Parasitological evidence of stocks of Paralichthys isosceles (Pleuronectiformes: Paralichthyidae) at small and large geographical scales in South American Atlantic coasts

•Parasites were used to distinguish between Brazilian and Argentinian stocks of Paralichthys isosceles.•The best discriminator species at small and large spatial scales were identified.•Selected tags were low specific, being applicable to population studies of other resources.•Artisanal fisheries face severe risks of overexploitation and collapse, needing stock assessment.•Parasite tags are valuable tools that should be included in holistic studies of stock assessment. Three stocks of the flounder Paralichthys isosceles Jordan, 1861 were identified in the southwestern Atlantic by using their parasites as biological markers. Comparisons (pemutational multivariate analyses of variance and canonical analysis of principal coordinates) of 78 flounders caught simultaneously at two close localities in Brazil (Cabo Frio and Niteroi) and previously published data of 51 fish from Argentina (Necochea) allowed testing the value of parasite tags as discriminators at both small and large spatial scales. A total of 5925 metazoan parasites belonging to 17 species were found in Brazilian samples. Parasite assemblages varied across localities in terms of species richness and presence and abundance of individual species. Multivariate analyses of presence and abundance of long-lived parasites resulted in clear dissimilarity patterns across all samples. Significant differences were also observed when the entire parasite assemblages were compared between both Brazilian samples. The best discriminator species at both spatial scales were represented by unspecific parasites, broadly distributed among fish species in the region. These indicator species apparently display recurrent spatial patterns across host species, being therefore expected to serve as suitable tags for population studies of other resources. Parasites constitute valuable tools to be included in further holistic stock identification studies, which may eventually allow proactive mitigation and conservation strategies for many short scale artisanal fisheries in the southwestern Atlantic coasts, which are facing severe risks of overexploitation and collapse.