Freshwater fish parasites and environmental quality: an overview and caution.

The use of free-living invertebrates as indicators of freshwater quality is long established and well documented. The basic approaches employed are 1) recognition of indicator species, 2) recognition of changes, usually reduction, in community diversity and 3) construction of biological indices, combining knowledge of 1) and/or 2) and taking into account changes in species abundance. These approaches employ invertebrates because of their immobility and longevity, and rely on a detailed body of ecological knowledge so that biotic changes can be related clearly to particular environmental changes. If we are to use parasites in the same way, we must show that pollution does affect parasites and then ask whether they can serve as indicator species, and whether changes in parasite communities can serve as indicators of specific changes in environmental conditions. We must also ask if parasites are better indicators than free living organisms. Parasites also pose additional difficulties: they are mobile because their fish hosts are, and the effects of pollution may be direct on the parasite or indirect on any of its hosts or act via host immune systems. No species has evolved and adapted to pollution, and presence or absence of a parasite species or changes in community may have many causes. Both presence and community structure vary considerably between localities and over time within localities and we cannot yet relate these changes to particular causes. We can record changes and differences, but with present lack of knowledge cannot relate them unequivocally to specific causal factors or environmental changes. Use of parasites as bio-accumulators shows promise, as does monitoring of a locality over time, but one-off surveys are of no use. Until we are prepared to stand up in a court of law and be cross-examined on our statement that these parasites clearly show a particular change in the environment, the answer to both questions above must be no.