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Defining loading limits of static ponds for catfish aquaculture
Aquacultural Engineering 28 (2003) 47 /63 www.elsevier.com/locate/aqua-online
Defining loading limits of static ponds for catfish aquaculture John A. Hargreaves a,1,*, Craig S. Tucker b a
Department of Wildlife and Fisheries, Mississippi State University, Box 9690, Mississippi State, MS 39762-9690, USA b Thad Cochran National Warmwater Aquaculture Center, Mississippi State University, P.O. Box 197, Stoneville, MS 38776, USA
Abstract Commercial channel catfish farming has emerged as the most important aquaculture industry in the United States. During the last two decades, industry growth has occurred by expansion in the number and area of facilities and through production intensification. Evidence suggests that catfish farming has apparently reached the limits of the production system as currently configured. The success of commercial catfish culture can be attributed in part to low production costs resulting from the inherent waste assimilation capacity of aquaculture ponds, although operating within this capacity is complex and associated with several poorly defined limitations and hidden costs. Loading limits for pond aquaculture are based on the waste assimilation capacity of ponds and tolerance limits of the cultured species. The important design and operational considerations affecting loading limits include temperature effects, oxygen requirements, fish water quality tolerance limits, organic matter decomposition, and nutrient removal. Engineering solutions for extending the loading limits of pond aquaculture must account for the highly dynamic and complex nature of the pond ecosystem, particularly processes related to phytoplankton and microbial dynamics. Characteristics of hypertrophic ponds amenable to engineering solutions include excessive phytoplankton biomass, dominance of phytoplankton communities by cyanobacteria, intense diurnal stratification, chronic undersaturation of dissolved oxygen at the sediment /water interface, and limitations of current aeration technology. Improvements in the profitability of
* Corresponding author. E-mail address: [email protected] (J.A. Hargreaves). 1 Current Address: Aquaculture Research Station, Louisiana State University, 2410 Ben Hur Road, Baton Rouge, LA 70820, USA. 0144-8609/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0144-8609(03)00023-2
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catfish pond aquaculture requires broad-scale implementation of a production paradigm based on measures of performance efficiency. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Channel catfish; Waste assimilation capacity; Pond aquaculture
1. Introduction Commercial farming of channel catfish in earthen ponds has emerged as the most important component of aquaculture in the United States. Although catfish farming began in the late 1940s, significant production levels were not achieved until the 1970s, and it was not until the 1980s that the industry grew rapidly and attained the currently prevalent production levels. Production practices vary greatly from farm to farm, due in part to different production goals among farms, but also to the paucity of research information on the economics of various production schemes. Recent weakness in domestic and world economies and competition in the global seafood market have also affected farm profitability. This context may catalyze significant changes in production practices as farmers seek more efficient strategies in the face of a new set of economic pressures. Notwithstanding differences in specific culture practices, three general features are common to most catfish farming strategies and broadly characterize the technical basis of the industry: the use of relatively large ponds, high fish stocking and feeding rates, and multiple-batch fish cropping. During the early years of the industry, pond size was highly variable but on average was large, with some ponds over 40 acres (16.2 ha). In one of the first economic studies conducted for the catfish industry, Foster and Waldrop (1972) examined the relationship between pond size and production costs. Although it vastly oversimplified the issue by focusing almost exclusively on construction cost as an economic variable, the study had a major impact on the catfish industry. Average pond size quickly decreased to about 18 acres (7.3 ha), which was the standard for many years. Farmers have since discovered that feeding, disease management, water quality management and many other common activities are easier and more effective in smaller ponds, and average pond size has continued to decrease. Most of the ponds constructed since 1995 have been between 10 and 12 acres (4 /5 ha). In the early 1970s, fish stocking densities ranged from 1000 to 2000 acre1 (2471 / 4942 ha1) and maximum daily feeding rates were restricted to 35 lb acre 1 (39 kg ha1). These limits were determined empirically by farmers who, lacking convenient and effective aeration devices, tried to limit culture intensity at a level where few problems with dissolved oxygen depletion would be encountered. Through the 1970s, stocking density gradually increased to 4000 acre 1 (9884 ha1) and episodes of critically low dissolved oxygen concentrations became more frequent. At first, when oxygen problems were still relatively rare, producers used aerators fabricated in on-farm machine shops and connected to the power take-off unit of a tractor, which was a readily available power source on farms. Tractor-powered aerators were mobile and could be moved around the farm depending on which pond was in the
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greatest need of emergency aeration. In the early 1980s, farmers continued to increase stocking densities and feeding rates, and the need for aeration increased to the point where most ponds on a farm needed aeration every summer night. Tractors were too expensive and difficult to maintain for routine, everyday use as a power source for aeration. Floating paddlewheel aerators powered by electric motors were much more efficient, and by the mid-1980s had become the most common aerator on catfish farms. A third characteristic of current catfish production is the widespread use of a unique fish cropping system wherein faster-growing fish are selectively harvested using a large-mesh seine and fingerlings or larger stockers are added (‘understocked’) to replace the harvested fish. The process of selective harvest and understocking may continue for many years without draining the pond. The multiple-batch cropping system was originally developed in the 1970s to provide a year-round supply of market-sized fish. The catfish industry has, however, become large enough to supply fish all year with single cropping in individual ponds, but the multiple-batch system remains popular because economic risks associated with development of algaerelated off-flavors are reduced. That is, if timely harvest of fish from a particular pond is constrained by the presence of off-flavors (or other factors, such as ongoing losses from an infectious disease), there is a greater probability of having acceptable fish to sell from another pond when all ponds on a farm are managed with the multiple-batch cropping system. Another benefit of the multiple-batch cropping system is that ponds can be operated continuously for many years without draining, which dramatically reduces pond effluent volume and the need for pumped water to refill ponds compared to ponds managed with the single-batch cropping system. The technical development of catfish farming in static ponds as described above has been characterized by increasing intensification. However, catfish farming has developed to the point where yield limits of the production system as currently configured and managed have apparently been reached. In the 1990s, annual production ranged from 5000 to 6000 lb acre 1 (5600 /6700 kg ha1) (Heikes, 1996) despite a fairly wide range of stocking densities (6000 /10000 acre 1; 14 826/24 710 ha1). Losses from infectious diseases, some of which are indicative of elevated stress associated with culture conditions, continue to plague catfish producers. Excessive eutrophication of catfish ponds caused by high waste loading rates often lead to the dominance of phytoplankton communities by blue /green algae, some of which may be toxic or impart off-flavors to cultured fish. Despite the emergence of yield limits, catfish farming during the last 2 to 3 decades has been generally profitable in most years. In part, the profitability of catfish farming can be related to the inherent waste treatment capacity of static ponds. Unlike recirculating and other intensive aquaculture production systems, there are few direct costs (e.g. supplemental aeration) associated with waste treatment because the pond ecosystem provides natural biological, chemical and physical waste treatment processes that appear, at first glance, to be free. However, when examined in a larger context, the waste treatment capabilities of ponds are not really a free service provided by natural ecological processes. Farmers do, in fact, pay a price for waste treatment in ponds because the system only
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functions effectively within certain operational limits based on loading rates for organic matter, nitrogen, and plant nutrients. Specific loading limits are ill-defined, but the implication for catfish culture in ponds is that rather large ponds are required to grow a certain biomass of fish. The other cost of allowing nature to operate unrestrained in response to high fish stocking densities and feeding rates is the low level of management control that farmers can exert over certain environmental variables such as off-flavor, infectious diseases, temperature, and dissolved oxygen. As a specific example, the various impacts associated with sporadic episodes of off-flavor in pond-raised catfish add about $US0.05 kg1 to the cost of production (Engle et al., 1995). The presence of off-flavor does not limit yield directly, but additional costs are incurred because harvest must be delayed until flavor quality is acceptable and feeding must continue to maintain fish weight. During the time that harvest is delayed, fish are also at increased risk of loss to disease and predation. Many farmers do not attempt to manage off-flavor problems, and simply assimilate the cost of off-flavor as part of the overall cost of growing catfish in ponds. Using two examples at the extremes of fish culture intensity, farmers are faced with a fundamental economic and engineering choice of using either small (in terms of land requirements), high-intensity fish culture systems that provide a high degree of control over most environmental variables or using pond systems with large land requirements and many hidden costs associated with the inability to control environmental conditions. At present, the latter appears to be the rational economic decision, at least for channel catfish. However, in the face of changing economic forcing factors, it may become necessary to more closely examine the basic pond culture paradigm that has developed for catfish and investigate economically feasible opportunities for better control of pond environments. Here we focus on the biological and physical factors that affect the loading limits and waste assimilation capacity of static ponds used for commercial catfish aquaculture. Loading limits are a function of the interaction between the biology of channel catfish, climate, and ecological characteristics of static ponds. Importantly, static pond production systems are also limited by farm management, business, and macroeconomic considerations (e.g. processing, marketing, farm financing, global seafood trade policy), but these are beyond the scope of this review. As the limits of currently configured static ponds are approached, some characteristics of ponds resulting from operation near loading limits and amenable to engineering solutions are identified. Creative, practical, and cost-effective solutions to the problems posed by these characteristics have the potential to increase the efficiency and profitability of catfish farming in static ponds.
2. Factors affecting loading limits in catfish pond aquaculture The technical success of catfish pond aquaculture in static ponds is based on the premise that natural physical, chemical, and biological processes within the pond assimilate or remove wastes that are generated as a result of nutrient input from
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feeds. A relative minority (15 /25%) of nutrients and organic matter contained in feed is retained by fish. Therefore, the majority of nutrients and organic matter added in feed is released to the pond environment. Natural process rates must be comparable to waste loading rates or water quality will deteriorate. If the concentration of water quality variables exceeds the tolerance limits of fish, stress can lead to reduced fish growth, immunocompetence, and survival. Thus, loading limits for ponds are based on two simultaneous sets of criteria. First, analogous to design criteria for aerobic waste treatment lagoons, process designs can be based on the assimilative capacity of the ponds, which, in turn, are based on nutrient and organic matter loading rates and hydraulic retention times. The second set of criteria consists of the tolerance limits of the cultured species. Despite several decades of research on pond dynamics, loading limits and the waste assimilation capacity of static ponds are not particularly well defined. In fact, definitions of maximum loading rates are at best approximations derived empirically from the practical experience of fish farmers, rather than through controlled studies. Biological and physical design and operational considerations for pond aquaculture include temperature effects, oxygen requirements, fish water quality tolerance limits, organic matter decomposition, and nutrient removal.
2.1. Temperature effects Water temperature exerts profound effects on fish metabolism and therefore feed consumption, growth and production. Temperature, particularly temperature extremes, can act as a stressor, thereby affecting immunocompetence and disease incidence. Temperature influences water quality by affecting the kinetics of chemical reactions, nutrient uptake by phytoplankton, and microbial growth. As temperature increases from some minimum at which biological process rates are minimum, rates increase according to the van’t Hoff relationship, i.e. the rate doubles for every 10 8C increase in temperature, to some temperature at which process rates are maximum. The range of temperature at which rates are maximum is characteristic of the reaction. For example, the temperature range for optimum channel catfish growth is 28/30 8C. Beyond this optimum temperature range, process rates will decline. Most channel catfish are grown in north Mississippi, north Alabama, and southern Arkansas where water temperatures vary considerably from summer to winter. In that region, catfish have a ‘growing season’ of about 200 days with average pond water temperatures above 20 8C. For the remainder of the year, fish grow little, if at all, and are essentially held in inventory over the winter. Although faster growth and greater fish production are possible in regions with a year-round growing season, the effect of water temperature on fish reproduction may limit the extent to which channel catfish can be cultured in tropical areas. Channel catfish evolved in a temperate climate and seasonal changes in water temperature exert primary control over the reproductive cycle (Davis et al., 1986). Thus, large-scale fingerling production relying on natural spawning of brood fish held in ponds is not possible in regions without relatively long, cool winters.
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As a practical matter, control of water temperature in large ponds is not possible. However, knowledge of water temperature is important as a determinant of feeding rate based upon (1) the quantity of feed that fish will consume and (2) the quantity of waste organic matter and nutrients that the pond can assimilate. Further research is necessary to better define feeding rate and waste assimilation capacity as a function of temperature. Research to date has focused on definition of a maximum waste assimilation capacity. This capacity is primarily a function of water temperature, but is also affected by other factors such as solar radiation, dissolved oxygen concentration, and water column mixing. The dynamic nature of the waste assimilation capacity of aquaculture ponds limits any definition to a generalized set of operational conditions. As fish are poikilothermic, feed consumption and growth are closely related to seasonal variation in temperature. A study of catfish ponds was conducted where daily measurements of water quality and meteorological variables and feeding rate were made (Taylor, 2003). Data reduction by factor analysis allowed inference of the relative importance of the processes controlling variation in water quality and feeding rate. Most of the variation in feed consumption during spring and fall was explained by variation in water temperature. Variation in feed consumption during summer was not related to variation in water temperature because water temperature was near optimum. (Rather, restricted access to pond levees during inclement weather was the most important factor affecting feed consumption during summer.) Water temperature was also the most important variable related to dissolved oxygen concentration at dawn. Counter-intuitively, the relationship between feeding rate and dissolved oxygen at dawn was weak. Water temperature affects dissolved oxygen concentration in two ways: (1) the solubility of dissolved oxygen is inversely related to water temperature and (2) respiration rates are directly related to water temperature. Therefore, solubility is minimum and respiratory oxygen demand is maximum during summer. 2.2. Oxygen requirements Supplying sufficient dissolved oxygen is a key operational consideration for pond aquaculture. There are two aspects to oxygen supply: (1) providing a sufficient quantity of oxygen to fulfill overall respiratory demand of the pond biota and (2) maintaining dissolved oxygen concentration above a minimum threshold for the cultured fish. Providing adequate oxygen to maintain aerobic conditions throughout the pond at all times will maximize the waste treatment function of ponds, in much the same way that maintaining aerobic conditions in waste treatment lagoons enhances waste treatment capacity over that provided in stabilization ponds operated without aeration. However, the economic benefit of providing sufficient oxygen to meet the total respiratory demand of the pond ecosystem is unknown and may not be justified. The cost of meeting overall oxygen demand is high because total oxygen demand is much greater than the oxygen requirements of the fish standing crop. For example, in a typical commercial catfish pond, the total oxygen consumed in respiration by
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fish, plankton, and sediment during summertime may exceed 15 kg O2 ha 1 h1 (1.5 mg l 1 h 1), the bulk of which is consumed by phytoplankton. In a study of commercial catfish ponds, whole pond respiration was partitioned among the water column (65%), sediment (20%), and fish (15%) (Steeby, 2002). Similarly, in two tropical fish ponds, water column respiration accounted for an average of 68% (range: 53 /76%) of whole pond respiration (Teichert-Coddington and Green, 1993). Thus, the catfish crop consumes only about 10 /20% of the total and phytoplankton respiration is the major sink for dissolved oxygen in ponds. Oxygen is supplied to ponds through photosynthesis, reaeration, and mechanical aeration. Photosynthesis supplies most of the oxygen to meet respiratory demand, although most of the oxygen produced in photosynthesis is consumed by phytoplankton respiration. Maximizing oxygen production by phytoplankton is one design objective of aerobic wastewater treatment ponds. Control of phytoplankton density to maximize oxygen production is difficult to achieve in practice. About 75% of nutrients added to catfish ponds in the form of feed is not recovered by fish and therefore released to the pond environment, stimulating luxuriant plant growth. As phytoplankton biomass increases, integrated water column gross oxygen production increases. However, algal turbidity also increases, reducing light penetration through the water column. Beyond an intermediate phytoplankton density (100 /300 mg chlorophyll a l 1), gross primary productivity does not increase because algal turbidity shades the water column. Net primary productivity reaches a maximum at intermediate phytoplankton density because respiration increases as a linear function of phytoplankton density whereas gross primary productivity increases as a curvilinear function of phytoplankton density (Smith and Piedrahita, 1988; Giovannini and Piedrahita, 1994). This important characteristic of eutrophic aquaculture ponds imposes a limit to oxygen production. More importantly, for operational purposes, maintaining an intermediate phytoplankton density is extremely difficult because the supply of plant nutrients derived from feeding is continuous. Practical and cost-effective reduction of plant nutrient supply or phytoplankton biomass in catfish pond aquaculture has not been achieved. Although photosynthetic oxygen production is the major source of oxygen in catfish ponds, reaeration from the surface can be an occasional and important oxygen source. The extent of oxygen diffusion from reaeration of static ponds is a function of wind velocity and duration, and the partial pressure differential of oxygen between water and the atmosphere. A shallow pond depth (B/0.5 m) promotes oxygen diffusion and mixing. A factor interpreted as ‘reaeration’ accounted for 10% of the variation in a data set consisting of daily water quality and meteorological measurements, and records of feed allowance (Taylor, 2003). The importance of the factor was episodic through time, corresponding to the occurrence of wind events in conjunction with low dissolved oxygen concentration. Analysis of data consisting of dissolved oxygen concentration measurements recorded every 15 min in three commercial catfish ponds indicated that reaeration was not an important contributor of oxygen until pond dissolved oxygen concentrations declined to B/2 mg l1 (Hargreaves and Steeby, 1999).
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Threshold minimum dissolved oxygen concentrations are similar for the maintenance of waste assimilation capacity and good fish growth and survival (/2/3 mg l 1). As phytoplankton density increases, the magnitude of fluctuations in dissolved oxygen concentration increases and the duration below critical threshold concentrations correspondingly increases. Maintenance of dissolved oxygen concentrations above this critical threshold is accomplished by mechanical aeration. In catfish ponds, mechanical aeration does not affect dissolved oxygen concentration throughout the pond. Rather, a zone of sufficient dissolved oxygen concentration necessary to meet the minimum physiological requirement for oxygen is maintained. Large areas of the pond may be otherwise uninhabitable at those times. Catfish farmers have developed aeration practices where oxygen transfer rates and the number and positioning of aerators are adequate only to meet the respiratory demands of the fish. Additional aeration would doubtless allow greater waste assimilation rates and, therefore, higher fish stocking and feeding rates. But, as mentioned above, the economic benefits of doing so are unknown.
2.3. Fish water quality tolerance limits Providing ponds with limited mechanical aeration (1 /2 hp acre 1; 1.8 /3.7 kW ha1) allows catfish farmers to feed up to about 100/125 kg feed ha1 day1 with good fish growth and relatively few problems with acute water quality deterioration. If summertime feeding rates exceed those levels, especially for an extended time, feed consumption by fish decreases, presumably because the waste assimilation capacity of the pond is exceeded and water quality deterioration suppresses fish appetite. Oddly, the specific water quality conditions responsible for decreased feeding activity at high waste loading rates (high feeding rates) have not been identified. Catfish have a fairly broad range of water quality tolerance. Similar to other fish species, below certain threshold or no-effect concentrations, response of fish exposed to a toxicant is not statistically different from control fish. Above the no-effect concentration, functional response is directly related to concentration and can be sufficiently impaired to cause mortality at high concentrations (LC50). Experience suggests that most water quality parameters are maintained within tolerance limits of fish by conventional pond management, primarily through aeration. With the exception of dissolved oxygen depletion and temporary accumulation of nitrite, there are few examples of water quality causing acute mortality in catfish ponds. Measurements of water quality in catfish ponds, coupled with information generated by controlled toxicity testing, suggest that water quality is more likely to cause sublethal effects, such as reduced growth and immunocompetence. As catfish ponds are extremely dynamic with respect to temperature, dissolved oxygen and carbon dioxide concentrations, and pH, fish may be exposed to transient toxicity associated with the extremes of diel water quality fluctuations. The effect of exposure to brief periods of toxicity on catfish growth, feed conversion and survival is not well understood. In one study, brief daily exposure to 0.91 mg l 1 NH3-N did not affect channel catfish growth and feed conversion ratio (Hargreaves and Kucuk,
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2001). Evidence from other fish species suggests that catfish may acclimate to transient exposures to water quality extremes. 2.4. Organic matter decomposition Organic matter dynamics in catfish ponds are complex. The sustainable operation of pond aquaculture is defined in large measure by the capacity of the pond to remove organic matter, a rate affected primarily by temperature and oxygen availability. During summer, organic matter loading rates from feeding of catfish ponds are high. Maximum sustainable feeding rates range from 100 to 125 kg ha1 day1, although short-term feeding rates can exceed 175 kg ha1 day1. Interestingly, this roughly corresponds to the design criteria for external BOD5 loading in high-rate aerobic waste stabilization ponds (Shieh and Nguyen, 1999). In catfish ponds and in aerobic wastewater stabilization ponds, autochthonous organic matter loading from photosynthesis in the form of phytoplankton biomass far exceeds intentional addition of organic matter. The amount of organic matter created from photosynthesis has been estimated to range from 2 to 3 times that added to catfish ponds from feeding (Boyd, 1985). Organic matter is decomposed in the water column and at the sediment /water interface. Compared to many lakes, reservoirs and the open ocean, aquaculture ponds are shallow so most organic matter settles to the sediment before it is decomposed. From the standpoint of microbial utilization, this settled organic matter is readily biodegradable because it consists of senescent phytoplankton and detritus of phytoplankton origin. Such high quality organic matter is rapidly and nearly completely mineralized when conditions are favorable. In aerobic wastewater treatment ponds, first order BOD5 removal rates of 0.05 /1.0 day1 (organic matter half-life of 0.7 /14 day) are typical (Metcalf and Eddy, Inc., 1991). Despite high rates of organic matter addition to catfish ponds, the accumulation of sediment organic matter is low (Tucker, 1985). Organic matter concentration in soils of newly constructed catfish ponds increases rapidly within the first 6/12 months and thereafter accumulates more slowly to an equilibrium concentration after 3/5 years (Steeby, 2002). The organic matter that accumulates in sediment consists of lowquality, recalcitrant forms that are resistant to decomposition (e.g. humic and fulvic acids) and which represents a very small fraction of the loaded and created organic matter. Although oxidation of organic matter proceeds most efficiently using oxygen as a terminal electron acceptor, organic matter can also be oxidized anaerobically using alternate electron acceptors. In the large volume of anaerobic sediment, the microbial decomposition of organic matter can be mediated by nitrate, manganicmanganese, ferric-iron, sulfate and carbon dioxide. Organic matter can be fermented to organic acids and carbon dioxide. Organic matter dynamics in the sediment of catfish ponds are poorly understood, in part because sediment organic matter is distributed heterogeneously across the pond bottom. Furthermore, mounting a spatially rigorous sampling effort sufficient to detect relatively small changes in sediment organic matter is logistically difficult.
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However, seasonal patterns in other water quality variables can be used to infer annual trends. In commercial catfish ponds, the availability of oxygen in the overlying water is the most important factor controlling the rate of sediment oxygen uptake during the growing season (Steeby, 2002), most of which is used for organic matter decomposition. Although few measurements of dissolved oxygen concentration near the sediment /water interface of aquaculture ponds have been reported (Losordo, 1991), the limited data suggest that near-bottom waters are chronically undersaturated with respect to dissolved oxygen concentration. Thus, in the absence of sufficient dissolved oxygen to fully mineralize settled organic matter during the summer, organic matter may temporarily accumulate. Additional evidence for the accumulation of organic matter during the growing season can be inferred from the annual pattern of ammonia concentration in catfish ponds. In general, ammonia is minimum during the summer and maximum during the late fall or early winter. Despite reduced feeding rates, ammonia concentration increases during late fall. Ammonia will increase if the rates of source processes (fish excretion, diffusion from the sediment) exceed the rates of sink processes (phytoplankton uptake, nitrification). Indeed, phytoplankton uptake is reduced during the late fall, but the availability of oxygen to support organic matter decomposition is greater than during summer. Therefore, organic matter that may have accumulated during the summer is decomposed during late fall to mid-spring, when feed input is minimal, thereby restoring sediment organic matter concentration to previous levels. This process also explains the long-term (10 /20 year) equilibrium concentration of sediment organic matter. Interactions between bacteria and algae contribute to the complexity of organic matter dynamics in catfish ponds. The relative importance of each of these groups varies, depending upon the rate of organic matter loading, season, and water turbulence. Although some inorganic carbon is fixed into algal biomass, phytoplankton release large quantities of soluble organic matter in the form of simple to complex polysaccharides that are readily utilized by heterotrophic bacteria. Algae also provide oxygen to support oxidative metabolism by bacteria. Bacteria provide algae with carbon dioxide and growth factors (e.g. vitamins) and release nutrients from the mineralization of organic matter. Algae may also compete with bacteria for substrates. For example, phytoplankton will compete with chemoautotrophic nitrifying bacteria for ammonia. Phytoplankton may be more effective competitors for ammonia than nitrifying bacteria when concentrations are low during the summer, but the reverse may be the case when concentrations are higher during the winter (Hargreaves, 1997). The importance of algae and bacteria in ponds has stimulated interest in assessing methods to manipulate the microbial community to favor one or the other group. Microbial activity can temporarily affect water quality in algae-dominated ponds by reducing dissolved oxygen concentration following sediment resuspension associated with wind, seining, or aeration. Some commercial catfish ponds develop a condition where non-algal or mineral turbidity will dominate and persist. Water quality in these ponds has been observed anecdotally to be more stable than algae-dominated ponds. The stability of microbial suspensions has not been well documented, but
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some threshold cell density may be necessary to maintain good water quality in bacteria-dominated ponds. There are some clear trade-offs associated with dominance by algae or bacteria. Phytoplankton are important oxygenators and provide a powerful mechanism for ammonia removal. Ponds dominated by bacteria require an external oxygen source that must be provided by a level of mechanical aeration that far exceeds the natural supply of oxygen in algae-dominated ponds. Although phytoplankton densities are lower in ponds dominated by non-algal turbidity, cyanobacteria, including those that may cause off-flavor, are favored in the dimly lit waters of such ponds. Finally, the quality of effluents discharged from ponds with turbidity dominated by mineral solids is likely to be worse than that from algae-dominated ponds. This discussion of organic matter dynamics raises the question of whether catfish ponds are characterized by net autotrophy or net heterotrophy. In the short term (days), productivity may exceed respiration. In one study in three commercial catfish ponds, average daily net productivity exceeded whole pond respiration by a factor of 1.3 (Hargreaves and Steeby, 1999). However, two lines of evidence point to net heterotrophy. First, respiration from decomposition of added organic matter exceeds gross photosynthesis in aquatic systems with high rates of allochtonous organic matter input, such as ponds used for wastewater treatment or semi-intensive aquaculture. Boyd (1985) measured greater respiration rates than production by photosynthesis in experimental catfish ponds. Inspection of seasonal trends in total alkalinity in catfish ponds provides additional inferential evidence of net heterotrophy. Alkalinity in catfish ponds declines during spring, suggesting a period of net autotrophy. However, alkalinity generally increases during the production season because more carbon dioxide is released during respiration than is fixed by photosynthesis. A net carbon dioxide release increases the dissolution of calcium carbonate in pond soils, thereby increasing alkalinity. Additional research is required to validate this mechanism because seasonal rainfall, evaporation, and groundwater pumping patterns may also contribute to an explanation of the observed seasonal trends in alkalinity. In summary, both aerobic and anaerobic microbial processes in catfish ponds oxidize large quantities of organic matter. Large-scale, commercial catfish aquaculture in ponds would not be possible without this high inherent capacity to oxidize organic solids generated by fish excretion and primary production. Photosynthesis provides the majority of oxygen used to decompose organic matter as a ‘free’ ecological service. 2.5. Nutrient removal Static hypereutrophic ponds have a high capacity to remove inorganic nitrogen and phosphorus. Ammonia is the inorganic nitrogen form of primary concern and fish excretion and sediment diffusion are the two principal sources. Phytoplankton uptake and nitrification are the major sinks for ammonia. Nitrogen is removed from ponds through mineralization of organic matter followed by coupled nitrification / denitrification. Some ammonia (as NH3) will volatilize to the atmosphere under
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warm, windy, high-pH conditions and a relatively small proportion of nitrogen accumulates in the sediment as recalcitrant organic matter. The sustained feeding rate at which total ammonia begins to rapidly accumulate can be used as an initial estimate of the pond’s capacity to assimilate nitrogen under summertime conditions. In one study (Cole and Boyd, 1986), that point occurred when the maximum feeding rate was 84 kg ha1 day1, which corresponds to a nitrogen loading of 430 mg N m 2 day1 for the 32% crude protein feed used in the study. Maximum feeding rates of 100 /150 kg ha1 day1 correspond to nitrogen loading rates of 500/750 mg N m 2 day1. In systems managed for autotrophic production, nitrogen loading rates of 700/800 mg N m 2 day1 appear to represent the upper limit to N loading of fish ponds (Schroeder et al., 1990; Knud-Hansen et al., 1991). The capacity of aquatic sediments to adsorb phosphorus is dependent on clay content. Most ponds are deliberately constructed in clay soils that retain water, so the phosphorus adsorption capacity of pond soils is very high. Inorganic phosphorus is removed from pond water by precipitation /dissolution reactions, which are controlled by pH, with aluminum controlling solubility at low pH and calcium controlling solubility at high pH. Sediment can also be a source of inorganic phosphorus. Under reducing conditions, reducible iron-bound phosphorus is soluble. In the absence of an oxidized barrier to diffusion at the sediment /water interface, phosphorus can diffuse into the overlying water. Low dissolved oxygen concentration at the pond bottom can enhance phosphorus diffusion from the sediment. Sediment phosphorus diffusion may be involved in a positive feedback mechanism related to the effect of inorganic phosphorus on phytoplankton density. Increased phytoplankton density increases the magnitude of the fluctuation of dissolved oxygen concentration, thereby increasing the duration of low dissolved oxygen concentration. Low dissolved oxygen concentration at the pond bottom promotes further diffusion of reductant-soluble phosphorus. This mechanism reinforces the need to maintain oxidized conditions at the sediment /water interface as a means to limit eutrophication. Additional research is required to confirm and evaluate the importance of this mechanism in eutrophic aquaculture ponds.
3. Characteristics of hypertrophic ponds amenable to engineering solutions The overwhelming majority of global aquaculture production of finfish occurs in earthen ponds. Therefore, it is surprising that more aquacultural engineers have not embraced the challenges associated with extending loading limits or waste assimilation capacity and improving the efficiency of fish culture in static ponds. There appears to be an inverse relationship between the interest in a particular culture system by aquacultural engineers and the aggregate fish biomass produced from those culture systems. Aquacultural engineers generally favor a reductionist approach, where fish culture consists of a system of integrated, but individual unit processes, the manipulation of which results in improved control over the production
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process. The main determinant of intensive system design is fish metabolism. In contrast, the complexities associated with highly dynamic ecosystems such as hypertrophic aquaculture ponds require a holistic, integrative approach where there is an appreciation and understanding that management interventions have multiple impacts. The dynamics of the pond ecosystem, especially process mediated by phytoplankton and bacteria, are more important than fish metabolism as determinants of water quality, loading limits, and waste assimilation capacity. Nonetheless, a broad range of engineering domains can contribute to solutions of problems associated with the inherent constraints and limitations of the static pond system. Some of the more critical and salient characteristics of hypertrophic aquaculture ponds that are amenable to engineering solutions are presented here. Many of the problems associated with conventional catfish pond aquaculture are associated with a lack of control over pond processes within the constraints of current pond configuration, operation and management. Additional problems can be attributed to management near the limits of the pond to assimilate wastes, leaving little margin for variation in system performance. Given the dynamic nature of ponds, these limits change over a range of spatial and temporal scales. 3.1. Excessive phytoplankton biomass Maximizing net oxygen production from photosynthesis requires the maintenance of an intermediate phytoplankton biomass. This objective is difficult to accomplish in practice because the release of plant nutrients forces an increase in phytoplankton biomass to the point where standing crop is limited by availability of light. Control of phytoplankton biomass is an elusive goal of pond culturists because of the continual input of nutrients derived from feeding. To a limited extent, phytoplankton biomass can be constrained by limiting nutrient supply, perhaps through chemical precipitation. Any techniques for harvesting cells, either mechanically or by using plankton-feeding fish, must be practical and efficient. 3.2. Dominance of phytoplankton communities by cyanobacteria Hypertrophic aquaculture ponds develop a number of characteristic conditions that provide selection pressures that promote the dominance of cyanobacteria. These include warm water temperature, a dim underwater light climate, calm poorly-mixed conditions, high total phosphorus concentration, and low carbon dioxide concentration. Engineering solutions that seek to alter phytoplankton community composition must reduce these selection pressures and also consider the numerous attributes that favor the dominance of cyanobacteria in hypertrophic aquaculture ponds. 3.3. Intense thermal and chemical stratification Algal turbidity limits the penetration of light, leading to elevated water temperature and photosynthetic oxygen production near the pond surface and
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lower water temperature and net oxygen consumption near the pond bottom. The impact of stratification on fish production is not known, but many fish species are known to seek areas of preferred water quality, at times forced into zones that restrict access to the full water column. For example, surface water temperature may increase to near-lethal levels during calm summer days and affect the voluntary consumption of floating feed. Stratification limits overall photosynthetic oxygen production to surface waters. Even in shallow (1.5-m deep) ponds, more than half the water column can be a net oxygen consumer. Szyper and Lin (1990) described an index of stratification intensity for eutrophic aquaculture ponds and concluded that the energy required to equalize the vertical density gradient in a stratified pond is low. However, this energy is diffuse in space, indicating a daunting engineering challenge. Thus, application of high-intensity, point sources of turbulence will not be as efficient as devices that produce a diffuse and uniform flow field. 3.4. Chronic dissolved oxygen undersaturation at the sediment /water interface Access to the pond bottom by culture species with a benthic orientation may be restricted by chemical (e.g. oxygen) stratification. In addition, the rate of aerobic decomposition of organic matter will be limited by the availability of oxygen, thereby affecting the waste assimilative capacity of the pond ecosystem. In bottom waters that are chronically undersaturated, any accumulation of organic matter represents a latent oxygen demand that will be exerted if sediments are resuspended or if oxygen concentration increases. Restricted decomposition of organic matter will limit nutrient regeneration. In addition, the thin oxic sediment /water interface that normally serves as a barrier to the diffusion of reduced substances with potential toxicity to culture animals (e.g. ammonia, sulfide) will be absent. 3.5. Limitations of current aeration technology Electric paddlewheel aeration is now standard technology in commercial catfish ponds, which are aerated at 1/2 hp acre 1 (1.8 /3.7 kW ha1). In large commercial catfish ponds, paddlewheel aeration cannot effect an increase in dissolved oxygen concentration throughout the pond. Rather, paddlewheel aeration produces zones of sufficient dissolved oxygen to maintain the fish standing crop. Paddlwheel aeration is not designed to meet the overall pond respiratory demand, but rather provides sufficient supplemental oxygen to satisfy fish respiratory demand when pond oxygen concentration declines to some critical threshold, usually 2 /3 mg l 1. At this threshold, limited oxygen is available to support plankton respiration and sediment organic matter decomposition. The standard oxygen transfer rate of paddlewheel aerators is high because the aerator produces vigorous turbulence as it moves a fairly large volume of water. Fish gathered in front of aerators are forced to swim in a strong water current, increasing respiratory demand simultaneous with the occurrence of low dissolved oxygen concentrations. A logical solution is the development of efficient aerators without the by-product of excessive water current.
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Creative approaches to aeration of large ponds are also needed. Ponds are aerated for approximately 1000 h during a typical production season of 210 /240 days. Thus, catfish farmers provide an annual direct input of electricity in the form of paddlewheel aeration of 1000/2000 hp-hr acre 1 (1843 /3685 kW-hr ha1). Assuming that catfish farmers are willing to continue to provide this level of electrical input, is paddlewheel aeration the most efficient application of this electrical input level? Will other devices or combinations of devices yield gains in input efficiency? Although results from the fair amount of research with pond mixers has been equivocal with respect to improvements in water quality, total energy use and fish yield, additional research on pond mixing is needed to firmly establish the value of this management technique.
4. Implementing an efficiency-based paradigm for catfish pond aquaculture As with any successful agricultural industry, potential profit is a primary motive for entry into catfish farming. Some catfish farmers have assumed that increased production and profits are synonymous. This way of thinking has been influenced by research intended to assess techniques for maximizing yield. For much of the brief history of catfish farming, temporary limiting factors periodically arose, but these were overcome by some technological innovation or new production practice. This limiting factor cycle */of encountering and overcoming a limiting factor, followed by the emergence of a new limiting factor*/is not unique to catfish pond aquaculture. Nonetheless, ultimate limits to static pond aquaculture as currently practiced have apparently emerged. Any changes to overcome limiting factors are severely constrained by current industry-standard practices related to pond size, fish stocking and feeding rates, water quality management, and stock management schemes. An efficiency-based paradigm for catfish pond aquaculture seeks to maximize profits by optimizing resource use, but this does not necessarily maximize production. Thus, the appropriate measure is one of efficiency (output per unit input) rather than production or yield (output per unit area or volume). As an example, economic analysis indicates that profit-maximizing catfish stocking densities (16 942/21 312 ha1) are lower than the yield-maximizing stocking density (30 000 ha1) (Losinger et al., 2000). This analysis further indicated that stocking rates and feeding rates interact to affect profitability. A separate analysis that included feed price (in addition to feeding rate and stocking density) indicated that profit is maximized at lower stocking rates when feed prices are high (Dasgupta et al., 2002). Conversely, profit is maximized at higher stocking rates when feed prices are low. Empirical evaluation of the interaction between stocking density and feeding rate through controlled experiments would be useful to validate the results obtained by modeling. Although catfish producers have long been aware of management techniques that improve efficiency of inputs, broad-scale implementation is lacking. Implementation of efficiency-based measures of performance can also profoundly impact feed utilization efficiency, water quality and pond effluents. The combined expertise of
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aquacultural engineers, biologists and economists should be brought to bear on a systematic evaluation of creative means to improve production efficiency. The focus of outreach programs should continue to emphasize increased efficiency.
Acknowledgements Approved for publication as Journal Article J-10180 of the Mississippi Agricultural and Forestry Experiment Station, Mississippi State University.
References Boyd, C.E., 1985. Chemical budgets for channel catfish ponds. Trans. Am. Fish. Soc. 114, 291 /298. Cole, B.A., Boyd, C.E., 1986. Feeding rate, water quality, and channel catfish production in ponds. Prog. Fish-Cult. 48, 25 /29. Dasgupta, S., Monestime, D., Engle, C., 2002. Can commercial catfish producers always feed efficiently? Aquacult. Econ. Manage. 6, 55 /64. Davis, K.B., Goudie, C.A., Simco, B.A., MacGregor, R., Parker, N.C., 1986. Environmental regulation and influence of the eyes and pineal gland on the gonadal cycle and spawning in channel catfish (Ictalurus punctatus ). Physiolog. Zool. 59, 717 /724. Engle, C.R., Pounds, G.L., van der Ploeg, M., 1995. The cost of off-flavor. J. World Aquacult. Soc. 26, 297 /306. Foster, T.H., Waldrop, T.E., 1972. Cost-size relationships in the production of pond-raised catfish for food. Bulletin 792, Mississippi Agricultural and Forestry Experiment Station, Mississippi State University, Mississippi. Giovannini, P., Piedrahita, R.H., 1994. Modeling photosynthetic production optimization for aquaculture ponds. Aquacult. Eng. 13, 83 /100. Hargreaves, J.A., 1997. A simulation model of ammonia dynamics in commercial catfish ponds in the southeastern United States. Aquacult. Eng. 16, 27 /43. Hargreaves, J.A., Kucuk, S., 2001. Effects of diel un-ionized ammonia fluctuation on juvenile hybrid striped bass, channel catfish and blue tilapia. Aquaculture 195, 163 /181. Hargreaves, J.A., Steeby, J.A., 1999. Factors affecting metabolism of commercial channel catfish ponds as indicated by continuous dissolved oxygen measurement. J. World Aquacult. Soc. 30, 410 /421. Heikes, D., 1996. Catfish yield verification trials. Final report. May 1993 /December 1996. Arkansas Cooperative Extension Program, University of Arkansas at Pine Bluff, Pine Bluff, Arkansas. Knud-Hansen, C.F., McNabb, C.D., Batterson, T.R., 1991. Application of limnology for efficient nutrient utilization in tropical pond aquaculture. Verh. Internat. Verein. Limnol. 24, 2541 /2543. Losinger, W., Dasgupta, S., Engle, C., Wagner, B., 2000. Economic interactions between feeding rates and stocking densities in intensive catfish Ictalurus punctatus production. J. World Aquacult. Soc. 31, 491 / 502. Losordo, T.M., 1991. Stratification and mixing in shallow aquaculture ponds. In: Brune, D.E., Tomasso, J.R. (Eds.), Aquaculture and Water Quality. Advances in World Aquaculture, vol. 3. World Aquaculture Society, Baton Rouge, Louisiana, pp. 396 /435. Metcalf and Eddy, Inc., 1991. Wastewater Engineering: Treatment, Disposal, and Reuse. third ed. McGraw Hill, New York, NY. Schroeder, G.L., Wohlfarth, G., Alkon, A., Halevy, A., Krueger, H., 1990. The dominance of algal-based food webs in fish ponds receiving chemical fertilizers plus organic manures. Aquaculture 86, 219 /229. Shieh, W.K., Nguyen, V.T., 1999. Ponds and lagoons. In: Liu, D.H.F., Liptak, B.G. (Eds.), Wastewater Treatment. Lewis Publishers, Boca Raton, Florida, pp. 206 /211.
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Smith, D.W., Piedrahita, R.H., 1988. The relation between phytoplankton and dissolved oxygen in fish ponds. Aquaculture 68, 249 /265. Steeby, J.A., 2002. Sediment accumulation, organic carbon content, and oxygen demand in commercial channel catfish (Ictalurus punctatus ) ponds. Ph.D. Dissertation. Mississippi State University, Mississippi State, MS. Szyper, J.P., Lin, C.K., 1990. Techniques for assessment of stratification and effects of mechanical mixing in tropical fish ponds. Aquacult. Eng. 9, 151 /165. Taylor, J.B., 2003. Modeling the interrelationships among feeding activity, water quality, and climatology in warmwater fish culture ponds. Ph.D. Dissertation. Mississippi State University, Mississippi State, MS. Teichert-Coddington, D., Green, B., 1993. Comparison of two techniques for determining community respiration in tropical fish ponds. Aquaculture 114, 41 /50. Tucker, C.S., 1985. Organic matter, nitrogen, and phosphorus content of sediments from channel catfish ponds. Research Report 10-7, Mississippi Agricultural and Forestry Experiment Station, Mississippi State University, Mississippi.
Defining loading limits of static ponds for catfish aquaculture John A. Hargreaves a,1,*, Craig S. Tucker b a
Department of Wildlife and Fisheries, Mississippi State University, Box 9690, Mississippi State, MS 39762-9690, USA b Thad Cochran National Warmwater Aquaculture Center, Mississippi State University, P.O. Box 197, Stoneville, MS 38776, USA
Abstract Commercial channel catfish farming has emerged as the most important aquaculture industry in the United States. During the last two decades, industry growth has occurred by expansion in the number and area of facilities and through production intensification. Evidence suggests that catfish farming has apparently reached the limits of the production system as currently configured. The success of commercial catfish culture can be attributed in part to low production costs resulting from the inherent waste assimilation capacity of aquaculture ponds, although operating within this capacity is complex and associated with several poorly defined limitations and hidden costs. Loading limits for pond aquaculture are based on the waste assimilation capacity of ponds and tolerance limits of the cultured species. The important design and operational considerations affecting loading limits include temperature effects, oxygen requirements, fish water quality tolerance limits, organic matter decomposition, and nutrient removal. Engineering solutions for extending the loading limits of pond aquaculture must account for the highly dynamic and complex nature of the pond ecosystem, particularly processes related to phytoplankton and microbial dynamics. Characteristics of hypertrophic ponds amenable to engineering solutions include excessive phytoplankton biomass, dominance of phytoplankton communities by cyanobacteria, intense diurnal stratification, chronic undersaturation of dissolved oxygen at the sediment /water interface, and limitations of current aeration technology. Improvements in the profitability of
* Corresponding author. E-mail address: [email protected] (J.A. Hargreaves). 1 Current Address: Aquaculture Research Station, Louisiana State University, 2410 Ben Hur Road, Baton Rouge, LA 70820, USA. 0144-8609/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0144-8609(03)00023-2
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catfish pond aquaculture requires broad-scale implementation of a production paradigm based on measures of performance efficiency. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Channel catfish; Waste assimilation capacity; Pond aquaculture
1. Introduction Commercial farming of channel catfish in earthen ponds has emerged as the most important component of aquaculture in the United States. Although catfish farming began in the late 1940s, significant production levels were not achieved until the 1970s, and it was not until the 1980s that the industry grew rapidly and attained the currently prevalent production levels. Production practices vary greatly from farm to farm, due in part to different production goals among farms, but also to the paucity of research information on the economics of various production schemes. Recent weakness in domestic and world economies and competition in the global seafood market have also affected farm profitability. This context may catalyze significant changes in production practices as farmers seek more efficient strategies in the face of a new set of economic pressures. Notwithstanding differences in specific culture practices, three general features are common to most catfish farming strategies and broadly characterize the technical basis of the industry: the use of relatively large ponds, high fish stocking and feeding rates, and multiple-batch fish cropping. During the early years of the industry, pond size was highly variable but on average was large, with some ponds over 40 acres (16.2 ha). In one of the first economic studies conducted for the catfish industry, Foster and Waldrop (1972) examined the relationship between pond size and production costs. Although it vastly oversimplified the issue by focusing almost exclusively on construction cost as an economic variable, the study had a major impact on the catfish industry. Average pond size quickly decreased to about 18 acres (7.3 ha), which was the standard for many years. Farmers have since discovered that feeding, disease management, water quality management and many other common activities are easier and more effective in smaller ponds, and average pond size has continued to decrease. Most of the ponds constructed since 1995 have been between 10 and 12 acres (4 /5 ha). In the early 1970s, fish stocking densities ranged from 1000 to 2000 acre1 (2471 / 4942 ha1) and maximum daily feeding rates were restricted to 35 lb acre 1 (39 kg ha1). These limits were determined empirically by farmers who, lacking convenient and effective aeration devices, tried to limit culture intensity at a level where few problems with dissolved oxygen depletion would be encountered. Through the 1970s, stocking density gradually increased to 4000 acre 1 (9884 ha1) and episodes of critically low dissolved oxygen concentrations became more frequent. At first, when oxygen problems were still relatively rare, producers used aerators fabricated in on-farm machine shops and connected to the power take-off unit of a tractor, which was a readily available power source on farms. Tractor-powered aerators were mobile and could be moved around the farm depending on which pond was in the
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greatest need of emergency aeration. In the early 1980s, farmers continued to increase stocking densities and feeding rates, and the need for aeration increased to the point where most ponds on a farm needed aeration every summer night. Tractors were too expensive and difficult to maintain for routine, everyday use as a power source for aeration. Floating paddlewheel aerators powered by electric motors were much more efficient, and by the mid-1980s had become the most common aerator on catfish farms. A third characteristic of current catfish production is the widespread use of a unique fish cropping system wherein faster-growing fish are selectively harvested using a large-mesh seine and fingerlings or larger stockers are added (‘understocked’) to replace the harvested fish. The process of selective harvest and understocking may continue for many years without draining the pond. The multiple-batch cropping system was originally developed in the 1970s to provide a year-round supply of market-sized fish. The catfish industry has, however, become large enough to supply fish all year with single cropping in individual ponds, but the multiple-batch system remains popular because economic risks associated with development of algaerelated off-flavors are reduced. That is, if timely harvest of fish from a particular pond is constrained by the presence of off-flavors (or other factors, such as ongoing losses from an infectious disease), there is a greater probability of having acceptable fish to sell from another pond when all ponds on a farm are managed with the multiple-batch cropping system. Another benefit of the multiple-batch cropping system is that ponds can be operated continuously for many years without draining, which dramatically reduces pond effluent volume and the need for pumped water to refill ponds compared to ponds managed with the single-batch cropping system. The technical development of catfish farming in static ponds as described above has been characterized by increasing intensification. However, catfish farming has developed to the point where yield limits of the production system as currently configured and managed have apparently been reached. In the 1990s, annual production ranged from 5000 to 6000 lb acre 1 (5600 /6700 kg ha1) (Heikes, 1996) despite a fairly wide range of stocking densities (6000 /10000 acre 1; 14 826/24 710 ha1). Losses from infectious diseases, some of which are indicative of elevated stress associated with culture conditions, continue to plague catfish producers. Excessive eutrophication of catfish ponds caused by high waste loading rates often lead to the dominance of phytoplankton communities by blue /green algae, some of which may be toxic or impart off-flavors to cultured fish. Despite the emergence of yield limits, catfish farming during the last 2 to 3 decades has been generally profitable in most years. In part, the profitability of catfish farming can be related to the inherent waste treatment capacity of static ponds. Unlike recirculating and other intensive aquaculture production systems, there are few direct costs (e.g. supplemental aeration) associated with waste treatment because the pond ecosystem provides natural biological, chemical and physical waste treatment processes that appear, at first glance, to be free. However, when examined in a larger context, the waste treatment capabilities of ponds are not really a free service provided by natural ecological processes. Farmers do, in fact, pay a price for waste treatment in ponds because the system only
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functions effectively within certain operational limits based on loading rates for organic matter, nitrogen, and plant nutrients. Specific loading limits are ill-defined, but the implication for catfish culture in ponds is that rather large ponds are required to grow a certain biomass of fish. The other cost of allowing nature to operate unrestrained in response to high fish stocking densities and feeding rates is the low level of management control that farmers can exert over certain environmental variables such as off-flavor, infectious diseases, temperature, and dissolved oxygen. As a specific example, the various impacts associated with sporadic episodes of off-flavor in pond-raised catfish add about $US0.05 kg1 to the cost of production (Engle et al., 1995). The presence of off-flavor does not limit yield directly, but additional costs are incurred because harvest must be delayed until flavor quality is acceptable and feeding must continue to maintain fish weight. During the time that harvest is delayed, fish are also at increased risk of loss to disease and predation. Many farmers do not attempt to manage off-flavor problems, and simply assimilate the cost of off-flavor as part of the overall cost of growing catfish in ponds. Using two examples at the extremes of fish culture intensity, farmers are faced with a fundamental economic and engineering choice of using either small (in terms of land requirements), high-intensity fish culture systems that provide a high degree of control over most environmental variables or using pond systems with large land requirements and many hidden costs associated with the inability to control environmental conditions. At present, the latter appears to be the rational economic decision, at least for channel catfish. However, in the face of changing economic forcing factors, it may become necessary to more closely examine the basic pond culture paradigm that has developed for catfish and investigate economically feasible opportunities for better control of pond environments. Here we focus on the biological and physical factors that affect the loading limits and waste assimilation capacity of static ponds used for commercial catfish aquaculture. Loading limits are a function of the interaction between the biology of channel catfish, climate, and ecological characteristics of static ponds. Importantly, static pond production systems are also limited by farm management, business, and macroeconomic considerations (e.g. processing, marketing, farm financing, global seafood trade policy), but these are beyond the scope of this review. As the limits of currently configured static ponds are approached, some characteristics of ponds resulting from operation near loading limits and amenable to engineering solutions are identified. Creative, practical, and cost-effective solutions to the problems posed by these characteristics have the potential to increase the efficiency and profitability of catfish farming in static ponds.
2. Factors affecting loading limits in catfish pond aquaculture The technical success of catfish pond aquaculture in static ponds is based on the premise that natural physical, chemical, and biological processes within the pond assimilate or remove wastes that are generated as a result of nutrient input from
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feeds. A relative minority (15 /25%) of nutrients and organic matter contained in feed is retained by fish. Therefore, the majority of nutrients and organic matter added in feed is released to the pond environment. Natural process rates must be comparable to waste loading rates or water quality will deteriorate. If the concentration of water quality variables exceeds the tolerance limits of fish, stress can lead to reduced fish growth, immunocompetence, and survival. Thus, loading limits for ponds are based on two simultaneous sets of criteria. First, analogous to design criteria for aerobic waste treatment lagoons, process designs can be based on the assimilative capacity of the ponds, which, in turn, are based on nutrient and organic matter loading rates and hydraulic retention times. The second set of criteria consists of the tolerance limits of the cultured species. Despite several decades of research on pond dynamics, loading limits and the waste assimilation capacity of static ponds are not particularly well defined. In fact, definitions of maximum loading rates are at best approximations derived empirically from the practical experience of fish farmers, rather than through controlled studies. Biological and physical design and operational considerations for pond aquaculture include temperature effects, oxygen requirements, fish water quality tolerance limits, organic matter decomposition, and nutrient removal.
2.1. Temperature effects Water temperature exerts profound effects on fish metabolism and therefore feed consumption, growth and production. Temperature, particularly temperature extremes, can act as a stressor, thereby affecting immunocompetence and disease incidence. Temperature influences water quality by affecting the kinetics of chemical reactions, nutrient uptake by phytoplankton, and microbial growth. As temperature increases from some minimum at which biological process rates are minimum, rates increase according to the van’t Hoff relationship, i.e. the rate doubles for every 10 8C increase in temperature, to some temperature at which process rates are maximum. The range of temperature at which rates are maximum is characteristic of the reaction. For example, the temperature range for optimum channel catfish growth is 28/30 8C. Beyond this optimum temperature range, process rates will decline. Most channel catfish are grown in north Mississippi, north Alabama, and southern Arkansas where water temperatures vary considerably from summer to winter. In that region, catfish have a ‘growing season’ of about 200 days with average pond water temperatures above 20 8C. For the remainder of the year, fish grow little, if at all, and are essentially held in inventory over the winter. Although faster growth and greater fish production are possible in regions with a year-round growing season, the effect of water temperature on fish reproduction may limit the extent to which channel catfish can be cultured in tropical areas. Channel catfish evolved in a temperate climate and seasonal changes in water temperature exert primary control over the reproductive cycle (Davis et al., 1986). Thus, large-scale fingerling production relying on natural spawning of brood fish held in ponds is not possible in regions without relatively long, cool winters.
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As a practical matter, control of water temperature in large ponds is not possible. However, knowledge of water temperature is important as a determinant of feeding rate based upon (1) the quantity of feed that fish will consume and (2) the quantity of waste organic matter and nutrients that the pond can assimilate. Further research is necessary to better define feeding rate and waste assimilation capacity as a function of temperature. Research to date has focused on definition of a maximum waste assimilation capacity. This capacity is primarily a function of water temperature, but is also affected by other factors such as solar radiation, dissolved oxygen concentration, and water column mixing. The dynamic nature of the waste assimilation capacity of aquaculture ponds limits any definition to a generalized set of operational conditions. As fish are poikilothermic, feed consumption and growth are closely related to seasonal variation in temperature. A study of catfish ponds was conducted where daily measurements of water quality and meteorological variables and feeding rate were made (Taylor, 2003). Data reduction by factor analysis allowed inference of the relative importance of the processes controlling variation in water quality and feeding rate. Most of the variation in feed consumption during spring and fall was explained by variation in water temperature. Variation in feed consumption during summer was not related to variation in water temperature because water temperature was near optimum. (Rather, restricted access to pond levees during inclement weather was the most important factor affecting feed consumption during summer.) Water temperature was also the most important variable related to dissolved oxygen concentration at dawn. Counter-intuitively, the relationship between feeding rate and dissolved oxygen at dawn was weak. Water temperature affects dissolved oxygen concentration in two ways: (1) the solubility of dissolved oxygen is inversely related to water temperature and (2) respiration rates are directly related to water temperature. Therefore, solubility is minimum and respiratory oxygen demand is maximum during summer. 2.2. Oxygen requirements Supplying sufficient dissolved oxygen is a key operational consideration for pond aquaculture. There are two aspects to oxygen supply: (1) providing a sufficient quantity of oxygen to fulfill overall respiratory demand of the pond biota and (2) maintaining dissolved oxygen concentration above a minimum threshold for the cultured fish. Providing adequate oxygen to maintain aerobic conditions throughout the pond at all times will maximize the waste treatment function of ponds, in much the same way that maintaining aerobic conditions in waste treatment lagoons enhances waste treatment capacity over that provided in stabilization ponds operated without aeration. However, the economic benefit of providing sufficient oxygen to meet the total respiratory demand of the pond ecosystem is unknown and may not be justified. The cost of meeting overall oxygen demand is high because total oxygen demand is much greater than the oxygen requirements of the fish standing crop. For example, in a typical commercial catfish pond, the total oxygen consumed in respiration by
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fish, plankton, and sediment during summertime may exceed 15 kg O2 ha 1 h1 (1.5 mg l 1 h 1), the bulk of which is consumed by phytoplankton. In a study of commercial catfish ponds, whole pond respiration was partitioned among the water column (65%), sediment (20%), and fish (15%) (Steeby, 2002). Similarly, in two tropical fish ponds, water column respiration accounted for an average of 68% (range: 53 /76%) of whole pond respiration (Teichert-Coddington and Green, 1993). Thus, the catfish crop consumes only about 10 /20% of the total and phytoplankton respiration is the major sink for dissolved oxygen in ponds. Oxygen is supplied to ponds through photosynthesis, reaeration, and mechanical aeration. Photosynthesis supplies most of the oxygen to meet respiratory demand, although most of the oxygen produced in photosynthesis is consumed by phytoplankton respiration. Maximizing oxygen production by phytoplankton is one design objective of aerobic wastewater treatment ponds. Control of phytoplankton density to maximize oxygen production is difficult to achieve in practice. About 75% of nutrients added to catfish ponds in the form of feed is not recovered by fish and therefore released to the pond environment, stimulating luxuriant plant growth. As phytoplankton biomass increases, integrated water column gross oxygen production increases. However, algal turbidity also increases, reducing light penetration through the water column. Beyond an intermediate phytoplankton density (100 /300 mg chlorophyll a l 1), gross primary productivity does not increase because algal turbidity shades the water column. Net primary productivity reaches a maximum at intermediate phytoplankton density because respiration increases as a linear function of phytoplankton density whereas gross primary productivity increases as a curvilinear function of phytoplankton density (Smith and Piedrahita, 1988; Giovannini and Piedrahita, 1994). This important characteristic of eutrophic aquaculture ponds imposes a limit to oxygen production. More importantly, for operational purposes, maintaining an intermediate phytoplankton density is extremely difficult because the supply of plant nutrients derived from feeding is continuous. Practical and cost-effective reduction of plant nutrient supply or phytoplankton biomass in catfish pond aquaculture has not been achieved. Although photosynthetic oxygen production is the major source of oxygen in catfish ponds, reaeration from the surface can be an occasional and important oxygen source. The extent of oxygen diffusion from reaeration of static ponds is a function of wind velocity and duration, and the partial pressure differential of oxygen between water and the atmosphere. A shallow pond depth (B/0.5 m) promotes oxygen diffusion and mixing. A factor interpreted as ‘reaeration’ accounted for 10% of the variation in a data set consisting of daily water quality and meteorological measurements, and records of feed allowance (Taylor, 2003). The importance of the factor was episodic through time, corresponding to the occurrence of wind events in conjunction with low dissolved oxygen concentration. Analysis of data consisting of dissolved oxygen concentration measurements recorded every 15 min in three commercial catfish ponds indicated that reaeration was not an important contributor of oxygen until pond dissolved oxygen concentrations declined to B/2 mg l1 (Hargreaves and Steeby, 1999).
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Threshold minimum dissolved oxygen concentrations are similar for the maintenance of waste assimilation capacity and good fish growth and survival (/2/3 mg l 1). As phytoplankton density increases, the magnitude of fluctuations in dissolved oxygen concentration increases and the duration below critical threshold concentrations correspondingly increases. Maintenance of dissolved oxygen concentrations above this critical threshold is accomplished by mechanical aeration. In catfish ponds, mechanical aeration does not affect dissolved oxygen concentration throughout the pond. Rather, a zone of sufficient dissolved oxygen concentration necessary to meet the minimum physiological requirement for oxygen is maintained. Large areas of the pond may be otherwise uninhabitable at those times. Catfish farmers have developed aeration practices where oxygen transfer rates and the number and positioning of aerators are adequate only to meet the respiratory demands of the fish. Additional aeration would doubtless allow greater waste assimilation rates and, therefore, higher fish stocking and feeding rates. But, as mentioned above, the economic benefits of doing so are unknown.
2.3. Fish water quality tolerance limits Providing ponds with limited mechanical aeration (1 /2 hp acre 1; 1.8 /3.7 kW ha1) allows catfish farmers to feed up to about 100/125 kg feed ha1 day1 with good fish growth and relatively few problems with acute water quality deterioration. If summertime feeding rates exceed those levels, especially for an extended time, feed consumption by fish decreases, presumably because the waste assimilation capacity of the pond is exceeded and water quality deterioration suppresses fish appetite. Oddly, the specific water quality conditions responsible for decreased feeding activity at high waste loading rates (high feeding rates) have not been identified. Catfish have a fairly broad range of water quality tolerance. Similar to other fish species, below certain threshold or no-effect concentrations, response of fish exposed to a toxicant is not statistically different from control fish. Above the no-effect concentration, functional response is directly related to concentration and can be sufficiently impaired to cause mortality at high concentrations (LC50). Experience suggests that most water quality parameters are maintained within tolerance limits of fish by conventional pond management, primarily through aeration. With the exception of dissolved oxygen depletion and temporary accumulation of nitrite, there are few examples of water quality causing acute mortality in catfish ponds. Measurements of water quality in catfish ponds, coupled with information generated by controlled toxicity testing, suggest that water quality is more likely to cause sublethal effects, such as reduced growth and immunocompetence. As catfish ponds are extremely dynamic with respect to temperature, dissolved oxygen and carbon dioxide concentrations, and pH, fish may be exposed to transient toxicity associated with the extremes of diel water quality fluctuations. The effect of exposure to brief periods of toxicity on catfish growth, feed conversion and survival is not well understood. In one study, brief daily exposure to 0.91 mg l 1 NH3-N did not affect channel catfish growth and feed conversion ratio (Hargreaves and Kucuk,
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2001). Evidence from other fish species suggests that catfish may acclimate to transient exposures to water quality extremes. 2.4. Organic matter decomposition Organic matter dynamics in catfish ponds are complex. The sustainable operation of pond aquaculture is defined in large measure by the capacity of the pond to remove organic matter, a rate affected primarily by temperature and oxygen availability. During summer, organic matter loading rates from feeding of catfish ponds are high. Maximum sustainable feeding rates range from 100 to 125 kg ha1 day1, although short-term feeding rates can exceed 175 kg ha1 day1. Interestingly, this roughly corresponds to the design criteria for external BOD5 loading in high-rate aerobic waste stabilization ponds (Shieh and Nguyen, 1999). In catfish ponds and in aerobic wastewater stabilization ponds, autochthonous organic matter loading from photosynthesis in the form of phytoplankton biomass far exceeds intentional addition of organic matter. The amount of organic matter created from photosynthesis has been estimated to range from 2 to 3 times that added to catfish ponds from feeding (Boyd, 1985). Organic matter is decomposed in the water column and at the sediment /water interface. Compared to many lakes, reservoirs and the open ocean, aquaculture ponds are shallow so most organic matter settles to the sediment before it is decomposed. From the standpoint of microbial utilization, this settled organic matter is readily biodegradable because it consists of senescent phytoplankton and detritus of phytoplankton origin. Such high quality organic matter is rapidly and nearly completely mineralized when conditions are favorable. In aerobic wastewater treatment ponds, first order BOD5 removal rates of 0.05 /1.0 day1 (organic matter half-life of 0.7 /14 day) are typical (Metcalf and Eddy, Inc., 1991). Despite high rates of organic matter addition to catfish ponds, the accumulation of sediment organic matter is low (Tucker, 1985). Organic matter concentration in soils of newly constructed catfish ponds increases rapidly within the first 6/12 months and thereafter accumulates more slowly to an equilibrium concentration after 3/5 years (Steeby, 2002). The organic matter that accumulates in sediment consists of lowquality, recalcitrant forms that are resistant to decomposition (e.g. humic and fulvic acids) and which represents a very small fraction of the loaded and created organic matter. Although oxidation of organic matter proceeds most efficiently using oxygen as a terminal electron acceptor, organic matter can also be oxidized anaerobically using alternate electron acceptors. In the large volume of anaerobic sediment, the microbial decomposition of organic matter can be mediated by nitrate, manganicmanganese, ferric-iron, sulfate and carbon dioxide. Organic matter can be fermented to organic acids and carbon dioxide. Organic matter dynamics in the sediment of catfish ponds are poorly understood, in part because sediment organic matter is distributed heterogeneously across the pond bottom. Furthermore, mounting a spatially rigorous sampling effort sufficient to detect relatively small changes in sediment organic matter is logistically difficult.
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However, seasonal patterns in other water quality variables can be used to infer annual trends. In commercial catfish ponds, the availability of oxygen in the overlying water is the most important factor controlling the rate of sediment oxygen uptake during the growing season (Steeby, 2002), most of which is used for organic matter decomposition. Although few measurements of dissolved oxygen concentration near the sediment /water interface of aquaculture ponds have been reported (Losordo, 1991), the limited data suggest that near-bottom waters are chronically undersaturated with respect to dissolved oxygen concentration. Thus, in the absence of sufficient dissolved oxygen to fully mineralize settled organic matter during the summer, organic matter may temporarily accumulate. Additional evidence for the accumulation of organic matter during the growing season can be inferred from the annual pattern of ammonia concentration in catfish ponds. In general, ammonia is minimum during the summer and maximum during the late fall or early winter. Despite reduced feeding rates, ammonia concentration increases during late fall. Ammonia will increase if the rates of source processes (fish excretion, diffusion from the sediment) exceed the rates of sink processes (phytoplankton uptake, nitrification). Indeed, phytoplankton uptake is reduced during the late fall, but the availability of oxygen to support organic matter decomposition is greater than during summer. Therefore, organic matter that may have accumulated during the summer is decomposed during late fall to mid-spring, when feed input is minimal, thereby restoring sediment organic matter concentration to previous levels. This process also explains the long-term (10 /20 year) equilibrium concentration of sediment organic matter. Interactions between bacteria and algae contribute to the complexity of organic matter dynamics in catfish ponds. The relative importance of each of these groups varies, depending upon the rate of organic matter loading, season, and water turbulence. Although some inorganic carbon is fixed into algal biomass, phytoplankton release large quantities of soluble organic matter in the form of simple to complex polysaccharides that are readily utilized by heterotrophic bacteria. Algae also provide oxygen to support oxidative metabolism by bacteria. Bacteria provide algae with carbon dioxide and growth factors (e.g. vitamins) and release nutrients from the mineralization of organic matter. Algae may also compete with bacteria for substrates. For example, phytoplankton will compete with chemoautotrophic nitrifying bacteria for ammonia. Phytoplankton may be more effective competitors for ammonia than nitrifying bacteria when concentrations are low during the summer, but the reverse may be the case when concentrations are higher during the winter (Hargreaves, 1997). The importance of algae and bacteria in ponds has stimulated interest in assessing methods to manipulate the microbial community to favor one or the other group. Microbial activity can temporarily affect water quality in algae-dominated ponds by reducing dissolved oxygen concentration following sediment resuspension associated with wind, seining, or aeration. Some commercial catfish ponds develop a condition where non-algal or mineral turbidity will dominate and persist. Water quality in these ponds has been observed anecdotally to be more stable than algae-dominated ponds. The stability of microbial suspensions has not been well documented, but
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some threshold cell density may be necessary to maintain good water quality in bacteria-dominated ponds. There are some clear trade-offs associated with dominance by algae or bacteria. Phytoplankton are important oxygenators and provide a powerful mechanism for ammonia removal. Ponds dominated by bacteria require an external oxygen source that must be provided by a level of mechanical aeration that far exceeds the natural supply of oxygen in algae-dominated ponds. Although phytoplankton densities are lower in ponds dominated by non-algal turbidity, cyanobacteria, including those that may cause off-flavor, are favored in the dimly lit waters of such ponds. Finally, the quality of effluents discharged from ponds with turbidity dominated by mineral solids is likely to be worse than that from algae-dominated ponds. This discussion of organic matter dynamics raises the question of whether catfish ponds are characterized by net autotrophy or net heterotrophy. In the short term (days), productivity may exceed respiration. In one study in three commercial catfish ponds, average daily net productivity exceeded whole pond respiration by a factor of 1.3 (Hargreaves and Steeby, 1999). However, two lines of evidence point to net heterotrophy. First, respiration from decomposition of added organic matter exceeds gross photosynthesis in aquatic systems with high rates of allochtonous organic matter input, such as ponds used for wastewater treatment or semi-intensive aquaculture. Boyd (1985) measured greater respiration rates than production by photosynthesis in experimental catfish ponds. Inspection of seasonal trends in total alkalinity in catfish ponds provides additional inferential evidence of net heterotrophy. Alkalinity in catfish ponds declines during spring, suggesting a period of net autotrophy. However, alkalinity generally increases during the production season because more carbon dioxide is released during respiration than is fixed by photosynthesis. A net carbon dioxide release increases the dissolution of calcium carbonate in pond soils, thereby increasing alkalinity. Additional research is required to validate this mechanism because seasonal rainfall, evaporation, and groundwater pumping patterns may also contribute to an explanation of the observed seasonal trends in alkalinity. In summary, both aerobic and anaerobic microbial processes in catfish ponds oxidize large quantities of organic matter. Large-scale, commercial catfish aquaculture in ponds would not be possible without this high inherent capacity to oxidize organic solids generated by fish excretion and primary production. Photosynthesis provides the majority of oxygen used to decompose organic matter as a ‘free’ ecological service. 2.5. Nutrient removal Static hypereutrophic ponds have a high capacity to remove inorganic nitrogen and phosphorus. Ammonia is the inorganic nitrogen form of primary concern and fish excretion and sediment diffusion are the two principal sources. Phytoplankton uptake and nitrification are the major sinks for ammonia. Nitrogen is removed from ponds through mineralization of organic matter followed by coupled nitrification / denitrification. Some ammonia (as NH3) will volatilize to the atmosphere under
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warm, windy, high-pH conditions and a relatively small proportion of nitrogen accumulates in the sediment as recalcitrant organic matter. The sustained feeding rate at which total ammonia begins to rapidly accumulate can be used as an initial estimate of the pond’s capacity to assimilate nitrogen under summertime conditions. In one study (Cole and Boyd, 1986), that point occurred when the maximum feeding rate was 84 kg ha1 day1, which corresponds to a nitrogen loading of 430 mg N m 2 day1 for the 32% crude protein feed used in the study. Maximum feeding rates of 100 /150 kg ha1 day1 correspond to nitrogen loading rates of 500/750 mg N m 2 day1. In systems managed for autotrophic production, nitrogen loading rates of 700/800 mg N m 2 day1 appear to represent the upper limit to N loading of fish ponds (Schroeder et al., 1990; Knud-Hansen et al., 1991). The capacity of aquatic sediments to adsorb phosphorus is dependent on clay content. Most ponds are deliberately constructed in clay soils that retain water, so the phosphorus adsorption capacity of pond soils is very high. Inorganic phosphorus is removed from pond water by precipitation /dissolution reactions, which are controlled by pH, with aluminum controlling solubility at low pH and calcium controlling solubility at high pH. Sediment can also be a source of inorganic phosphorus. Under reducing conditions, reducible iron-bound phosphorus is soluble. In the absence of an oxidized barrier to diffusion at the sediment /water interface, phosphorus can diffuse into the overlying water. Low dissolved oxygen concentration at the pond bottom can enhance phosphorus diffusion from the sediment. Sediment phosphorus diffusion may be involved in a positive feedback mechanism related to the effect of inorganic phosphorus on phytoplankton density. Increased phytoplankton density increases the magnitude of the fluctuation of dissolved oxygen concentration, thereby increasing the duration of low dissolved oxygen concentration. Low dissolved oxygen concentration at the pond bottom promotes further diffusion of reductant-soluble phosphorus. This mechanism reinforces the need to maintain oxidized conditions at the sediment /water interface as a means to limit eutrophication. Additional research is required to confirm and evaluate the importance of this mechanism in eutrophic aquaculture ponds.
3. Characteristics of hypertrophic ponds amenable to engineering solutions The overwhelming majority of global aquaculture production of finfish occurs in earthen ponds. Therefore, it is surprising that more aquacultural engineers have not embraced the challenges associated with extending loading limits or waste assimilation capacity and improving the efficiency of fish culture in static ponds. There appears to be an inverse relationship between the interest in a particular culture system by aquacultural engineers and the aggregate fish biomass produced from those culture systems. Aquacultural engineers generally favor a reductionist approach, where fish culture consists of a system of integrated, but individual unit processes, the manipulation of which results in improved control over the production
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process. The main determinant of intensive system design is fish metabolism. In contrast, the complexities associated with highly dynamic ecosystems such as hypertrophic aquaculture ponds require a holistic, integrative approach where there is an appreciation and understanding that management interventions have multiple impacts. The dynamics of the pond ecosystem, especially process mediated by phytoplankton and bacteria, are more important than fish metabolism as determinants of water quality, loading limits, and waste assimilation capacity. Nonetheless, a broad range of engineering domains can contribute to solutions of problems associated with the inherent constraints and limitations of the static pond system. Some of the more critical and salient characteristics of hypertrophic aquaculture ponds that are amenable to engineering solutions are presented here. Many of the problems associated with conventional catfish pond aquaculture are associated with a lack of control over pond processes within the constraints of current pond configuration, operation and management. Additional problems can be attributed to management near the limits of the pond to assimilate wastes, leaving little margin for variation in system performance. Given the dynamic nature of ponds, these limits change over a range of spatial and temporal scales. 3.1. Excessive phytoplankton biomass Maximizing net oxygen production from photosynthesis requires the maintenance of an intermediate phytoplankton biomass. This objective is difficult to accomplish in practice because the release of plant nutrients forces an increase in phytoplankton biomass to the point where standing crop is limited by availability of light. Control of phytoplankton biomass is an elusive goal of pond culturists because of the continual input of nutrients derived from feeding. To a limited extent, phytoplankton biomass can be constrained by limiting nutrient supply, perhaps through chemical precipitation. Any techniques for harvesting cells, either mechanically or by using plankton-feeding fish, must be practical and efficient. 3.2. Dominance of phytoplankton communities by cyanobacteria Hypertrophic aquaculture ponds develop a number of characteristic conditions that provide selection pressures that promote the dominance of cyanobacteria. These include warm water temperature, a dim underwater light climate, calm poorly-mixed conditions, high total phosphorus concentration, and low carbon dioxide concentration. Engineering solutions that seek to alter phytoplankton community composition must reduce these selection pressures and also consider the numerous attributes that favor the dominance of cyanobacteria in hypertrophic aquaculture ponds. 3.3. Intense thermal and chemical stratification Algal turbidity limits the penetration of light, leading to elevated water temperature and photosynthetic oxygen production near the pond surface and
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lower water temperature and net oxygen consumption near the pond bottom. The impact of stratification on fish production is not known, but many fish species are known to seek areas of preferred water quality, at times forced into zones that restrict access to the full water column. For example, surface water temperature may increase to near-lethal levels during calm summer days and affect the voluntary consumption of floating feed. Stratification limits overall photosynthetic oxygen production to surface waters. Even in shallow (1.5-m deep) ponds, more than half the water column can be a net oxygen consumer. Szyper and Lin (1990) described an index of stratification intensity for eutrophic aquaculture ponds and concluded that the energy required to equalize the vertical density gradient in a stratified pond is low. However, this energy is diffuse in space, indicating a daunting engineering challenge. Thus, application of high-intensity, point sources of turbulence will not be as efficient as devices that produce a diffuse and uniform flow field. 3.4. Chronic dissolved oxygen undersaturation at the sediment /water interface Access to the pond bottom by culture species with a benthic orientation may be restricted by chemical (e.g. oxygen) stratification. In addition, the rate of aerobic decomposition of organic matter will be limited by the availability of oxygen, thereby affecting the waste assimilative capacity of the pond ecosystem. In bottom waters that are chronically undersaturated, any accumulation of organic matter represents a latent oxygen demand that will be exerted if sediments are resuspended or if oxygen concentration increases. Restricted decomposition of organic matter will limit nutrient regeneration. In addition, the thin oxic sediment /water interface that normally serves as a barrier to the diffusion of reduced substances with potential toxicity to culture animals (e.g. ammonia, sulfide) will be absent. 3.5. Limitations of current aeration technology Electric paddlewheel aeration is now standard technology in commercial catfish ponds, which are aerated at 1/2 hp acre 1 (1.8 /3.7 kW ha1). In large commercial catfish ponds, paddlewheel aeration cannot effect an increase in dissolved oxygen concentration throughout the pond. Rather, paddlewheel aeration produces zones of sufficient dissolved oxygen to maintain the fish standing crop. Paddlwheel aeration is not designed to meet the overall pond respiratory demand, but rather provides sufficient supplemental oxygen to satisfy fish respiratory demand when pond oxygen concentration declines to some critical threshold, usually 2 /3 mg l 1. At this threshold, limited oxygen is available to support plankton respiration and sediment organic matter decomposition. The standard oxygen transfer rate of paddlewheel aerators is high because the aerator produces vigorous turbulence as it moves a fairly large volume of water. Fish gathered in front of aerators are forced to swim in a strong water current, increasing respiratory demand simultaneous with the occurrence of low dissolved oxygen concentrations. A logical solution is the development of efficient aerators without the by-product of excessive water current.
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Creative approaches to aeration of large ponds are also needed. Ponds are aerated for approximately 1000 h during a typical production season of 210 /240 days. Thus, catfish farmers provide an annual direct input of electricity in the form of paddlewheel aeration of 1000/2000 hp-hr acre 1 (1843 /3685 kW-hr ha1). Assuming that catfish farmers are willing to continue to provide this level of electrical input, is paddlewheel aeration the most efficient application of this electrical input level? Will other devices or combinations of devices yield gains in input efficiency? Although results from the fair amount of research with pond mixers has been equivocal with respect to improvements in water quality, total energy use and fish yield, additional research on pond mixing is needed to firmly establish the value of this management technique.
4. Implementing an efficiency-based paradigm for catfish pond aquaculture As with any successful agricultural industry, potential profit is a primary motive for entry into catfish farming. Some catfish farmers have assumed that increased production and profits are synonymous. This way of thinking has been influenced by research intended to assess techniques for maximizing yield. For much of the brief history of catfish farming, temporary limiting factors periodically arose, but these were overcome by some technological innovation or new production practice. This limiting factor cycle */of encountering and overcoming a limiting factor, followed by the emergence of a new limiting factor*/is not unique to catfish pond aquaculture. Nonetheless, ultimate limits to static pond aquaculture as currently practiced have apparently emerged. Any changes to overcome limiting factors are severely constrained by current industry-standard practices related to pond size, fish stocking and feeding rates, water quality management, and stock management schemes. An efficiency-based paradigm for catfish pond aquaculture seeks to maximize profits by optimizing resource use, but this does not necessarily maximize production. Thus, the appropriate measure is one of efficiency (output per unit input) rather than production or yield (output per unit area or volume). As an example, economic analysis indicates that profit-maximizing catfish stocking densities (16 942/21 312 ha1) are lower than the yield-maximizing stocking density (30 000 ha1) (Losinger et al., 2000). This analysis further indicated that stocking rates and feeding rates interact to affect profitability. A separate analysis that included feed price (in addition to feeding rate and stocking density) indicated that profit is maximized at lower stocking rates when feed prices are high (Dasgupta et al., 2002). Conversely, profit is maximized at higher stocking rates when feed prices are low. Empirical evaluation of the interaction between stocking density and feeding rate through controlled experiments would be useful to validate the results obtained by modeling. Although catfish producers have long been aware of management techniques that improve efficiency of inputs, broad-scale implementation is lacking. Implementation of efficiency-based measures of performance can also profoundly impact feed utilization efficiency, water quality and pond effluents. The combined expertise of
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aquacultural engineers, biologists and economists should be brought to bear on a systematic evaluation of creative means to improve production efficiency. The focus of outreach programs should continue to emphasize increased efficiency.
Acknowledgements Approved for publication as Journal Article J-10180 of the Mississippi Agricultural and Forestry Experiment Station, Mississippi State University.
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Smith, D.W., Piedrahita, R.H., 1988. The relation between phytoplankton and dissolved oxygen in fish ponds. Aquaculture 68, 249 /265. Steeby, J.A., 2002. Sediment accumulation, organic carbon content, and oxygen demand in commercial channel catfish (Ictalurus punctatus ) ponds. Ph.D. Dissertation. Mississippi State University, Mississippi State, MS. Szyper, J.P., Lin, C.K., 1990. Techniques for assessment of stratification and effects of mechanical mixing in tropical fish ponds. Aquacult. Eng. 9, 151 /165. Taylor, J.B., 2003. Modeling the interrelationships among feeding activity, water quality, and climatology in warmwater fish culture ponds. Ph.D. Dissertation. Mississippi State University, Mississippi State, MS. Teichert-Coddington, D., Green, B., 1993. Comparison of two techniques for determining community respiration in tropical fish ponds. Aquaculture 114, 41 /50. Tucker, C.S., 1985. Organic matter, nitrogen, and phosphorus content of sediments from channel catfish ponds. Research Report 10-7, Mississippi Agricultural and Forestry Experiment Station, Mississippi State University, Mississippi.