Animal Welfare Information Center Bulletin, Winter 1999/2000, Vol. 10 No. 3-4

Animal Welfare Issues

Responsible Care And Health Maintenance Of Fish In Commercial Aquaculture

T.E. Schwedler, Aquaculture Health Specialist
 Aquaculture, Fisheries, and Wildlife Department, G08 Lahotsky Hall
Clemson University, Clemson, South Carolina 29634


S. K. Johnson, Fish Disease Specialist
Wildlife and Fisheries Science, Texas A & M University, College Station, Texas  77841
current address: Aquatic Animal Disease Specialist, Texas Medical Diagnostic Laboratory, Drawer 3040
Texas A & M University, College Station, Texas  77841-3040


Production agriculture has enabled the United States to become the best fed society in the world and is vital in domestic and international trade. An important segment of production agriculture, livestock production, has successfully provided consumers with a low-cost, wholesome protein source. Consumers have dictated low prices and high quality. Consequently, to maintain competitiveness, producers have set management objectives that maximize production while minimizing costs. In addition to public concerns about cost and wholesomeness of animal products, there is growing concern for the well-being of the animals while in the care of the producers. Most animal farming systems are designed to maximize production; however, proper care and good husbandry practices are linked not only with high productivity, but also with animal health and well-being. Producers must make a conscious effort to ensure the well-being of animals in their care. The argument is often made that attention to animal welfare can adversely affect profitability. In most systems, however, improved health and well-being translate into better animal performance. Both animal welfare and environmental quality protection are the responsibility of producers, and appropriate management inputs should be factored into production costs. If production costs increase significantly, the consumer will either have to pay higher prices or face limited supplies caused by producers being forced out of business.

Aquaculture, the raising of animals or plants in an aquatic environment, has received considerable attention during the past two decades as an alternative farming practice. Aquaculture has enabled society to enjoy fish for food, pets, and recreation and contributes to the preservation of certain threatened aquatic species. Concurrent with aquaculture development, concern for the welfare of the animals grown in aquatic systems has become an increasingly important issue with certain segments of society. The goals of the aquaculturist and concerns over animal welfare are not necessarily at odds. With careful planning and proper management, fish can be cultured to meet production and profit goals while maintaining aquatic animal health and well-being.

Both the general public and the producers must understand the needs and health status of the animals being produced. However, concern should be based on scientific facts about the animals’ well-being and not solely on perceptions. It is generally easier to identify with the welfare needs of animals that are more closely related to humans, such as primates and other mammals. Understanding of the well-being and care of lower vertebrates, such as fish, is usually less. Assessment of animal well-being should be based on subtle behavioral and physiological changes as well as established environmental limits.

The number of aquatic species is vast and their needs vary greatly; here we mainly discuss finfish. New species of fish from the approximately 20,000 species worldwide are being evaluated and adopted as candidates for aquaculture. The optimum health requirements for major farm-raised species are known. However, requirements for other species are being determined by ongoing research that aims at defining the unique limits of each. Consequently, the amount of information available concerning health requirements varies considerably depending on the species. An understanding of the health requirements for a species increases with the length of time it is commercially cultured and its economic importance. We know much more about how to evaluate the well-being of traditionally grown species, such as channel catfish, goldfish, fathead minnows, golden shiners, and rainbow trout than we do about newer aquaculture species.

This article is intended to provide aquaculture producers and the general public with scientifically based information on which to base procedures for the care and husbandry of aquatic animals raised in commercial production systems.

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Finfish Aquaculture Classifications

Finfish aquaculture is commonly classified according to (1) consumer use of the farm-raised product or (2) the environmental requirements of the fish being produced.

Consumer use

Environmental requirements

Aquaculture classifications based on the environmental requirements of the fish being cultured commonly use the criteria of water temperature and salinity. Each classification has an optimal range of environmental conditions where a fish species thrives and a larger range where they survive.

While these classifications are somewhat arbitrary, they are helpful when discussing basic environmental requirements and the well-being of fish grown in different aquaculture settings.

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Production Systems

Regardless of the type of aquaculture based on the previous classifications, fish can be cultured in many different systems. Each type of system can have different effects on cultured animals. To address the management inputs required to maintain the health and well-being of the fish within a system, it is important to understand the type of system in which the fish are grown. Each of these systems has specific sets of conditions that can be controlled by the producer, resulting in a graded level of management responsibility.

Pond Culture

Pond systems may be classified by the level of intensification or the degree of management necessary to produce the quantity and quality of fish desired. The least intensive system offers the producer few control options, and management requirements are low. Management inputs include stocking and harvesting control to establish a balanced relationship between predator and prey species (for example, a bass and bluegill sunfish pond). The major management strategy is to control the species ratios of the original stocking and to control subsequent harvests. The productivity of fish in the culture system depends on the natural fertility of the pond.

The next level of intensification involves use of inorganic fertilizer, consisting of nitrogen, phosphorus, and potassium (NPK). Management efforts are similar to those described previously, but now the fertility and production of the pond are enhanced. The fertilizer increases production of plants (primary productivity) and of the small aquatic animal life that feeds on these plants. This increased food supply results in as much as a fivefold increase in fish production.

The productive capacity of the system can be further increased through supplemental feeding. By providing commercial feed, the number and weight of fish per unit volume of water can be greatly increased. The factor limiting production is usually dissolved oxygen. Oxygen supply usually limits the total weight of fish to about 1,500 pounds per acre per year. While the weight limit varies by species, the likelihood of oxygen depletion increases as the total weight of fish increases. It is common at this level of intensification and management to grow a single species of fish. Stocking, reproduction, and feeding rates are managed to ensure that overpopulation or excessive fish weight does not create high-risk conditions.

Management at the next intensification level includes greater control of dissolved oxygen, with the objective of obtaining even higher production rates. This type of system may accommodatae 5,000 to 10,000 pounds of fish per acre, depending on the species grown and the availability of water quality management equipment. Greater inputs of feed cause increased production of waste products by the fish. If the dissolved oxygen is managed effectively, nitrogenous waste products produced by the fish usually become the next production- limiting factor. If stocking and feeding rates are not carefully controlled, the concentration of un-ionized ammonia and nitrites can increase to undesirable or dangerous levels. Nitrogenous waste becomes a limiting factor because of the limited capacity of the pond biota (primarily algae and bacteria) to convert the waste products into less harmful byproducts. The amount of nitrogenous compounds that can be effectively processed and removed on a daily basis in this type of system is about 3 pounds of nitrogen per acre per day. This translates into about 100 pounds of 32-percent protein feed per acre per day. These values can change with different climates, environmental conditions, and fish species.

All of the systems described thus far are pond culture systems that require little or no water exchange. They rely on physical, microbial, and/or photosynthetic processes to remove waste products released by the fish.

Intensive Culture

Other aquaculture systems for commercial and research use require specific management practices and typically contain aquatic stocks of high density. These systems are referred to as intensive culture systems. Fish density is usually expressed in number of fish or weight of fish per cubic foot of water and/or by the flow rate. These intensive culture systems require the highest degree of management, which is aided by system design. Intensive culture systems include net pens or cages, raceways, and recirculating systems.

Net pen or cage culture systems involve the stocking of high numbers of fish per cubic foot into enclosures placed in large bodies of water. Water quality management within the cage is one of the producer’s main tasks. For good water quality, water must flow through the cage at a rate sufficient to remove the water containing the fish wastes and replace it with cleaner water containing suitable concentrations of dissolved oxygen. The next management task is to ensure that the fish are stocked at the proper density. This provides for adequate lateral swimming space and limits aggressiveness resulting from dominance behavior. Under good management, certain fish species (such as catfish) can be grown at densities as high as 10 pounds per cubic foot of enclosure. Another important management strategy is making sure that the fish are fed a complete diet — one that contains all the essential nutrients. The quality of the feed used in any intensive culture system is critical because the fish have limited access to natural food sources.

In raceway production, continuously flowing water provides fish with a high-quality environment. The water flows through the system and is discharged before the water quality degrades. Fish density of a raceway system is determined by the flow rate and quality of the incoming water. Stocking rates are high in these systems, and the lateral swimming space requirements for each species has to be known. Water flow velocities must be maintained below a critical level to avoid excessive exercise, which can cause stress. Trout, for example, are commonly grown at densities as high as 2 to 3 pounds per cubic foot of water and catfish at 10 to 15 pounds per cubic foot of water without any adverse effects, if suitable water quality is maintained. Supplemental aeration and oxygen injection are commonly used to enhance production in raceways. Because water quality is controlled more by physical factors than biological factors, problems that result from environmental stressors are usually limited. Generally, problems in raceway systems are caused by system failure (reduction or cessation of waterflow) or the introduction of disease organisms into the facility.

Recirculating culture systems can be complex and are the most difficult aquaculture system to manage. The usual intent of this culture method is to limit new water inputs to about 5 to 10 percent replacement per day. The purpose of this control is either to control water temperature within a specific range or to limit water usage. The water that flows through the tank or trough is collected and filtered both mechanically and biologically to remove waste products before returning to the fish culture unit. Though the basic principle of this type of system is sound, backup systems are required to maintain water movement and quality within established critical limits. The most common problem with this type of system is biofilter overload or failure. Additionally, health management can be difficult because practical, legal applications of certain chemicals and drugs are constrained by unique functional features of the system. Most compounds that will control disease agents also have a detrimental effect on the bacteria that are responsible for removing or converting waste products to nontoxic forms within this system.

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Fish And Their Environment

Sensory Reception and Response

The nervous system of fish is similar to that of birds, amphibians, reptiles, and mammals. Their central nervous system consists of a brain and spinal cord capable of receiving and reacting to external stimuli. The central nervous system receives information from the external environment via sensory organs and peripheral nerves. The information is processed in the brain or spinal cord, and the appropriate reactions to the stimuli are initiated. The nervous system transmits both voluntary and involuntary signals to control the action of muscles and glands. Upon stimulation, the nervous system and the endocrine glands integrate to control functions and processes such as feeding and digestion, reproduction, respiration, circulation, osmoregulation, growth, excretion, buoyancy regulation, avoidance behavior, disease resistance, and even body temperature.

While most of the endocrine and nervous system functions found in land animals are also found in fish species, there are important anatomical, physiological, and biochemical differences. A major difference between mammals and birds and most species of fish is that fish cannot control their body temperature. The temperature of a fish varies with the temperature of the water; thus also do its biochemical, physiological, and behavioral responses. While there is some variation, fish generally double their metabolic rate for each 10o C rise in temperature, within their acceptable range. This becomes important when assessing the health of fish that appear listless, a common response to low temperatures.

An understanding of how fish perceive their environment is helpful when managing their care properly. Fish are finely attuned to their environment by the senses of taste, touch, sight, smell, hearing, and additional senses unique to fish. Sense organs of fish are adapted for life in an aquatic environment and have many sensory structures and functions that differ somewhat or completely from those of land animals.

Sensory functions of fish can be grouped according to the type of physical or chemical stimuli that are detected. The detection of chemical stimuli by the senses of smell and taste may overlap because water is very different from air as a means of transport for chemical substances. Some fish have taste buds on their body that detect the taste of food at a distance. The sensitivity of detection increases as the fish gets closer to the food source. This allows them to locate food even under conditions when it cannot be seen. Fish also have sensory organs called nares, which are similar in structure and function to those in nasal passages of land animals, but it is the water rather than the air that carries the smell.

The perception of physical disturbances by fish is also different from that of higher vertebrates because the density of water is greater than that of air. Orientation and pressure recognition, along with buoyancy control, are important parts of a fish’s physical sensory capacity. Hearing in fish is different from that in land animals because sound waves are received in a liquid medium, and there is no need for specialized structures to translate sound waves from the air to liquid (the ear drum). There is also little need for the external structures that are used in land animals to concentrate sound waves from the air. A fish’s lateral line system, which is a sensing system for low-frequency pressure waves, can be thought of as “touch at a distance.” This system provides fish with important information about food or predators while some distance away. Additional sensory capabilities in some species can recognize and react to very low levels of electricity. The organs that receive the electrical impulses from the water help the fish to find their prey and avoid predators. This can be important when considering the possible effects of stray electrical currents that can occur in fish culture units.

Sight in fish is similar to vision in land animals. Lens shape varies considerably among species, but the eyes are functionally similar. In some fish species, the small pineal gland in the brain has sensory function in light perception. This function is thought to be responsible for circadian rhythms (biorhythms based on a 24-hour cycle) that control maturation and spawning activity. These senses may require the regulation of light intensities and daily light/darkness regimes to avoid stress in fish.

We do not know the extent to which fish perceive pain as a sensory function. We do know, however, that when fish are presented with conditions that cause pain in humans, they display an avoidance behavior. Pain, as defined in Webster’s New World Dictionary, is “a sensation of hurting or strong discomfort, in some part of the body, caused by an injury, disease, or functional disorder, transmitted through the nervous system.”  The difficulty in assuming similarities between what fish experience and what humans experience is based on our inability to find structures in fish that are similar to those known to sense pain in humans. It is also impossible to ascribe to fish the process of conscious recognition of pain so well developed in humans. While evidence that fish have pain receptors identical to mammals is disputed (Nickum 1988), their ability to identify irritants appears to be well documented. Thus it appears important to avoid conditions that cause a violent response from fish or more subtle physiological changes that are indicative of stress.

It is impossible for humans to understand completely how fish perceive and respond to their environment. Some differences that are not part of our own experiences are how fish perceive acoustical and electrical stimuli and their ability to taste the environment with external taste buds. Possibly even more difficult to understand is how fish perceive touch. An important question to answer might be whether fish have the ability or need to discriminate between tactile stimuli that humans describe as “pleasurable” or “painful.” This question is certainly important when considering the well-being of higher vertebrates that have the ability to display their pleasure or discomfort. Because we do not understand the fish’s perception, a prudent policy would be to assume that conditions that cause pain in higher vertebrates should be avoided with fish whenever possible.

Environmental Stress and Disease

Fish, like other animals, have both generalized and specific responses to prolonged or repeated exposure to less than favorable environmental conditions. In a manner similar to other vertebrates, fish respond with a specific set of biochemical and physiological changes that help them survive bad conditions. Some of the changes that occur when a fish is exposed to a stressor are similar regardless of the type of stressor. The types of stressors that can occur in aquaculture are chemical, physical, or behavioral. Because the net effect of a stressor is costly to the fish’s energy, stressful environmental conditions become costly to the producer and may result in lower production efficiencies and a poor survival rate.

The overall effect of a stressor on an animal depends on the nature of the stressor and the degree and duration of exposure. Three recognizable stages are common in animals forced to tolerate sub-optimal conditions: a stage of adaptation, a stage of recovery, and/or a stage of exhaustion. The degree and duration of the stressor generally dictates the outcome of the stress event.  If the stress event is limited, fish are often able to adapt to the conditions and reestablish normal function under the new set of conditions. If the stress is removed, fish will generally go through a process of recovery, where they reestablish normal function over a period of time. If the stress is too great for the fish to compensate through adaptation, the fish will enter the stage of exhaustion and eventually die.

 Even if fish recover from a stressful experience, important physiological and immunological changes can cause the animal to become more susceptible to disease organisms. The response pattern of fish is less understood than that of mammals and birds, but its key elements are similar. The basic response of fish to a stressor or adverse condition is to adopt an emergency survival status. While some of the responses that occur have obvious benefits to the fish, such as mobilization of energy reserves, other responses appear to have negative effects on long-range survival, such as decreased immune function. It appears that when fish are presented with a stressor, they sacrifice long-term survival strategies to concentrate their efforts on short-term survival.

The overall effect of a stressful environment to fish stocks is reduced performance. Reduced performance may be measured in poor survival, poor feed conversion rates, poor reproduction, and poor feeding and growth. Thus, raising fish in sub-optimal conditions is not to the advantage of the aquaculturist. Understanding the environmental requirements of the fish species and providing proper care and health maintenance to avoid stressful conditions are the keys to the success of the producer and the well-being of the fish.

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Responsible Management

The basic requirements for the well-being of fish that are raised in an aquaculture facility must be provided by the producer. While fish in the wild are capable of migrating and changing behavioral patterns to meet their needs, fish in an aquaculture facility often cannot seek out optimum or more suitable conditions. To provide fish with a healthy environment, it is important to have both a properly designed facility and a management plan that addresses the needs of the fish. Fish should be provided with their basic needs: sufficient lateral swimming space; good water quality; a nutritionally complete diet; limited physical disturbance; and careful, prudent handling. The producer should also have a health management program that focuses on both infectious and noninfectious diseases. The program should be based on sound information and a thorough understanding of environmental requirements of the fish species and the culture system.

Because there is so much diversity in culture species and culture systems, a responsible management strategy has to be developed for  individual aquaculture operations. For example, the management inputs necessary for a less intensive pond system raising an environmentally tolerant species such as the common carp would be low. However, raising the more environmentally sensitive rainbow trout in a recirculating system would require a very high level of management input. Proper management is a requirement for achieving high fish performance in any culture system. A well-designed and properly managed aquaculture facility can produce fish consistent with production goals while maintaining the well-being of the fish.

Stocking Rates

 The number of fish stocked in the culture unit is very important to production goals and the well-being of the fish. Enough fish must be stocked to meet production goals but not so many that management cannot maintain proper health. If stocking rates exceed the carrying capacity of the system, then management to maintain acceptable conditions may be impossible. The influence of stocking rate is expressed in two ways: (1) effects fish have on the environment, and (2) effects fish have on each other. Greater fish densities will result in greater release of waste products into the culture environment. To avoid water quality problems associated with stocking rates, the capacity of the system to remove waste products should be understood by the producer. The carrying capacity of the system is limited by reliable physical and biological processes that have the capacity to remove specific amounts of waste on a reliable basis. Stocking rates should match the quantity of fish to be produced with the carrying capacity of the system. Additionally, the producer should provide the equipment necessary to maintain a healthy environment, and the management necessary to ensure production goals and the well-being of the fish stock.

Assuming that the stocking rate is within the carrying capacity of the system, the next important consideration is fish interactions. Most fish species of commercial aquaculture are characteristically tolerant of the presence of other fish of their own species. This is important in the selection of a candidate species for aquaculture. The lateral swimming space of high fish densities is most important in culture systems such as raceways, tanks, or cages. Depending on the species, limited swimming space may or may not cause stress. For example, catfish have been grown in cages in excess of 10 pounds of fish per cubic foot without a reduction in performance (Davis et. al. 1991). There is evidence that intermediate stocking rates of catfish (below 4 fish per cubic foot) results in fighting and injury. Thus, catfish raised in intensive systems should be stocked at rates that do not exceed the carrying capacity of the system and are above the threshold where fighting commonly occurs. Studies on coldwater fish (salmonids) have demonstrated that an elevated cortisol level (an indicator of stress in fish) depends more on dominance factors and interspecies fighting than on rate of stocking (Li and Brockman 1977).

A fish’s natural behavior influences its density requirement. For example, adverse effects of crowding are often experienced with open-water pelagic species and predatory species, but occur infrequently with schooling or socially oriented fish. Consequently, naturally tolerant species are ones often selected for aquaculture. It is recommended that producers carefully investigate stocking rates to establish criteria that minimize aggression among cultured fish and maintain good water quality.

Water Quality

Management of good water quality is necessary to maintain good production and the well-being of farm-raised fish. Two sets of water quality conditions must be managed. The first set consists of factors that are generally provided within an optimal range for the culture species. Examples are dissolved ions (sodium, chloride, calcium, and bicarbonate), temperature, pH, and dissolved oxygen. The second set consists of water quality factors that, in excess, are potentially harmful to the fish and should be maintained below a specific threshold. This set can be divided into (1) external or introduced toxicants, such as heavy metals, pesticides, and supersaturated gases and (2) natural substances, such as ammonia, nitrites, carbon dioxide, hydrogen sulfide, and suspended solids.

To maintain the health of the fish in the culture unit, it is important to select a water source that meets the requirements of the fish. A culture unit’s water supply will often limit the range of species that can be grown. Not only does the ionic content of the water determine the aquatic environment where aquaculture can occur (i.e., saltwater, brackishwater, or freshwater), it can also affect management practices. For example, in freshwater aquaculture, calcium, sodium, and chlorides are very important ions to fish physiology. If they are not present in concentrations high enough for the fish to efficiently utilize them from the water, then a fish can have osmoregulatory (salt water balance) problems. The dissolved ion complex of bicarbonate/carbonate is very important in management because its buffering capacity (total alkalinity) helps control changes in water pH. While all of the important ions can be added to aquaculture water supplies, cost and logistics of such additions make certain water sources impractical for aquaculture.

Temperature of the source water is also very important in selection of production sites. As mentioned earlier, temperature is important in classifying aquaculture systems. Species-specific temperature requirements also make certain climates and water sources preferred for optimal growth (table 1). While temperature of the water can be changed to meet the requirements of almost any fish species, the cost is often excessive. Rapid water temperature changes will also cause stress in fish. It is generally recommended to change the water temperature slowly at a rate of less than 3o C per hour. This allows the fish to adapt to a new water quality condition.

Table 1. Preferred water temperature ranges for optimal growth for various fish species of different temperature classifications.
Temperature Classification Fish Species Optimal Temperature Range  o C
Coldwater Rainbow trout 7-13
Coolwater Yellow perch 24-27
Warmwater Channel catfish 28-31
Tropical Tilapia 27-32

The type and concentration of dissolved ions in water must be compatible with the species of fish that are grown within the system. Salinity is a measurement of the ionic concentration of water, primarily sodium and chloride. The salinity of the water greatly affects the physiology of the fish being cultured. Waters can be broadly classified into three basic categories: saltwater — >20 parts per thousand (ppt); brackishwater — 5 to 20 ppt; and freshwater — <0.5 ppt. The strategy that different fish species have developed to maintain internal salt concentrations (osmoregulation) depends on the salt concentration of their natural environment. Saltwater fish have developed mechanisms that help to remove or exclude ions from internal tissues. Freshwater fish have developed mechanisms to concentrate or retain internal ions within their bodies. In fresh water, sodium and chloride should be maintained at a level of at least 10 parts per million (ppm) and calcium at 20 ppm for most fish species. Selection of a fish species that is compatible with the water source is necessary if fish are to be raised under healthy conditions.

The pH of the water in the culture unit should be maintained within a desired range (generally 5 to 9) for the health and well-being of the fish. The pH of the water is dependent on both the buffering capacity (usually total alkalinity) and the biological activity within the unit, including the fish. The buffering capacity of the water controls the degree of pH change in the water which is caused by photosynthesis and respiration. Photosynthesis by plants in the system removes carbon dioxide (the major source of acidity in most natural waters) from the water, causing the pH to rise. Respiration, on the other hand, adds carbon dioxide to the water, thus lowering the pH. The changes in pH that occur in the system are dynamic and can differ from hour to hour depending on conditions. As with other water quality conditions, maintenance of pH within the acceptable range must be considered during facility design and managed during production.

Possibly the most important management task of a producer is to maintain dissolved oxygen at acceptable levels (above 4-5 ppm). The level of management changes dramatically with the intensity of the culture system and is also affected by the fish species raised. There are two basic approaches to managing dissolved oxygen in aquaculture systems:

The passive management approach is to control stocking rates so that dissolved oxygen concentrations in the water do not reach critical levels (below 4-5 ppm). Oxygen can be managed by stocking and feeding fish at low levels, as with low intensive pond culture (feeding under 30 pounds per acre per day) or by designing a raceway system so adequate water replacement keeps dissolved oxygen at desired levels. The critical level depends on the species and their health status.

The active management approach is to introduce supplemental oxygen by mechanical or other means. There are many different designs and approaches, but all supply oxygen to the fish at a rate that will prevent stressful conditions. The two major strategies for supplying oxygen to the fish are, (1) aeration, where the diffusion of oxygen is mechanically enhanced, and (2) oxygenation, where pure oxygen is delivered into the water. Regardless of the method used, dissolved oxygen should be maintained at acceptable levels to ensure good production and the well-being of the fish.

Of the compounds that are directly toxic to fish, the types that come from sources outside of the system (external toxicants) are the most diverse. It is necessary to prevent the occurrence of these compounds in production systems by proper site selection, water source evaluation, selection of nontoxic materials, and avoidance of any harmful contaminants.

A second group of compounds that are toxic to fish are the compounds that are produced within the system. Some of these are released by the fish as metabolic byproducts (ammonia and carbon dioxide). Others are products of decomposition of the waste products, such as nitrites and hydrogen sulfide. A third group of compounds, produced by other organisms within the system, include bacterial and algal metabolites. Fish waste products are very soluble in water and quickly become incorporated into the water. Metabolites and their breakdown products become environmental problems for the fish if released in excess of a culture system’s ability to convert them to harmless forms (table 2). When more fish are raised per unit of water, the release of metabolic wastes also increases. Fish cultured at high densities without proper waste management can cause poor water quality. This increases the risk that the water will become degraded to the point where fish will experience discomfort. The metabolic byproducts of primary concern are the nitrogenous compounds; of these, ammonia and nitrites are the most important. Proper management of waste products requires careful design of the system to ensure that the waste produced by the fish is disposed of in an efficient and environmentally sound manner. It is also important to stock fish within the waste disposal carrying capacity of the system so the system does not become overloaded. To maintain proper fish health, good water quality must be provided by source and system design and through proper management based on the needs of each species.

Table 2. Critical levels of naturally occurring waste products in fish culture.
Compound Critical Level
Ammonia >0.05 ppm NH3-N
Carbon dioxide >10 ppm
Hydrogen sulfide >0.005 ppm H2S-S
Nitrite >20 percent of Cl- concentration

Nutrition and Feeding

The complete dietary requirements for all commercial aquaculture species are not known. Generally, the longer a species has been raised in aquaculture, the more is known about its specific dietary requirements. Recommendations on the protein, energy, amino acids, essential fatty acids, vitamins, and minerals are published in the scientific literature and by the National Research Council (1983) for catfish and trout. While the feed manufacturer is usually responsible for providing feed of adequate quality, producers should know the nutritional needs of their fish. Nutritionally complete rations are required for fish reared in intensive culture conditions, while those grown in the least intensive conditions can consume more natural foods that contribute to their nutrition.

 Feeding practices are also very important and can change with size and developmental stage of the fish. It is important to feed the fish on a prescribed schedule according to specific nutritional needs. The amount to be fed should be adjusted as the fish grow so they receive the proper quantity of feed daily. Additionally, temperature and water quality conditions that exist prior to and at the time of feeding can also affect feeding response. Feeding activity is a very important observation in management and is often the first indication that one or more problems exist with the fish or in the production system. Any sudden decrease in feeding activity not attributed to natural variation (such as a change in temperature) should be investigated immediately, because it is likely that management action is required.

Physical Disturbances

Because fish are so attuned to their environment, it is important that tranquility be maintained by minimizing physical disturbances. For indoor systems, this should include provisions for necessary photoperiod (daylight cycle) manipulation and no sudden changes in light intensities. Avoidance of loud or startling noises is important. Care should be taken to not disturb fish by casting shadows over them or tapping on tanks. Care should also be taken to prevent stray electrical currents in production units, especially with highly sensitive species. Restricted access should be maintained to facilities where fish are raised in tanks to prevent excessive physical disturbances. Fish can also be stressed by excessive water velocity in raceways; the critical swimming velocity should be investigated for the species being cultured in these systems. Studies with trout demonstrate that water pH of less than 5 and more than 10 has a negative effect on the maximum critical swimming speed (Ye and Randell 1990). The velocity of the water in a raceway should be set at a rate (usually expressed in body lengths per second) that will effectively remove wastes but does not over-exercise the fish. Excessive turbulence caused by water flow or aeration should also be avoided, especially when culturing very small fish.

Handling Fish

Handling and harvesting can cause some of the most stressful episodes in the life of a cultured fish. This is because during handling, fish are often restrained or confined for periods of time outside water and many times are held in suboptimal water quality conditions. It is therefore very important to handle fish as infrequently as possible and with great attention to proper handling practices. The proper salt content, temperature, and other water quality conditions should be maintained when fish are handled or transported. In some cases, approved anesthetics can be used to reduce excitement of fish during transport. This can reduce fish metabolic rates and relieve stress. The addition of salt to transport tanks for freshwater fish can also reduce the effects of stress by improving the efficiency of salt balance mechanisms. Every effort should be made to minimize the amount of time that fish are restrained or held out of water.

Health Management

Disease management in aquaculture systems begins with creating and maintaining a good living environment for the fish. Proper design and good management are necessary to minimize health risks by reducing stress to the fish. Once the system is designed properly and the management practices are directed to reduce stress, it is important to minimize the contact of the fish with infectious disease agents. Prevention is the best approach for avoiding diseases, and management plans should include a vigorous health management program including quarantine, hygiene, health monitoring, and disinfection when appropriate. Treatments should be used only after a proper diagnosis of a treatable infectious disease has been made. Use only drugs and chemicals that are FDA-approved and proven to be safe and effective. Many disease treatments can have an adverse effect on the water quality within the system. Monitor water quality and be prepared to implement management action when necessary.

Disease Prevention

Disease prevention is an important part of any animal production system. Two aspects of prevention are especially important in a health management program.

Good management practices minimize introductions of disease agents by recognizing their potential sources. The most common means of infectious disease entry is introduction of infected fish from contaminated sources. Screen new fish for important diseases that affect the species being raised. This can be accomplished in part by review of historical evidence provided by reputable suppliers and through inspection (Thoesen 1991). Quarantine the fish in an isolated portion of the facility for 4 to 6 weeks at a temperature that allows outbreaks of specific diseases. Do not share equipment with other facilities, and disinfect it between uses. Personnel should take preventive action before entering a facility or areas of a facility where they can potentially spread or carry harmful disease organisms. Buy feed from a reputable source and store it properly until used. These practices are general, but they will help reduce the potential of disease introduction into the production system or farm.

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Aquaculture producers should use good management practices to ensure that the animals within the culture systems meet production goals and are cared for properly. Successful production and profitability require an understanding of the needs of the fish and the use of management practices that reduce stress. The most pressing task of new producers is to learn the specific requirements of the species selected and the limitations of their culture system and water sources. This task is easy for some commercial species and systems because of past commercial successes and available literature. For new species and new kinds of systems, however, the track record and scientific information are lacking. Prospective producers should learn as much as possible about the aquatic animal and the chosen production system. Making sure that the species selection is compatible with the culture system is the first ingredient necessary for success. The producer then must make a commitment to proper system design and management. If the information on a particular species and system is sparse, the venture will be risky to the producer as well as to the fish.

Development of procedures to ensure the well-being of farm-raised fish is a dynamic process that will require ongoing research to provide new information on how to successfully culture aquatic species with minimal stress. Stress prevention, which contributes to an aquatic animal’s well-being, also improves the profitability of an aquaculture enterprise. Producers should pay close attention to ensuring, through proper management practices, maintenance of a suitable environment. Proper design and management of aquaculture systems can help ensure the well-being of the fish and production efficiencies. There are many excellent books available on the culture of aquatic organisms, design and management of aquaculture systems, water quality management, stress in fish, and health maintenance procedures. Consult with an aquaculture specialist on how to select a species and a system that has a high probability of success.

 [Contents | References]


The authors wish to thank the following people for providing critical reviews and valuable comments during the development of this article: Marty Brunson, Fred Conte, David Erickson, Mike Freeze, Reginal Harrell, Jeff Hinshaw, Gary Jensen, John Jensen, Hugh Johnson, Randy MacMillan, James McVey, Richard Michaud, Gary Moberg, Joe Morris, John Nickum, Richard Reynnells, Nathan Stone, and Hugh Warren.



  1. Davis, S.A., T.E. Schwedler, J.R. Tomasso, and J.A. Collier (1991). Production characteristics of pan-sized channel catfish in cages and open ponds. Journal of World Aquaculture Society 22(3):183-186.

  2. Li, W.H. and W. Brocksen. (1977). Approaches to the analysis of energetic cost of intraspecific competition for space in rainbow trout (Salmo gairdneri) Journal of Fish Biology 11: 329-341.

  3. National Research Council. (1983). Nutrient requirements of warmwater fishes and shellfishes. Washington D.C.: National Academy of Sciences.

  4. Nickum, J.G. (1988). Guidelines for use of fishes in field research fisheries. American Fisheries Society 13(12):16-22.

  5. Thoesen, J.C. (editor) (1991). Bluebook suggested procedures for the detection and identification of certain finfish and shellfish pathogens. Fish Health Section, American Fisheries Society.

  6. Ye, X. and D.J. Randell (1991). The effect of water pH on swimming performance in rainbow trout (Salmo gairdneri). Fish Physiology and Biochemistry. 9 (1): 15-21.

This article appeared in the Animal Welfare Information Center Bulletin, Volume 10, Numbers 3-4, Winter 1999/2000

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