Parasites are integral components of the ecosystem in which they live. They are not merely superimposed on food webs but they flow through them (Marcogliese & Cone, 1997a, b; Marcogliese, 2002). Any effects that they have on their host populations, invertebrate and vertebrate, can affect the ecosystem. Because of this intimate relationship, they can also be affected by any changes in the free-living components of the ecosystem. Under some circumstances, therefore, they can provide information on the condition and functioning of an ecosystem (Marcogliese & Cone, 1997b). In particular they may be able to provide information on the biology of some of the species within it, especially on their movements and diets, and on changes that have taken place in that ecosystem. They can in effect serve as biological tags and as indicators of the state of an ecosystem.
The use of parasites as biological tags to indicate the geographical origin of a host is particularly applicable to marine fish. Margolis (1963, 1965) was among the first people to fully appreciate the value of parasites in this context; he demonstrated that it was possible to distinguish the country of origin of Pacific salmon of the genus Oncorhynchus caught at sea by their parasite fauna: some species of parasites were found only in fish from North America and others only in fish from Russia.