Fish Hatchery Management - fisheries & aquaculture

FISH IIEALTH N'IANAGEN'IEN13l lboil, red plague, or salt water furunc:ulosis. Vibrio anguillarum is now consideredto be the etiologic agent of the disease. Although vibriosis generallyis a disease of cultured marine fishes, it also occurs in wild populations. Itcan occur any time of year, even in water temperatures as low as 39'F.However, it is most prevalent in the temperate zones during the warmersummer months and epizootics can be expected when water temperaturesreach 57"F.Signs of the disease usually do not become evident until the fish havebeen in salt water for two weeks or more under crowded conditions. Diminishedfeeding activity is one of the first noticeable signs. Lethargic fishgather around the edges of holding units; others swim in erratic, spinningpatterns. Diseased fish have hemorrhages around the bases of their pectoraland anal fins or a bloody discharge from the vent. When a fish is openedfor necropsy, diffuse pin-point hemorrhages of the intestinal wall and livermay be evident. The spleen frequently is enlarged and may be two to threetimes its normal size.Diagnosis of vibriosis caused by V. anguillarum reqluires isolation of agram-negative, motile, rod-shaped bacterium on salt medium. The organismmay be slightly curved and produces certain biochemical reactionsunder artificial culture. There is no reliable presumptive diagnosis of vibriosisbecause of its similarity to other septicemic diseases caused bygram- negative bac teria.The organism is ubiquitous in marine and brackish waters and infectionsprobably are water-borne and may be spread by contact. Salmonids usuallydie within I week after exposure; fish of all ages are susceptible.Vibriosis is worldwide in its distribution, but it usually is most severe inmariculture operations. Virtually all species of marine and estuarine fishesare susceptible. Among salmonids, pink salmon and chum salmon are themost susceptible but serious epizootics have occurred in coho salmon, rainbowtrout, and Atlantic salmon. Stresses associated with handling, low oxygen,and elevated temperature predispose fish to vibriosis.Prevention of vibriosis depends on good sanitation, no crowding, andminimal handling stress. Immunization is an effective means of combattingthe disease. Bacterins now are available from commercial sources and appearto provide long-term protection. Hyperosmotic procedures utilizingbacterins appear most suitable for large numbers of small fingerlings. Injectionsmay be preferable for larger fish. In theory, long-term selection andbreeding for resistance to the bacterium may be a means of control.Sulfamerazine (registered by the Food and Drug Administration) used atthe rate of 17 grams per 100 pounds of fish per day for l0 days has controlledvibriosis. Terramycin (also registered) at 5.0 to 7.5 grams per 100pounds of fish per day for l0 days also has been successful.