The use of parasites as biological tags allowed the identification of three stocks of Argentine hake Merluccius hubbsi in the south‐west Atlantic. A total of 344 fish caught in the Argentine‐Uruguayan Common Fishing Zone, San Matías Gulf, San Jorge Gulf and Patagonian Shelf, were examined for parasites and 26 species were found. Univariate analysis on both parasite populations and infracommunity descriptors, as well as a discriminant analysis, allowed the identification of three separate stocks, one composed of hake from the Argentine‐Uruguayan Common Fishing Zone, a second by fish from San Matías Gulf and a third comprising samples from both San Jorge Gulf and the Patagonian Shelf. Northern samples were characterized by low values of prevalence for most parasite species, with the exception of Corynosoma australe and Grillotia sp. 1. Among Patagonian hake, Anisakis simplex showed similar values of prevalence in all zones, fish from San Matías Gulf were characterized by Kudoa rosenbuschi, Grillotia sp. 1 and Contracaecum sp. Among all compared areas, San Jorge Gulf and the Patagonian Shelf were the most similar, sharing most of their biological indicators (Grillotia sp. 2, Pseudophyllidea sp. 1 plerocerci and Hysterothylacium aduncum); therefore, the hypothesis on the presence of a separate stock in San Jorge Gulf could not be confirmed. Varying environmental conditions and their influence on the distribution of both zooplankton and other hosts involved in the life‐cycles of parasites could be determinant factors for the differences observed among hake stocks.

Sum m ary :Hysterothylacium aduncum (Rudolphi, 1802) is reported from five fishes and one invertebrate species. Third-stage larvae were found in the crustacean Themisto gaudichaudii and in mesenteries of the fishes Engraulis anchoita and Merluccius hubbsi ; fourth-stage larvae were recovered from the digestive tract of M . hubbsi and Scomber japonicus and adult specimens were obtained from the stomach and intestine of M. hubbsi, S. japonicus, G enypterus blacodes and Genypterus brasiliensis. Nematodes are described, measured and illustrated. Parasitic prevalence, mean intensity and range were calculated in relation to different geographic zones, from the Argentinean-Uruguayan Common Fishing Zone to Patagonic areas. An increase of parasitism from the northern areas southwards was observed. The life-cycle of H. aduncum, involving the host species considered, is also postulated.

In Argentina, studies on Varroa destructor resistance to coumaphos are still unknown. At present, high infestation levels of V. destructor are being detected in colonies of Apis mellifera after treatment with this acaricide. The aim of the present study was to determine the LC50 of coumaphos in V. destructor from four apiaries with high mite density after treatment with coumaphos. The LC50's were 112, 319, 127 and 133 microg/Petri dish for mites from the four apiaries. Significant LC50 differences were detected between resistant and susceptible mites. LC50 increased 197-559-fold when compared to the corresponding baseline, suggesting the development of resistance. These results are the first report of resistance to coumaphos in V. destructor in Argentina.