Fish Hatchery Management - fisheries & aquaculture

FISH HEALTH MANAGEMENT 271seconds, that depends on the type of chemical, its concentration, and thespecies of fish being treated. Metal containers should not be used to holdthe treatment solution because some chemicals can react with the metaland form toxic compounds, particularly if the water is acid.This method of treatment is dangerous because the difference betweenan effective dose and a killingdose often is very small. However, if doneproperly, it is very effective for treating small numbers of fish. Other disadvantagesto this method include its high labor costs and stress on the fishdue to handling.PROLONGED BATHFor prolonged-bath treatments, the inflowing water is cut off and the correctamount of chemical is added directly to the unit being treated (AppendixG). After a specified time, the chemical is flushed out quickly with freshwater. This treatment can be used in any unit that has an adequate supplyof fresh water and can be flushed out within 5 to l0 minutes.Several precautions must be observed with this method to prevent seriouslosses: (1) Because the water flow is turned off, the oxygen concentrationof the water may be reduced to the point that the fish are stressed andlosses occur. The more fish per unit volume of water, the more likely this isto occur. Aerators of some type must be installed in the unit being treatedto insure an adequate oxygen supply or must be available if needed. (2)Regardless of the treatment time that is recommended, the fish alwaysshould be observed throughout the treatment and, at the first sign of distress,fresh water must be added quickly. (3) The chemical must be uniformlydistributed throughout the unit to prevent the occurrence of "hotspots" of the chemical. Fish being treated may be killed or severelyinjured by overdoses if they swim through hot spots. Conversely, fish thatavoid these hot spots may not be exposed to a concentration high enoughto be effective. The method used for distributing the chemical throughoutthe unit will depend on the kind of chemical being used, type and size ofunit being treated, and equipment and labor available. Common sensemust be used as it is impossible to lay down hard and fast guidelines thatwill cover every situation.INDEFINITE BATHIndefinite baths usually are used to treat ponds or hauling tanks. A lowconcentration of a chemical is applied and left to dissipate naturally. Thisgenerally is one of the safest methods of treatment. One major drawback,however, is that the large quantities of chemicals required can be exPensiveto the point of being prohibitive. Another drawback relates to the