The effects of pollution on fish health.

Potentially harmful substances-e.g. pesticides, heavy metals and hydrocarbons-are often released into the aquatic environment. When large quantities of pollutants are released there may be an immediate impact as measured by large-scale sudden mortalities of aquatic organisms, e.g. fish kills resulting from contamination of waterways with agricultural pesticides. Lower levels of discharge may result in an accumulation of the pollutants in aquatic organisms. The end results, which may occur long after the pollutants have passed through the environment, include immunosuppression, reduced metabolism, and damage to gills and epithelia. However, the link between adverse water quality and fish diseases is not proven. Alleged pollution-related diseases include epidermal papilloma, fin/tail rot, gill disease, hyperplasia, liver damage, neoplasia and ulceration. Many surveys have indicated a greater proportion of diseased fish in polluted compared to non-polluted marine sites. Yet, the value of such surveys may be questioned. Specific examples of fish diseases thought to reflect the effects of pollution include surface lesions attributed to Serratia plymuthica, fin and tail rot caused by Aeromonas hydrophila and Pseudomonas fluorescens, gill disease resulting from the actvity of Flavobacterium spp., vibriosis as caused by Vibrio anguillarum, and enteric redmouth (causal agent, Yersinia ruckeri). Research indicated that some of the diseases caused by Aeromonas, Flavobacterium and Pseudomonas resulted from generally adverse water quality, i.e. higher than usual quantities of organic material, oxygen depletion, changes in pH values and enhanced microbial populations. Some infections with Serratia and Yersina may well have reflected contamination of waterways with domestic sewage, e.g. leaking septic tanks. At least one outbreak of vibriosis was linked to high concentrations of copper, which may have debilitated the fish making them more susceptible to disease.