|Abstract||Parasites have been used to identify and trace the migrations of different intraspecific populations of commercially important marine fish since 1939, Literature research shows how the numbers of publications involving this method of tagging has increased in every successive decade since then, reflecting increasing interest. More recently the same approach has been applied to population studies of marine invertebrates, such as squid and prawns and of marine mammals. The efficiency of the method improves as research adds to our knowledge of the biology of marine parasites, particularly their life cycles. This paper describes the general principles of using parasites as biological tags and highlights the advantages and disadvantages of the method. It offers guidelines to the selection of the most appropriate tags for the main tubes of population study ; stock discrimination, recruitment studies and seasonal migrations. It also gives a guide to the interpretation of results following analyses of the parasitological data. These results are often open to more than one interpretation; the correct one is arrived at by a process of elimination.
Tow main approaches to the use of parasites as tags are recognised. In one, a small number of parasite species are selected according to established criteria and a large number of host individuals are examined specifically for these species. In the other, entire parasite assemblages are analysed using sophisticated statistical methods. Examples are given of each approach and of the use of parasite tags for each type of fish population study, together with examples of the use of parasites as tags in population studies of marine invertebrates and cetaceans.