Conceptual design of a sustainable pond-based shrimp culture system

Aquacultural Engineering, Vol. 15, No. 1, pp. 41-52, 1996 Elsevier Science Limited Printed in Great Britain 0144-8609(95)00003-8 ELSEVIER

Conceptual Design of a Sustainable Pond-based Shrimp Culture System Paul A. Sandifer

& J. Stephen

Hopkins

James M. Waddell, Jr Mariculture Research and Development 809, Bluffton, SC 29910, USA (Received

9 July 1994; accepted

16 December

Center, P. 0. Box

1994)

ABSTRACT Estuaries are used as both water source and effluent disposal areas for shrimp farms. Feeding rates rise as stocking densities and production levels increase; howeveq much of the feed is not assimilated into shrimp tissue, but instead becomes dissolved and particulate waste that may be discharged as shrimp pond water is exchanged or drained for harvesting. Such nutrient enrichment of coastal waters by aquaculture efluents may be a significant concern in some areas. In the hypothetical intensive shrimp farm described here, water is recycled within the system, with discharges only to release excess water from precipitation or to maintain acceptable salinity. Water is not discharged at harvest but is held over winter between crops and then used again the next growing season. The primary crop of shrimp is co-cultured with oysters and herbivorous fish (mullet) to enhance solids removal and deposition and create other cash crops. The amount of nitrogen provided as feed is reduced by using feeds with less protein and better feed management. The farm plan is modular with each 4-ha module consisting of (1) three 1 *O-hashrimp monoculture ponds; (2) one I sO-haoyster-mullet polyculture and water treatmentpond; (3) a small phytoplankton inoculation pond; (4) a solids settling basin; (5) sludge drying beds; and (6) upland use of pond sludge to improve agricultural land. The system is designed to produce 40 mt of whole shrimp, 7 mt of mullet, and approximately 0.5 million single select oysters/module/ year

INTRODUCTION Expansion of aquaculture products increases through

is inevitable as demand for seafood population and income growth. However,

42

P A. Sandijeq J. S. Hopkins

improperly managed aquaculture may poorly utilize or damage land, water, energy and protein resources. The collapse of poorly regulated intensive shrimp aquaculture industries in Taiwan (Liao, 1992), Thailand (Stanley, 1993) and China (Anonymous, 1993; Wang et aZ., 1995) provides stark examples of what can occur when unsustainable, highly resource-dependent production systems are allowed to proliferate. Estuaries are used as both water source and effluent disposal areas for shrimp farms. Feeding rates rise as stocking densities and production levels increase; however, much of the feed is not assimilated into shrimp tissue, but instead becomes dissolved and particulate waste that may be discharged as shrimp pond water is exchanged. Such nutrient enrichment of coastal waters by aquaculture effluents may be a significant concern in some areas. This paper describes a conceptual model for a sustainable shrimp aquaculture technology which, after full development, should: (1) be appropriate for the United States; (2) minimize and balance utilization of land, water, energy and protein resources; (3) be adaptable by existing commercial aquaculture farms, in part or in whole, while maintaining or increasing profitability; and (4) serve as an example of sustainable coastal aquaculture technology for use in the design of new farms and establishment of best management practices. It is also intended as an early step in the development of an integrated water and waste management approach for shrimp aquaculture, similar to the move toward more integrated, sustainable terrestrial agriculture.

THE ELEMENTS The hypothetical, shrimp farming system described here embraces six major elements. (1) Minimization of land production technology

use

through

application

of intensive

The target production goal for the model farm is 10 000 kg of whole shrimp/ha/crop. This goal substantially exceeds the current production levels for most commercial farms in the US, but yields of this order have been regularly produced in research and demonstration projects (see Sandifer et al., 1991a,b).

Conceptual design of a shrimp-culture ystem

43

(2) Reduced or eliminated discharge of waste water Traditional intensive shrimp culture typically uses substantial water exchange, often as much as lo-50% of the pond volume/day or more (Sandifer et al., 1987, 1988, 1991a, b, 1993; Hopkins et al., 1991, 1993b). However, (Hopkins et al., 1995a, b) effectively eliminated water exchange and associated discharges of BOD, solids and nutrients, and still produced 6000-8000 kg/ha/crop without sacrificing shrimp survival or growth.

(3) Reduced energy requirements for aeration and water circulation Paddlewheel aerators generally have been found to be the most efficient mechanisms for oxygen transfer in aquaculture ponds (Boyd and Ahmad, 1987; Ahmad and Boyd, 1988; Fast and Boyd, 1992; et al., 1991). Aerators also create water currents that Ruttanagosrigit can be directed to concentrate settleable matter in deposition zones where it can be removed from the pond bottom. Our model farm uses low-hp vertical fans to supplement paddlewheels, since they may perform similar mixing and circulation functions with less energy costs. A microcomputer-based monitoring and feedback system is included to activate more energy-intensive aeration equipment only on an as-needed basis.

(4)

Reduced nitrogen input as feed

Following the Aranyakananda and Lawrence (1993) report that 15% protein was sufficient for Penaeus vannamei when fed to excess in laboratory experiments, Hopkins et al. (1995b) found that a 20% protein diet performed as well as a 40% protein formulation in pond culture of p vannamei at densities of 38 and 79 shrimp/m2. Production levels ranged from 5807 to 8163 kg/ha/crop. Use of a 20% protein ration rather than the 40% formulations currently in widespread use in the US would reduce feed nitrogen input by 50%. should also lead to better feed Improved feeding strategies utilization. For example, intensive production systems generally target food conversion ratios (FCRs) of around 2:l. However, Hopkins et al. (1995b, c) demonstrated FCRs as low as 1.5 and 1*6:1 in intensive culture of E! vannamei, indicating that there is much room for improvement in feed management.

44

l? A. SandifeG .l. S. Hopkins

(5) Recycling of water and improved protein transformqtions

Protein transformation efficiency may be improved in numerous ways, including production of additional useful crops without a proportional increase in feed input and the recycling of water and its organic load. Water from intensive shrimp ponds is rich in organic particulates and inorganic nutrients derived primarily from the feed. This enriched water may sustain a dense biota of bacteria, phytoplankton and many other organisms, directly enhance shrimp growth (Bowen, 1987; Leber and Pruder, 1988), and provide opportunities for polyculture. Particularly promising in this latter regard are combinations of a fed shrimp or fish crop and a filter-feeding bivalve mollusc which feeds on the phytoplankton blooms that occur as a result of nutrient enrichment. Combinations tested include fish and the Manila clam (Shpigel and Fridman, 1990), fish and oysters (Shpigel et al, 1993a, b), shrimp and oysters (Wang, 1990; Hopkins et al, 1993a; Jakob et al, 1993), shrimp and clams (Hopkins et al., 1993a) and shrimp and scallops (Walker et aZ., 1989). More recently, Enander and Hasselstrom (1994) described a simple wastewater treatment system for a shrimp farm in Malaysia that incorporated a filter feeding bivalve (the hairy cockle, Scapharca inaequivaZvis),and a seaweed (Gracilaria sp.). After 1 month of operation, the system reduced phosphorus and nitrogen levels in the effluent water by 61 and 72%, respectively. They suggested that the oyster might be a better bivalve to use because of its higher value. High quality, single select oysters can be produced in pond effluent waters (Scura et aZ., 1979) or directly in pond culture (Waddell Mariculture Center, unpublished data), and oysters tolerate a wide range of salinities allowing them to be grown at most sites now commercially culturing shrimp in the US. Oysters efficiently filter large volumes of water, ingest a portion of the particulate matter and deposit considerable quantities of pseudofeces (Haven and MoralesAlamo, 1966). Thus, they effectively capture suspended organic matter and utilize it or deposit it so that it may be removed from the system. However, oysters also produce notable amounts of nitrogenous waste products. Combining a fed crop of shrimp with herbivorous or omnivorous fish such as tilapia (Meriwether et aZ., 1984) or mullet (Waddell Mariculture Center, unpublished data) is also appealing. In South Carolina, mullet commands a higher market value than tilapia; roe from wild mullet harvested in late fall bring very high prices in export

Conceptual design of a shrimp-culture system

markets, and limited quantities seafood and bait markets.

45

of mullet flesh may be sold into fresh

(6) Reclamation and utilization of pond sludge Aquaculture pond sludge may be applied to soils to increase tilth, organic matter and fertility (Bergheim et al., 1993; Westerman et al., 1993). Hopkins et al. (1994) demonstrated that frequent removal and high-land disposal of sludge from shrimp ponds significantly lowered levels of inorganic nutrients in the pond water column and reduced the impact of drain harvest effluent on the receiving stream. They reported that weekly removal of sludge deposits from an intensive shrimp pond accounted for about 700 kg N/ha/crop or about twothirds of the nitrogen added as feed. Some of this sludge was added to an area of highly impoverished, very sandy soil where it improved growth of a cover crop the following winter. There should be fewer use restrictions on fish farm sludge than treated sewage sludge, since the aquaculture sludge does not have to be sterilized to inactivate human pathogens and there is little likelihood of encountering hazardous substances. However, the degree of salt retention in sludge from seawater ponds, which is likely influenced by the porosity of the underlying soil, must be considered.

THE MODEL The six elements described above were combined in a conceptual design of a modular, environmentally-friendly shrimp farm. Each module consists of four l-O-ha production units [three for monoculture of shrimp (Penaeus vannamei or rl setifems) and one, a water treatment pond, for polyculture of oysters (Crussostrea and mullet (Mugil cephalus)]; a small phytoplankton virginica) inoculation pond; a settling basin; and a sludge drying bed (Fig. 1). Water is recycled within the system, and once all ponds are filled, no water is added except by precipitation or to maintain salinity. ‘Used’ water from the production ponds is cycled through the oystermullet polyculture pond and then pumped back to the production units. Water transfers among ponds are carried out as needed via a system of interconnecting pipes and pumps. Periodically, sludge deposits are pumped out of the production ponds, dewatered, dried and used as a fertilizer for row crops such as tomatoes and broccoli or as a soil amendment. The recycling and transformations of the

I! A. Sand$e< J. S. Hopkins

46

1.O,ha shrimp pond

1.0 ha shrimp pond

1.0 ha shrimp pond

baseline

I paddelwheel aerator 0 vertical fan mixer 0 pump; chying bed to pond + elec./mech. control panel

ng bed compartments mid-water connection to common sump &pump oyster racks in 1.0 ha. oyster 81mullet pond settling basin

A pump, pond to settling basin 0

pump, settling basin to drying bed

??pump, mocu. pond to product. pond

phytoplankton inoculation pond

Fig. 1. Schematic design of one 4-ha module of a sustainable, pond-based shrimp culture system showing major system components.

pond water and its nutrient load are illustrated schematically in Figure 2. Shrimp are stocked into three of the four production ponds at a density of 100 shrimp/m2. This density has been shown to yield 2 10 000 kg/ha/crop reliably (Sandifer et aZ., 1991b). Shrimp are stocked in April or May for a 140-160-day growing season. The fourth pond is stocked with oysters (lo6 seed/ha) and mullet (7000/ha). Hatchery-produced single oysters representing 2 year classes are placed on 6 m2 racks at a minimum density of 500/m2 in October preceding shrimp stocking. One year class is harvested annually and more seed then added to the system. Similarly, the

Conceptual design of a shrimp-culture system output input

Legena:

Fig. 2.

0

assimilation dIgestIon and/or

02 03

excretion

harvest

input

output

04 mineralization 05 dew c9 denitrlfication @S,$$ghmaa and volatlllzation

Inputs and outputs, principal in the conceptual

reservoirs and recycling pathways for nitrogen shrimp production system.

initial stocking of juvenile mullet includes animals of 2 year classes (approximately 20 and 300 mm). To counter changes in phytoplankton dominance that may occur as the oysters graze down the desired species, the farm module includes a small pond where an algal bloom can be developed for inoculating the larger ponds as needed to effect a shift in phytoplankton community structure. Water is pumped from this pond to the production units as needed. The settling basin and sludge drying bed are fashioned after similar units used in the sewage treatment industry (Vesilind et cd., 1988; Eckenfelder, 1991). The basin receives the water-sludge mixture from the ponds, solids are settled out, and water is returned to the

48

R A. Sand$eq J. S. Hopkins

production system. The thickened sludge is then pumped to a sludge drying bed, which is divided into 20 compartments and plastic-lined to prevent seepage. Each compartment has a perforated underdrain embedded in gravel. Layers of successively smaller gravel and sand overlay the drains, and low sand berms separate compartments. One compartment is filled with sludge daily. When the moisture content has been reduced from the original > 90% to < 50%, the sludge can be handled as a solid. Water from the sludge dewatering process is piped back to the production units. The dried pond sludge is used in cultivation of row crops or incorporated into impoverished soils to improve soil fertility and capacity for water retention. To facilitate shrimp harvest, the water is pumped to adjacent ponds (utilizing the freeboard space) with a hydraulic fish pump, and the shrimp are captured in a dewatering device. Since the normal pond operating level leaves about 50 cm of freeboard, three of the four l-ha ponds can store about 15 000 m3 of water for short periods. This should readily accommodate the roughly 12 000-13 000 m3 that is moved from the pond being drain-harvested. The oysters, supported 60 cm off the pond bottom on racks, are exposed to air at regular intervals by pumping water to adjacent ponds. This exposure is needed to control oyster parasites such as the boring worm (Polydora sp.) and boring sponge (Chione sp.). A costeffective and easily managed rack system has been developed for off-bottom oyster culture in ponds (Hopkins, unpublished). During winter, when shrimp are not being grown, an ‘aquatic pasture’ is allowed to develop in the production ponds, sustained by residual nutrients from the summer crop. Initial investigations indicate this assemblage is dominated by a seaweed (Enteromorpha sp.), gammarid amphipods, copepods (e.g. Acatiia tonsa), and various species of benthic polychaetes (including Capitelh cupitelh and PoZydoru corn&a) (Maier, 1991; Hopkins et al., 1995d). Many of these organisms are heavily preyed upon by shrimp shortly after the postlarvae are stocked in the spring (Hopkins et al., 1988), thereby recycling residual nutrients from one crop into shrimp tissue in the subsequent crop. Target production levels for each module are as follows: shrimp, survival of +75%, 18 g mean weight at harvest, total yield of + 40 mt whole shrimp/year; oysters, survival > 95%, growth to single selects in 24 months, and yield of 500 000 single oysters/year; and mullet, survival 190% and harvest biomass of > 7 mt/year.

Conceptual design of a shrimp-culture system

49

DISCUSSION Nutrient enrichment of natural waterways by effluents is perhaps the most obvious and often-cited adverse environmental impact of aq‘uaculture (see Hopkins et aZ., 1995a, for a review of the literature). Discharges from shrimp ponds are typically high in suspended solids, particulate organic nutrients, dissolved inorganic nutrients, and biochemical oxygen demand (BOD), with the concentrations largely determined by the feed inputs. These, in turn, are related to stocking density and estimated standing crop biomass (Ziemann et al., 1990, 1992; Pruder, 1992; Hopkins et cd., 1993b, 1995a, c). Often shrimp ponds are completely drained to facilitate harvest. This drain effluent may be particularly high in BOD, suspended solids and particulate nutrients due to mechanical disturbance of the pond bottom and the large volume of discharge (Boyd, 1978). The model farm described here builds on earlier suggestions for storing this drain water for later reuse (Schwartz and Boyd, 1992; Hopkins et al., 1995~). Moreover, it not only eliminates waste water discharges, it recycles nutrients, reduces the amount of land needed for commercial shrimp culture operations, and produces additional products with appreciable market value. In addition, management of the organic-rich waste sludge is addressed. A financial analysis of this approach is expected to show better economic viability than traditional monoculture, at least when risks and costs related to obtaining and maintaining discharge permits are included. An assessment of adjacent estuarine waters is expected to show that the sustainable system has little, if any, measurable environmental impact.

ACKNOWLEDGEMENTS This is Contribution No. 359 from the South Carolina Marine Resources Center. We gratefully acknowledge the diligence, dedication, and hard work of the entire Waddell Mariculture Center staff. This work was supported by the US Department of Agriculture, CSRS, through a subcontract from grants to the Oceanic Institute for the US Marine Shrimp Farming Program of the Gulf Coast Research Laboratory Consortium and by the State of South Carolina.

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