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I 46 ROGER L. HERMAN In fish kills caused by parasitic or infectious agents, losses are seldom abrupt. Rather, there is a gradual buildup in the rate of loss as the weakest, most severely affected animals die first. Often only a single species is affected. Occasionally, an affected population is subjected to a second stressor and a seemingly abrupt fish kill may develop after a linger- ing, chronic loss has persisted for some time. In all such disease cases, moribund fish are heavily in- fected with the pathogen or parasite. Lesions may be present, but microscopic examinations and bac- terial or cell cultures are usually required to iden- tify the causative pathogen. Occasionally, when the pathogen is a highly viru- lent bacterium or virus, the mortality rate may begin slowly, but increase logarithmically and reach catas- trophic levels in a relatively short time. Even so, the course of the fish kill is not as abrupt as in an oxygen depletion or situations related to toxic substances. A variety of infectious agents have been identified as the cause of fish kills in natural waters, among which viruses, bacteria, fungi, and parasitic organ- isms are prominent. The likelihood that they may be involved in a fish kill is discussed here. Viral Agents Viruses have seldom been documented as the causes of major fish kills in nature. However, they most often infect very early life stages of fish, and major losses of fry and fingerlings could occur without visible evidence. Examples of instances in which viral agents have been involved follow. Infectious pancreatic necrosis virus has often been isolated from captive and wild freshwater and marine fishes and from some invertebrates. This virus is best known as the cause of a disease of young cultured salmonids that destroys the pancreas. It is also the cause of spinning disease in wild Atlantic menhaden, so-named because of the erratic swim- ming of infected fish. Outbreaks of infectious pan- creatic necrosis in menhaden are usually associated with low dissolved oxygen and changes in water temperature. Infectious hematopoietic necrosis virus has killed 2-year-old, wild kokanees and an unidentified virus was implicated in a large mortality of wild rainbow smelt in late summer in Canada. The isolation of a virus requires the inoculation of infectious material onto living cell cultures. Not all cell lines will support each virus. When dealing with a suspected, but unknown, viral disease, several dif- ferent types of cell cultures must be innoculated, just as different types of bacteriological media must be inoculated when an unknown bacterial disease is suspected. Unlike bacteriological media that can be prepared and stored for extended periods in anticipation of need, cell cultures must be maintained in an active, fresh condition. Therefore, a fish kill investigator who is not associated with a laboratory routinely working with fish cell cultures will be unable to pro- cess samples for virological assay at the kill site. In- stead, the investigator should select and properly package fish for shipment to a laboratory equipped to isolate and identify fish viruses. Moribund animals showing lesions and aberrant behavior should be selected for analysis. Fish dying from viral infections may have hemorrhagic lesions, but ulcerated, ne- crotic lesions are rare. The fish should be bagged or wrapped in plastic and packed with wet ice. They must not be frozen. The samples should be trans- ported to the laboratory as soon as possible. Bacterial Agents Most bacterial diseases of fish are stress-related. This means that fish kills related to bacterial patho- gens are associated with a significant environmen- tal situation or change. Usually the stressful, but sublethal, situation occurred 10 to 14 days before the start of the epizootic. An investigator should be alert for seasonal stresses related to climate or weather or to normal physiological changes in fish, such as those related to migration or spawning. Massive winter and spring kills of gizzard shad are classic examples of fish kills associated with the bacterium Aeromonas hydrophila. This organism is a ubiquitous facultative pathogen that frequently causes disease when the defense systems of fish are compromised by stressful environmental conditions, nutritional deficiencies, low temperatures, or re- duced winter feeding. When spring water tempera- tures increase rapidly, the pathogen responds more rapidly than the immune system of the fish. As a result of this difference in physiological responses, outbreaks of hemorrhagic septicemia due to A. hydrophila may occur. The disease name is descriptive of the gross appearance of infected fish-hemorrhages and hyperemic (red) areas on the