ABSTRACT
Parasites1 are ubiquitous components of natural and managed ecosystems. Water provides a particularly suitable medium for the support and movement of directly transmitted ectoparasites, and the central role played by fish in aquatic food webs make them ideal intermediate hosts for indirectly transmitted infections. Consequently, parasites use fish as hosts. By definition, parasite infections negatively impact host fish, though very
often these detrimental effects are unquantified. However, parasites rely on their hosts for nutrition; so they impose at least energetic demands, and very often they have other physiological impacts. Parasites can be classified as environmental stressors of fish, not only because of the direct impacts they impose after infection, but also because their presence in aquatic environments can reduce the efficiency with which fishes function. The latter effects may arise because of constraints imposed on, for example, the habitat selection or food selection choices that are available to fish attempting to avoid contact with infectious agents.
