Fish Hatchery Management - fisheries & aquaculture

FISH HEALTH MANAGEMENT 265distribution in a pond or raceway, such as swimming at the surface, alongthe 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, andloss of ability to withstand stresses during handling, grading, seining, loading,or transportation.In addition to changes in behavior, disease may produce physical signsand lesions, or be caused by parasites that can be seen by the unaided eye.Signs observed may be external, internal, or both. For microscopic examination,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 orgills; popeye; hemorrhages; and cysts containing parasites or tumors.Gross internal signs of disease are color changes of organs or tissue (paleliver or kidney or congested organ.); 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 contactedfor assistance in isolating and identifying the causative agent. If avirus is suspected, contact a laboratory for analysis of tissues.Two other classes of disease are important to fish culturists, in additionto those caused by pathogenic organisms. One is nutritional in origin, andthe other concerns environmental factors, including bad hatchery practicesand poor water quality, that stress the fish.Stress and lts Relationship to DiseaseStress plays a major role in the susceptibility of fish to disease. The differencebetween health and sickness depends on a delicate balance resultingfrom the interactions of the disease agent, the fish, and the environment(Figrr.e 78). For example, although bacteria such as species of Aeromonas,Pseudomonas, and Flexibacter are present continuously in most hatchery watersupplies, disease seldom occurs unless environmental quality or the defensesystems of the fish have deteriorated.Fish in intensive culture are affected continuously by environmental fluctuationsand management practices such as handling, crowding, hauling,and drug treatment. All of these, together with associated fright, can imposesignificant stress on the limited disease defense mechanisms of mostfishes. Table 36 presents a list of infectious diseases together with thestress factors known to be predisposing conditions. In addition to sophisticatedphysiological measurements, behavioral changes, production traits(growth, weight gain or loss, food conversion), morbidity, and mortality arefactors that can be used to evaluate the severity of stresses.