T he spread of fish diseases is an event of opportunity. That is, when conditions such as overpopulation, high water temperature, or low oxygen levels occur, fish become stressed. The disease-causing organisms can then take advantage of the opportunity to attack the fish's system while it is most vulnerable. O ther causes of fish mortalities are usually not related directly to water quality at all. Several years ago, large numbers of fish died in coastal Louisiana when water temperatures dropped to record lows.
Fish are cold-blooded, and their body temperature is dependent on their surroundings.
If they are caught in shallow water when the air temperature approaches freezing, entire schools of fish can perish. I n another unusual fish kill case near Ruston in 1986, DEQ field personnel spent hours of diagnostic work before it was finally determined that sunfish were dying from formic acid poisoning in a local lake because they were gorging themselves on fire ants driven from their nests by flood waters. O nce the cause of a fish kill is categorized, the investigation takes a decidedly more complex direction. The investigator must use all observations, laboratory data, and technical expertise to decide what is the most probable cause. If a discharge is implicated, enforcement actions are possible, provided that all rules of evidence have been followed meticulously.