Fish Hatchery Management - fisheries & aquaculture

266 FISH HATCHERY MANAGEMEN'I'ABFtcuRE 78. (A) Frequently, a fish population (l) must interact with a pathogen(2) in an unfavorable environment (3) for an epizootic (l-2-3) to occur. (B)Interaction of more than three factors may be required. In carp hemorrhagic septicemia,a chronic virus infection (1) of the common carp (2), followed by exposureto Aeromonas liquefaciens (3) i" a stressful environment (+), *uy be prerequisitesto an epizootic (t-Z-S-l). (Sorrrce, Snieszko 1973.)Whereas some pathogens of fish are highly virulent and cause disease assoon as they invade a fish, most diseases are stress-related. Prevention ofthese diseases best can be done through good hatchery management. Environmentalstresses and associated disease problems are minimized byhigh water quality standards, optimum rearing densities, and adequate nutrition.Management stresses such as handling, stocking, drug treatments, hauling,or rapid temperature fluctuations of more than 5'F frequently are associatedwith the onset of several physiological diseases. Table 37 gives a partiallisting of these fish cultural practices, their associated disease problems,and stress mitigation procedures if known.Disease TreatmentA complete rearing season seldom passes during which fish do not requiretreatment for one disease or another. Every treatment should be considereda serious undertaking, and caution should be taken to avoid disastrous