Myxozoans as biological tags for stock identification of the Argentine hake, Merluccius hubbsi (Gadiformes: Merlucciidae).

Myxozoans have been successfully used as tags for fish stock identification around the world. However, few studies using myxozoan tags have been carried out in the Southern Atlantic, a region with complex oceanography that constitutes a potentially suitable scenario for testing the utility of myxozoans as indicators. Its usefulness was tested using six samples of Merluccius hubbsi in two different regions of the Argentine Sea. Generalized linear models were performed to assess the effects of fish size and sex, and year and region of capture and selected using the Information Theoretic approach. Three myxozoan species were recorded: Kudoa rosenbuschi, Myxoproteus meridionalis and Fabespora sp. Results of modelling species individually showed differential capabilities for detecting geographical population structure at different spatial scales, with K. rosenbuschi and Fabespora sp. allowing the discrimination of northern and southern stocks, but Fabespora sp. also as a promissory indicator of intrapopulation sub-structure due to different migratory routes during non-reproductive periods. This work confirms that myxozoans offer a set of suitable markers at different spatial scales, which can be selected individually or in any combination, depending on the geographical extent of the study, constituting tools adaptable to the objectives of further research on fish population structure.

Marine parasites as biological tags in South American Atlantic waters, current status and perspectives.

Many marine fisheries in South American Atlantic coasts (SAAC) are threatened by overfishing and under serious risk of collapsing. The SAAC comprises a diversity of environments, possesses a complex oceanography and harbours a vast biodiversity that provide an enormous potential for using parasites as biological tags for fish stock delineation, a prerequisite for the implementation of control and management plans. Here, their use in the SAAC is reviewed. Main evidence is derived from northern Argentine waters, where fish parasite assemblages are dominated by larval helminth species that share a low specificity, long persistence and trophic transmission, parasitizing almost indiscriminately all available fish species. The advantages and constraints of such a combination of characteristics are analysed and recommendations are given for future research. Shifting the focus from fish/parasite populations to communities allows expanding the concept of biological tags from local to regional scales, providing essential information to delineate ecosystem boundaries for host communities. This new concept arose as a powerful tool to help the implementation of ecosystem-based approaches to fisheries management, the new paradigm for fisheries science. Holistic approaches, including parasites as biological tags for stock delineation will render valuable information to help insure fisheries and marine ecosystems against further depletion and collapse.

Fish trophic level and the similarity of non-specific larval parasite assemblages.

Whereas the effect of parasites on food webs is increasingly recognised and has been extensively measured and modelled, the effect of food webs on the structure of parasite assemblages has not been quantified in a similar way. Here, we apply the concept of decay in community similarity with increasing distance, previously used for parasites in geographical, phylogenetic and ontogenetic contexts, to differences in the trophic level (TL) based on diet composition of fishes. It is proposed as an accurate quantitative method to measure rates of assemblage change as a function of host feeding habits and is applied, to our knowledge for the first time, across host species in marine waters. We focused on a suite of 15 species of trophically-transmitted and non-specific larval helminths across 16 fish species (1783 specimens, six orders, 14 families) with different sizes and TLs, gathered from the same ecosystem. Not all host species harboured the same number and types of parasites, reflecting the differences in their ecological characteristics. Using differences in TL and body length as measurements of size and trophic distances, we found that similarity at both infracommunity and component community levels showed a very clear decay pattern, based on parasite abundance and relative abundance, with increasing distance in TL, but was not related to changes in fish size, with TL thus emerging as the main explanatory factor for similarity of parasite assemblages. Furthermore, the relationships between host TL and assemblage similarity allowed identification of fishes for which the TL was under- or over-estimated and prediction of the TL of host species based on parasite data alone.