| Summary: | The parasite fauna of Spanish mackerel, Scomberomorus commerson, from ten sites across northern Australia and one site in Indonesia, was examined to evaluate the degree of movement and subsequent stock structure of the fish. Data from temporary (gill copepods and monogeneans) and permanent (juvenile helminths) parasites indicated that S. commerson in the waters of north Australia are relatively sedentary and are structured into distinct stocks. Kupang fish had very few Terranova spp., Grillotia branchi, Otobothrium cysticum, and Pterobothrium sp. compared to Australian fish, indicating that no Australian fish enter the Kupang fishery. Univariate and discriminant function analysis of four temporary parasite species (Pseudocycnoides armatus, Gotocotyla bivaginalis, Pricea multae, and Pseudothoracocotyla ovalis) demonstrated little similarity between areas of northern Australia, thus indicating minimal short-term exchange between neighbouring groups of S. commerson. Analyses of five permanent parasite species (Grillotia branchi, O. cysticum, Pterobothrium sp., Callitetrarhynchus gracilis and Paranybelinia balli) indicated at least six parasitological stocks across northern Australia (Fog Bay and Bathurst Island; Cape Wessel; Groote Eylandt and the Sir Edward Pellew Group; Mornington Island; Weipa; and the Torres Strait), though the occurrence of a few irregular fish suggested that up to 5% of fish had moved during their lifetime. Within-school variability was similar to that between schools, showing that the fish do not form long-term school associations.
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