Fish Hatchery Management - fisheries & aquaculture

264 FISH HATCHERY MANAGEMENTa diagnosis is required from a pathology laboratory. Broad-spectrum treatmentsbased on a poor diagnosis are ill-advised, but treatment based onkeen observation and awareness of signs can mean the difference betweenlosing just a few fish or losing tens of thousands.Disease CharacteristicsD isease - C a u s ing Orga nis msOrganisms that cause diseases in fish include viruses, bacteria, fungi, protozoans,and a wide range of invertebrate animals. Generally, they can becategorized as either pathogens or parasites, although the distinction is notalways clear. For our purposes, we consider subcellular and unicellular organisms(viruses, bacteria) to be pathogens. Protozoans and multicellularorganisms (invertebrate animals) are parasites, and can reside either insidethe host (endoparasites) or outside it (ectoparasites). Low numbers of eitherpathogens or parasites do not always cause disease signs in fish.Viruses are neither plant nor animal. They have been particularly successfulin infecting fish. viruses are submicroscopic disease agents that arecompletely dependent upon living cells for their replication. All knownviruses are considered infective agents and often have highly specific requirementsfor a particular host and for certain tissues within that host.Deficiencies or excesses in the major components of the diet (proteins'amino acids, fats, carbohydrates, and fiber) often are the primary cause ofsecondary bacterial, fungal, and parasitic diseases. Fish with a diet deficientin protein or any of the indispensable amino acids will not be healthy andwill be a prime target for infectious agents. The same is true of deficienciesof fatty acids or excesses of digestible carbohydrates' Secondary diseaseagents may infect a fish in which biochemical functions are impaired. Nutritionaldeficiences are discussed in more detail in Chapter 4.Disease RecognitionDisease can be defined briefly as any deviation of the body from its normalor healthy state causing discomfort, sickness, inconvenience, or death.When parasites become numerous on a fish, they may cause changes inbehavior or produce other obvious signs.Individual diseases do not always produce a single sign or characteristicthat is diagnostic in itself. Nevertheless, by observing the signs exhibitedone usually can narrow down tire cause of the trouble to a particular tyPeof causative agent.Some of the obvious changes in behavior of fish suffering frgm a disease,parasite, or other physical affliction a." (t) loss of appetite; (2) abnormal