Aquaculture is the science or art of cultivating the water. There are many uses for the various animal products of aquaculture, but the potential of aquaculture is in the production of human food.  Aquaculture production in this country is less than 2 percent of a total world production of 6 million metric tons.(1) In the United States, the major established industries for food production are represented by catfish, salmonid fish and crawfish. The production of oysters also figures significantly, but the methodology used in this country varies somewhat from what many consider to be true aquaculture. The United States has baitfish and pet fish as established non-food industries.

Aquaculture has developed rapidly during the latter half of this century. As aquaculture commodities commercialized, they also intensified. Efforts to become more efficient in attaining maximum levels of production have been accompanied by crop loss from disease. Aquatic animal medicine has developed over the years in response. This specialized field developed as certain individuals decided to devote their careers to the study and management of diseases of cold-blooded animals.

Aquatic animal medicine is made up of a variety of professionals who serve aquaculture in many ways. Most specialists have a partial role in direct services; the remainder of their time is committed to research, teaching or other occupational activities.

A great variety of people also represent the aqua-culturist. Although there are governmental researchers and hatchery men, the most common aqua-culturist is involved in some form of commercial culture. He may be the individual entrepreneur trying to make a living for his family on his own or rented land, a manager of a farm controlled by investor groups, a member of an aquaculture cooperative, a manager of a farm controlled by a large corporation, or a manager of a culture operation that uses leased public water. His background may range from being illiterate with invaluable practical experience and skill, to being well trained by years of study at the best schools of aquaculture. Aqua-culturists have something in common -- they enjoy farming the water and have a love for aquatic life.

Basic Problems

Growing Pains. As many have taken the opportunity to point out, losses due to disease (used here in its broadest sense) increase as animals are concentrated in unnatural environments. However, there are many facets of the disease problem that may go unmentioned. That is, the loss of aquatic animals from disease is very much the result of growing pains in aquaculture. Animals are lost because of: pursuit of unproven culture methodology; pursuit of proven culture methodology for one set of circumstances, but not the one at hand; inexperienced or careless management; unpredictable natural events; contamination by toxins; and even holdups in the marketing or distribution process. Blended in with all of these are the familiar pathogen-host-environment imbalances that favor disease development.(2) It is important to understand that much of the disease loss experienced in aquaculture results from temporary imperfections in culture methodology and management. For example, imperfections in management could include inexperience in regulating water exchange or handling a common parasitic infestation.

Trouble Spots. Many disease situations stand out in overview, in the current practice of aquaculture. Many invertebrate aquacultures require a special culture period to rear larval stages. The fragile larvae may be overwhelmed by bacteria, fungi, viruses, or protozoa in the intensive rearing environment.  The time of exposure to larval rearing is important. Marine shrimp, for example, are more susceptible to disease in hatchery culture than freshwater shrimp are, but the longer time of freshwater shrimp's larval rearing provides more opportunity for problem development.

Most invertebrate larvae will become infected with bacteria when poor nutrition or adverse environmental conditions cause larval weakening. Enriched culture water will support excess numbers of potentially invasive bacteria. In crustacean culture, even epibionts have been a problem.(3)

Crustacean larvae have also had problems with several genera of fungi. Once the invasion process begins, larval lots are merely discarded because no effective corrective measures are available. Discarding is also a practice used when bacteria] or viral disease occurs. If a system is operating at its maximum effectiveness and mortality begins, most or all of the crustacean lot dies. This results from accentuated enrichment of the culture medium by tissue fluid released from dead animals, and a consequent sudden rise in bacteria] numbers. The diseases that molluscan larvae experience are similar to those of larval crustaceans.(4)

Under normal cultural practice, juvenile and adult invertebrates are less prone to disease than the larval stage. Larger crustaceans fare very well where water circulation is maintained in grow-out ponds. A fungal disease (Fusarium) is commonly encountered in juvenile and adult marine shrimp, which are reared intensively in specialized systems.(5)  American oysters are damaged by Dermocystidium on the Atlantic and Gulf Coasts of the United States.(6) This disease is considered to be related to atypical temperature and salinities.

The role of disease in finfish culture is greatest during the rearing of fingerlings for stocking purposes. In cold water fish, viruses and bacteria have been noted to play a major role, whereas in semi-tropical and tropical areas, parasites, particularly protozoa, have the major role.(7) The protozoan lchthyophthirius, is a seasonal problem of fall and spring in United States catfish farming. Ichthyophthirius and other protozoans (Ichthyobodo, Chilodonella, Trichodina and Epistylis) are a global problem with fingerlings of all species.

Culture practices in which disease is a major factor in the grow-out phase have failed to become widespread in aquaculture. Fox example, bacteria infections cause serious damage in cage cultures of catfish and salmonids. The importance of disease during the grow-out stage varies among different aquacultures. Disease is considered of relatively little importance in crawfish, freshwater shrimp and tilapia, when these cultures are compared to grow-out of catfish, trout and oysters.

It is difficult enough to make aquatic creatures produce in an unnatural culture setting, but adding to this difficulty is the necessity for the stock to endure certain aqua-cultural practices which have particular potential for predisposition to disease. These practices occur when aquatic animals are hatched, handled, hauled or harvested. Guidelines are available for what animals can endure, but the variety of ways in which the practices are undertaken could be judged anywhere from advanced to backward.

Toxicity is a constant problem for animals living in aquatic environments. Pesticides and other forms of pollutants are of obvious concern in certain aquacultures, such as shellfish, where animals develop in 'natural environments. Certain pesticides are particularly toxic to fish and may severely affect normal growth. Toxaphene, a pesticide which is widely used in the southern United States, has been shown to act as a drain on vitamin C in channel catfish when the pesticide was present in sublethal quantities.(8) Because insects and crustaceans are both arthropods, insecticides can be a particular threat to crustaceans, where sensitivity is high because of similar body functions.

Perhaps of more practical concern to aquaculture are the toxins produced as biological wastes of animals and decomposition processes, and certain unidentified toxins that are produced by aquatic plant or microbial life. Knowledge of the adverse effects of simple compounds such as ammonia, carbon dioxide and hydrogen sulfide is well documented for most aquaculture species, but lethal and sublethal effects of certain chemical compounds, which are released into the water, remain mysteries.

Basic Needs

Development of Management Plans for Prevention. Aquaculture's needs from aquatic animal medicine are greatest in the area of prevention. There is a great need to increase the resistance of stocks, so that they may overcome predictable and unpredictable exposures to sub-optimum conditions.  The use of bacterins, vaccines,(9) improved diets and genetics are valuable in many instances, but a wide variety of disease management plans designed to increase resistance needs to be demonstrated and integrated into the broader management scheme. In addition, management plans to reduce environmental stress need to be derived and demonstrated with input from aquatic animal medicine professionals. Aqua-cultural engineering has much to offer, and we really stand at the threshold of the day where environmental parameters previously considered as limiting will be manipulated to provide opportunities for intensive culture far beyond what was considered possible several years ago. As this happens, aquatic animal medicine must be ready to respond to keep disease as an un-limiting factor. For example, larval invertebrate aquaculture needs assistance in developing prevention strategies against opportunistic pathogens. Fingerling finfish aquaculture needs assistance in establishing probabilities of disease pro-disposition where certain culture practices are undertaken.

Development of Disease Management Plans for Contagious Diseases. There is also the continuing need to develop plans to prevent and control contagious diseases.  At present, there is much hope in the development of immunization, which should be approached as one too among others. As is the case of other tools, immunization must be accepted by the aqua-culturist. This will come about by the exposure to convincing demonstrations of effectiveness. The aqua-culturist views his animals as a group or crop, and will judge the effectiveness of technique on its overall result. Several of the supposedly "uncontrollable" viruses can be partially controlled by environmental manipulation, but such practices need to be established as clear-cut guidelines.

In certain situations, it may be best for the disease agent to be totally eliminated. Stock destruction and system disinfection are always possibilities for elimination, but effective chemical control is the most practical method for certain disease agents (e.g., Ichthyophthirius). Procedures for screening new stock and monitoring existing stock should be developed and put into practice wherever needed. The procedures should be kept as simple as possible, and their benefits documented so that they will have the best opportunity to be integrated into the existing aquaculture plan.

Technology Transfer. Aqua-culturists are constantly developing technology to fit their culture systems. They are also implementing management plans received from others. Aqua-culturists have a special need for the disease specialists to share their disease management technology. It is also important that what is related as practical is in fact practical. Furthermore, disease specialists have a role in assisting aqua-culturists to attain and sustain competence in the awareness of, and the critical identification of, disease conditions in their stock. The contemporary aqua-culturist is quite capable of personally handling most disease problems, once they are identified.

In order to accomplish these things, aquatic animal medicine's knowledge must include a comprehensive grasp of the aquacultures, and there are many, that we are dealing with. We will come closer to meeting the needs of aquaculture by being inside experts, rather than outside experts.

Special Needs

Drug lnavailability. Aquaculture is a small and growing industry which has the misfortune of developing in this country at a time when the nation is reassessing agricultural chemicals and drugs. What is presumed to be best or tolerable for the more important agricultural industries is not necessarily the fare for aquaculture; thus aquaculture has had to roll with the punches.

The present stage of transition in regulation places the aqua-culturist in a position of either closing his operations, or being routinely dishonest about various legal restraints that he may or may not be familiar with, or more often, cannot understand. The result of this situation is a continuing erosion of credibility for regulation.

The development of aquaculture requires that the aqua-culturist have at his immediate disposal certain agricultural chemicals and drugs that can be purchased over-the-counter. The cost of preventatives and correctives must be low enough to allow them to be usable. This is particularly the case for preventatives, and the recent inflationary trend has caused aqua-culturists to take a second look at many chemicals. Aquaculture as an industry is relatively a low-user of chemicals and drugs, and has very little effect on the price of these products. Consequently, if the chemicals and drugs are too expensive, they will not be effective because they will be impractical.

Stock Introduction and Certification. The spread of disease by transporting stock from one area or country to another has been actively considered for many years.  Society task forces and special conferences have developed lists of diseases of special concern. Those listed as highest priority are usually the ones with strong host specificity.(10) Typical examples are viruses of salmonid and cyprinid fishes, and myxomotosis in rainbow trout.

Eradication of a disease in a geographical area must include eradication in both domestic and wild stock. In many cases, the status of the wild stock is unknown, and may present problems when certifying facilities as being free of certain communicable diseases. Certification most commonly refers to a statement on the disease-free status of a specific lot. Consequently, certification should be viewed from two perspectives: inter-farm traffic, where avoidance is practiced as part of the recipient farm's management program; and international or interregional traffic, where the recipient geographical area is considered disease-free.

There is a need in aquaculture for a more comprehensive look into the spread of communicable disease. Actions based solely on the opinions of representatives of aquatic animal medicine can retard aquaculture's progress. Opinions and actions should reflect the desires of aquaculture representatives, aquatic animal medicine professionals, and others indirectly affected by stock transfer.

Diagnostic Services. People engaged in diagnostic service in this country commit only a fraction of their time to diagnosis. The remainder of their time is devoted to research, education, resource management or other occupational activity. Diagnostic services are most frequently utilized in the geographical concentrations of aquaculture. Mississippi, for example, has over 22,000 acres of aquaculture concentrated among twelve counties in the Mississippi River Delta (T. L. Wellborn, pers. com.). Larger aqua-cultural enterprises will support a staff large enough to have a biologist with some training in disease diagnostics. This person will conduct routine diagnosis for his farm, and will usually assist neighboring aqua-culturists.

Although diagnostics is a critical part of training professionals in aquatic animal medicine, aquaculture's need for diagnostic assistance is often overstated. The major shortcoming of diagnostic assistance is not the limitations in equipment that prevent searching a case. Rather, it is the reduced competence of the diagnostician that results from a lack of exposure to concentrated diagnostic activity in the formative years.

Recommendations

Disease Management Plans Should be Developed. The more important disease conditions occur in intense rearing situations, such as the rearing of larvae and fingerlings.  Because of the intense effort involved in these cultures, there is a high interest in implementing new preventative techniques.  Too often the preventative techniques are inferior to others, because the aqua-culturist or his disease specialist is unaware of the benefits of a superior method.  The time is ripe for new prevention management plans in aquaculture, and their comparative evaluation. Methodology for control of contagious diseases should be developed by time tested results in the field. Too often the main flow of this information comes from the field to the disease specialist, who only acts as a sounding board. Aquaculture needs sound control methods for communicable disease, and is eager to respond to efforts of professionals in aquatic animal medicine.

Technology Transfer Should be Improved. The success of aquaculture requires that aqua-culturists have specialized skills and experience.(11) A part of the role of aquatic animal medicine should be its extension. High priority must be given to assure that the aqua-culturist or his advisor has more offered to him than a "fifth pass paraphrase." Information must be practical and should be transmitted in an understandable manner. Many of us have attended aquatic animal disease "workshops" with over fifty people, and listened to speakers who could be understood by not more than one or two. Unless effectiveness of practices can be demonstrated, practices will rarely be adopted by aqua-culturists. Aquatic animal medicine's contribution to aquaculture will be measured by its ability to transfer research findings into actual aqua-cultural practice. Applied research and other services should be designed with this in mind.

Communication on Direction of Research Should be Improved. Applied research on diseases affecting aquaculture most often follows trends set by aquaculture research in general. This has its benefits, but the diversity of aquacultures makes it difficult for research to be directed in a practical manner. The diversity of specialization of professionals may add further complexity. We need a proper balance of research emphasis, and unbiased communication on aquaculture's primary problems.

The Importance of Availability of Treatment Chemicals Should be Recognized. Aquaculture's use of treatment chemicals is too slight to encourage manufacturer's expenditures on registration procedures. Governmental regulations usually blanket aquaculture and place many chemicals in an illegal status. Professionals in aquatic animal medicine should recognize the importance of having certain treatment chemicals available and where possible, encourage clearance for their use.

References

1. U.S. Congress, Senate 1978.  Aquaculture in the United States.  Committee on agriculture, nutrition and forestry. 95th Congress, 2nd session.
2. Snieszko, S. F. 1974. J. Fish. Biol. 6: 197-208.
3. Fisher, W. S. 1977. Pages 673-684 in J. Avault, ed. Proceedinqs of the Eighth Annual Meetinq of the World Mariculture Society, San .Jose Costa Rica, January, 1977.
4. Loosanoff, V. L. 1972. University of Washington Publications in Fisheries, New Series, 5: 165-179.
5. Lightner, D V 1977. Pages 10-77 in C. J. Sinderman, ed.  Develop ments in aquaculture and fisheries science. Vol. 6 Elsevier Sci. Publ.  Co., Amsterdam.
6. Quick, J. A., Jr. and J. G. Mackin. 1971. Fla.  Dept. Nat. Resource Prof. Pap. Ser. No. 13, 55 pp.
7. Food and Agriculture Organization. 1976.  FAO fisheries and report 188, Rome.
8. Mayer, F. L., P. M. Mehrle, and P. L. Crutcher. 1978.  Trans. Am.  Fish Soc. 107: 326-333.
9. Fryer, J. L. ed. 1977. Oregon State University Sea Grant College Program Pub].  No. ORESU-T-77-012. 10 pp.
10. Food and Agriculture Organization. 1977.  FAO fisheries report no. 1
11. Pillay, T. V. R. 1972. University of Washington Publications in Fisheries, New Series, vol. 5. Pages 203-208.