EFFECT OF POLLUTION ON FISH DISEASES - POTENTIAL IMPACTS ON SALMONID POPULATIONS

Anthropogenic factors have contributed to the precipitous decline of w ild Pacific salmon stocks, although the mechanisms and processes at wo rk are largely unknown. Pollution may be one of these factors. Sedimen ts in estuaries are known to act as repositories for contaminants, and estuaries are important habitats for ocean-and river-migrating salmon . We have shown that juvenile salmon Oncorhynchus spp. and their prey bioaccumulate chlorinated hydrocarbons and aromatic hydrocarbons-impor tant classes of toxic xenobiotics. Furthermore, we have shown that exp osure to these pollutants can lead to immunosuppression and increased disease susceptibility in juvenile salmon. Whether pollution influence s natural disease outbreaks in host populations, including salmon, is currently unknown. It is postulated that the occurrence of disease dep ends on the interaction of the host, the environment, and the pathogen . Absence of pathogens would reduce the potential for adverse environm ents to influence disease outbreaks. However, a recent reconnaissance survey of juvenile chinook salmon Oncorhynchus tshawytscha from Oregon coastal rivers revealed that pathogens were an integral component in all systems studied, although the prevalence of the pathogens varied. Furthermore, recent studies of natural fish populations have demonstra ted that infectious-disease-induced mortality can significantly reduce the size of the host population. By creating adverse environments (e. g., polluted estuaries) which alter the susceptibility of the host to pathogens that are integral and ubiquitous components of the habitat, pollution increases the probability of disease-related impacts on fish populations.