Department of Microbiology and Immunology


Avoidance of Disease

Fish are aquatic animals, they live in the water. Water provides the oxygen they breath, the food they eat, and the means to dispose of their wastes (e.g., carbon dioxide, urine, feces). The quality of the water determines how well the fish will grow and, indeed, if they will even survive. Maintenance of suitable water quality greatly reduces the likelihood of a disease problem. Critical water quality parameters include temperature (particularly sudden and dramatic shifts), dissolved oxygen, pH, alkalinity, hardness, nitrogenous wastes (ammonia, NH3; nitrites, N02-), and toxic substances (e.g., heavy metals, pesticides, carbon dioxide). Water quality should be monitored frequently and corrective measures initiated if conditions become stressful (e.g., dissolved oxygen below 3 ppm for warm water fishes or 5 ppm for cold water fishes, unionized ammonia above approximately 0.05 ppm, elevated nitrite concentrations and temperature above 21o C for cold water fishes).

Use of high quality feeds provides fish with the nutrients that they need to remain healthy and grow rapidly. Fish fed a nutritionally complete diet are better able to cope with stress and resist disease. Culturists should remember that even high quality feeds will deteriorate if improperly stored or kept too long. Feed should be purchased from a reputable supplier, stored in a cool and dry place, and used within 90 days of manufacture.

Light (excessive or rapid changes in intensity), noise and other disturbances can stress fish and should be minimized. Routine maintenance, stocking, and harvest require handling the fish. When fish are removed and processed (e.g., weighed, transported) they compensate physiologically. To reduce the trauma of handling make sure all necessary materials (e.g., nets, hauling tanks, scales) and adequate personnel are immediately available. Use of salt (0.1-0.5% by weight in the water, see Treatment of Diseased Fish section for precautions), aeration or oxygen, and anesthetics can reduce the stress associated with handling. Handle the fish gently and for as short a time as possible. If possible, do not handle fish that are already stressed or when environmental conditions are marginal.

Responding to Disease Problems

Regardless of how careful one is, if you culture fish long enough you will inevitably encounter a fish disease problem. When a disease problem develops, a quick and effective response is essential. There is no better preparation than to know your fish.

Under routine aquaculture conditions, healthy fish display "normal" behavior. Fish feed vigorously when food is presented or shortly thereafter. In ponds and cages fish are usually invisible, except when feeding. Therefore, it is important for the aquaculturist to note the feeding behavior of the fish being cultured even when automatic feeders are used. A reduced feeding activity should serve as a notice to the aquaculturist that immediate further investigation for the cause of the noted behavior is warranted. In raceways they normally swim leisurely, either in masse or singly, depending on the species. Distribution in raceways varies for species, but is usually constant (e.g., some species prefer covered areas and others prefer uncovered areas, some concentrate toward the water inflow and others are more randomly distributed). As a culturist, you should become familiar with the normal behavior of your fish. If their behavior changes (e.g., they stop feeding, swim near the water surface, dart or scratch on objects), something has occurred and you need to find out what! The first response to a disease is abnormal behavior; to recognize what is abnormal, you must first be familiar with what is normal.

Routine monitoring of water quality in a production system is imperative. When abnormal behavior is observed, culturists should check their water quality (e.g., dissolved oxygen, nitrogenous wastes, temperature). A sudden change in weather (e.g., a cold front moves through) can cause a change in behavior. If water quality is a problem (e.g., low dissolved oxygen, high ammonia) then corrective measures should be initiated. If abnormal behavior persists for several days or mortalities are observed, culturists should seek professional assistance. Water quality data should be provided to the diagnostic laboratory as well as information concerning the fish in question. The following sections describe what you should do and who you should contact. Become familiar with the process, have the necessary supplies available, and know your diagnostic personnel and where they are located before a problem arises so you can address quickly and effectively the situation before a catastrophic dieoff occurs.