Parasites as indicators of fish population structure at two different geographical scales in contrasting coastal environments of the south-western Atlantic

Fish parasites are widely used as biological tags of stocks and populations of their hosts at large spatial scales, but less frequently at small geographical scales. The application of this methodology at small scales requires heterogeneity in the availability of infective stages of parasites and hosts with high site fidelity. Here we tested the value of parasite tags at both large and small spatial scales using the Argentine silverside, Odontesthes argentinensis (Valenciennes, 1835) as a model. This is a marine fish that can tolerate a wide range of salinities and has great phenotypic plasticity, which has promoted adaptive divergences at reduced spatial scales across populations, in the absence of geographical barriers isolating them, which could be reflected in their parasite assemblages. A total of 142 specimens were examined for parasites. Samples were collected at two close localities from each of two different biogeographical regions of the south-western Atlantic. The structure and composition of both parasite infracommunities and component communities were analysed through uni and multivariate statistics. All sites were significantly different in their infracommunity structure, even those that were at shorter distances from each other. However, infracommunity composition and component community structure showed higher similarities within each region than between samples from different ones. These results proved that fish parasites are valuable tools to assess fine-scale population structure of their hosts when site fidelity and adaptations to local conditions prevail. The inclusion of parasite indicators as complementary tools in such kind of studies can provide valuable evidence for those resources requiring stock assessments and management at fine-scale, in order to prevent negative impacts over exploited populations.