 1998; Ogle and Lotz 1998; Bratvold and Browdy 1999; Leung and Moss 1999; Moss 1999; Lotz and Lightner 2000).  Greenhouse enclosed systems also provide opportunities for inland culture operations (Scarpa 1998; Main and Van Wyk 1999) and year round production can be achieved (Browdy et al. 2001).

At the Waddell Mariculture Center (WMC) experiments have been carried out on the application of greenhouse enclosed super-intensive raceways for nursery and growout culture of Litopenaeus vannamei. This article will provide a brief description of the systems, operational procedures, and present some of the production data.

System Description and Operational Procedures

To date, several juvenile and growout production trials have been conducted using pilot and commercial-scale greenhouse-enclosed super-intensive raceway systems.  The pilot scale system consists of two 55 m² HDPE-lined, sediment-free raceways; the commercial-scale system consists of a single 282 m² unit.  Each can be equipped with AquaMats™ for vertical surface enhancement at a density of about 1 m² of mat for each m³ of raceway volume.  Aeration and water movement has been supplied by blown air,   and 0.75-kw paddlewheel and propeller-aspirator units.  Propane-based water heating is achieved using heat exchange units in-situ (pilot-scale system) or externally with a recirculating water pump system (commercial-scale system), which also aids in water movement.  Temperature is moderated during warmer months via shade cloth covering and thermostat-controlled fans.  Through 2002, no external biological or mechanical filtration has been employed during the production trials.  However, during the last growout trial in the 282 m² commercial-scale system, a 25 m³ bead filter was operated during the latter half of the cycle.  Also in place of the paddlewheel and aspirator units used in previous trials, oxygen supplementation was provided via injection by a Air Products Specialty VSA oxygen generator (Model No. A-150 L).  Two weeks prior to stocking of PL-10/20, raceway units are filled with UV-sterilized water and fertilized to establish planktonic communities.  For juvenile production trials, PL are stocked and then reared using standard feeding and husbandry protocols for 25-100 d.  For the most recent growout production trials, juvenile animals (1g) were stocked at 300 m² and then reared for 75-90d.

Results and Discussion

Nursery
Results obtained thus far confirm the technical feasibility of the application of these technologies for the nursery production of juvenile L. vannamei. Results are shown in Table 1.  Survival of animals produced in each trial has been > 90%, with mean

Table 1. At-harvest results of greenhouse-based trials for the production of juvenile Litopenaeus vannamei.

harvest weight exceeding 0.25 g at 30 d post-stocking.  As densities are increased, risks associated with mechanical failure increase.  This necessitates investment in personnel, emergency backup and alarm systems to manage the additional risk of catastrophic crop loss. In a recent review, Samocha et al (2002) summarized similar results from a range of commercial and experimental scale super-intensive nursery systems.  Clearly, the technologies for the large scale biosecure super-intensive nursery culture of L. vannamei have matured such that commercial implementation can be quickly implemented with a minimum of risk. Transfer of juveniles to growout systems can be quickly and efficiently carried out.  Transfer mortality is minimal particularly if oxygen is applied during the transfer process.  The degree of intensification of the nursery can be balanced with factors such as target growth rate and harvest size, capital investments and application of water treatment and oxygen injection technologies.  The advantages of the use of biosecure super-intensive nursery systems vary according to the site and type of growout operation.  In more temperate zones and in areas where shrimp growth improves seasonally, these systems allow the grower to gain three to four weeks of growing time by stocking 1g juveniles into the ponds at the start of the optimal growing season. Typically PL availability is limited during optimal stocking windows and the use of the nursery system to “head start” and/or stockpile PL can improve the outlook for stock availability during optimal stocking windows (Rhodes et al. 1995). For operations in which culture can be carried out year round, the use of the head start system greatly increases farm productivity allowing much more efficient use of space and increasing the number of cycles which can be produced per year and thus overall returns per unit area. Holding of stock in raceways for up to 30 days provides a powerful quarantine option to assure stock health before stocking into the farm. This requires proper nursery system site selection, planning and design along with an effective health monitoring protocols but it can be one of the most important components in an overall disease risk management and farm biosecurity program. Survival of nursed animals in growout is typically much higher and more predictable when compared to direct stocked PL.  This allows for more efficient feed management and can be an important factor in overall farm profitability. In areas where viral pathogens are endemic, use of nursed juveniles can improve survival rates by allowing the animals to grow larger and by improving the condition of the shrimp before exposure to endemic pathogens.

Growout
A total of five growout production trials were completed through 2002 in the Waddell Mariculture Center super-intensive production systems and were summarized by Weirich et al. (2002). The trials demonstrated the biological feasibility of the systems while documenting several bottlenecks which must be overcome for successful commercialization.  While market size shrimp were produced using minimal exchange systems without external filtration, survival and yield have been inconsistent. The most important factor in maintaining crop health and growth is adequate supplemental aeration in the system. In some trials, technical difficulties with liner designs or power failures in the absence of proper alarm and backup systems caused catastrophic failures. Clearly, one of the most important guiding principles for the design and implementation of commercial scale systems must be risk management through appropriate engineering technologies and management systems.  By designing in computer control systems and adequate redundancy, crop loss due to aeration failure should be eliminated.

Table 2 shows production results for trials 2 and 5. Both trials were stocked at 200/m² as PL8-10 for a 20 week growout cycle.  Trends towards increasing production levels and reduced feed conversion ratios reflect improvements through experience gained in completion of initial trials. The results obtained suggest that the system can operate without any outside filtration or water exchange up to a carrying capacity of 3kg/m².

Table 2. Harvest results of greenhouse-based trials for the production of Litopenaeus vannamei.

Water quality parameters from trial 5 are shown in Table 3. Temperature was maintained at a stable optimal levels by the heat exchange system despite outside temperatures which neared freezing on several occasions.  Data on temperature inside and outside the greenhouse and in the raceway coupled with information on propane usage will form the basis for system engineering to optimize passive heat gain and to provide a basis for system financial feasibility analyses for varying geographical locations.  In trial 2 and in raceway 2 in trial 5, water from the previous run was stored at harvest and reused for the subsequent cycle.  The reuse of water coupled with a relatively fast turn around and re-flooding around the Aquamat material resulted in consistently low ammonia and nitrite levels.  In contrast, freshly filled and bloomed tanks or use of new Aquamat material resulted in midseason ammonia increases followed by nitrite spikes. Thus, system designs which reuse water between crops may serve to stabilize microbial communities and associated water quality parameters while enhancing opportunities for application of these technologies away from the coastal zone.

Table 3. Water quality parameters recorded during Litopenaeus  vannamei  superintensive raceway production trial 5, December 2001-April 2003.

Results from the most recent growout trial continued to demonstrate trends towards improvement in production parameters (Table 4). At harvest production results were:survival, 91%; mean weight, 16.6 g; FCR, 1.54; avg. growth/week, 1.44 g; and yield, 4.50 kg/m² 10.0pt'. For this trial the raceway was stocked with 1g juveniles from a nursery system.  Together with improved growth rates, growout time was reduced to 11 weeks.  Use of juveniles may also have contributed to the improvement in survival rates.

Table 4. At-harvest results of greenhouse-based trials for the production of Litopenaeus vannamei (March 2003).

As described above, important modifications were made to system operations during this trial including use of a bead filter and removal of solids during the backwash cycle and injection of oxygen. Figure 1 shows the growth of the shrimp and system dissolved oxygen levels before and after addition of oxygen and filtration. The apparent increase in growth during the latter half of the cycle is in stark contrast to the typical plateau effect observed in previous trials as solid levels continued to increase late in the growing season. The use of oxygen can have positive benefits in terms of improved growth and condition of the target crop and increased system carrying capacity, however, negative effects of C02 buildup and associated pH drops must be addressed.

Figure 1. Relationship between dissolved oxygen levels and growth of Litopenaeus vannamei in a superintensive raceway system.

Preliminary Economic Feasibility Analysis

The economic feasibility of commercial shrimp culture in South Carolina using the state of the art indoor recirculating biosecure systems has also been initiated. The purpose of the analysis is two-fold: (1) to develop bioeconomic model specific to South Carolina that will assist researchers in investigating the sensitivities of projected profits and risks to various culture techniques and related variables (e.g. market prices), and (2) to provide commercial feasibility analysis information germane to private sector needs (e.g. preparation of business plans). As an initial starting point of this analysis, a "base case" production system along with production and economic assumptions were selected based upon experimental work at the Waddell Mariculture Center (Table 4) and other information sources. The hypothetical, base case production unit in the initial analysis is comprised of five modules each of which is comprised of six, 750/m² raceways covered by biosecure interlinked greenhouses. It was assumed that seawater would be pumped in and treated from nearby source (e.g. an estuarine creek) and this treated seawater would be heated via a heat exchanger system during the colder months. Major preliminary base case production assumptions included direct stocking of shrimp 1g animals at 300/m², overall survival to harvest size of 85%, mean harvest size of 17g, growth rate of 1.3 g/wk allowing four crops per raceway over a 12 month period, and 1.50 FCR. Major economic assumptions included juvenile stocking costs of $11.50/1,000 and feed costs of $0.50/kg.  The projected total estimated cost of production per kg is $4.36 with variable costs making up $3.27 per kg.  This preliminary analysis indicates that one of the most important parameters relative to reducing production costs is growth rate.  For example maintaining growth averaging 1.44g/wk (table 4) rather than the 1.3 g/wk in the base case scenario would reduce projected total production costs by $0.21 per kg.  Consequently, it appears that cost-effective techniques that can improve growth rates such as the selection of fast growing shrimp strains could significantly improve the financial performance of indoor recirculating biosecure systems.

The WMC prototype systems provide an insight into alternative culture technologies that have excellent potential to allow development and expansion of the US shrimp farming industry. By embracing these technologies, the environmental impact of discharged effluents would be reduced substantially if not eliminated, biosecurity protocols and procedures could be implemented to prevent the spread of disease agents to and from the culture facility, and year round culture could be achieved irrespective of location. In addition, systems could be located inland, potentially near market centers, opening up new opportunities for value added fresh or live product sales. However, in order for greenhouse-based minimal exchange shrimp production systems to be adopted commercially, they must be proven under a research setting to be biologically and economically sound on a consistent basis. To do this a commercial scale prototype system with a minimum of six raceways to allow for controlled studies must be constructed such that commercial feasibility can be demonstrated while continuing research and development to improve system performance. Additional research will be needed to explore effects of aeration type, water flow patterns, deployment of substrates, diets, feeding regimes, and other factors. Appropriate use of oxygen supplementation, filtration, and solids removal will require additional replicated studies to develop optimal protocols to assure maximum growth and production with the highest efficiency of protein use. Future studies will also have to address the disposal or treatment options for solids removed from these superintensive systems.  Although some studies have demonstrated the use of these solids as fertilizers and soil amendments further research in this area is needed. As discussed, data obtained from previous, ongoing, and future trials are being subjected to appropriate economic feasibility analyses to develop baseline cost and return information. These data will be reported in a separate publication and will be used to project profitability of various multi-unit culture system layouts and to drive establishment of ongoing research objectives.

In summary, it appears that the commercial development of greenhouse, superintensive shrimp culturing is favorable in the US, but it will remain dependent upon applied research and development of cost-effective production techniques coupled with marketing approaches that can buffer US commercial shrimp farm products from the vicissitudes of major US shrimp market segments, especially those segments that are apparently subject to shrimp import induced price reductions.

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