FISH HEALTH MANAGEMENT 265 



distribution in a pond or raceway, such as swimming at the surface, along 

 the tank sides, or in slack water, or crowding at the head or tail screens; 

 (3) flashing, scraping on the bottom or projecting objects, darting, whirling, 

 or twisting, and loss of equilibrium; and (4) weakness, loss of vitality, and 

 loss of ability to withstand stresses during handling, grading, seining, load- 

 ing, or transportation. 



In addition to changes in behavior, disease may produce physical signs 

 and lesions, or be caused by parasites that can be seen by the unaided eye. 

 Signs observed may be external, internal, or both. For microscopic exami- 

 nation, it may be necessary to call in a fish pathologist. 



Gross external signs of disease include discolored areas on the body; 

 eroded areas or sores on the body, head, or fins; swelling on the body or 

 gills; popeye; hemorrhages; and cysts containing parasites or tumors. 



Gross internal signs of disease are color changes of organs or tissue (pale 

 liver or kidney or congested organs); hemorrhages in organs or tissues; 

 swollen or boil-like lesions; changes in the texture of organs or tissues; 

 accumulated fluid in body cavities; and cysts or tumors. 



If a serious disease problem is suspected, a pathologist should be con- 

 tacted for assistance in isolating and identifying the causative agent. If a 

 virus is suspected, contact a laboratory for analysis of tissues. 



Two other classes of disease are important to fish culturists, in addition 

 to those caused by pathogenic organisms. One is nutritional in origin, and 

 the other concerns environmental factors, including bad hatchery practices 

 and poor water quality, that stress the fish. 



Stress and Its Relationship to Disease 



Stress plays a major role in the susceptibility of fish to disease. The differ- 

 ence between health and sickness depends on a delicate balance resulting 

 from the interactions of the disease agent, the fish, and the environment 

 (Figure 78). For example, although bacteria such as species of Aeromonas, 

 Pseudomonas, and Flexibacter are present continuously in most hatchery wa- 

 ter supplies, disease seldom occurs unless environmental quality or the de- 

 fense systems of the fish have deteriorated. 



Fish in intensive culture are affected continuously by environmental fluc- 

 tuations and management practices such as handling, crowding, hauling, 

 and drug treatment. All of these, together with associated fright, can im- 

 pose significant stress on the limited disease defense mechanisms of most 

 fishes. Table 36 presents a list of infectious diseases together with the 

 stress factors known to be predisposing conditions. In addition to sophisti- 

 cated physiological measurements, behavioral changes, production traits 

 (growth, weight gain or loss, food conversion), morbidity, and mortality are 

 factors that can be used to evaluate the severity of stresses. 



