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<jats:p>Four stocks of Argentine anchovy <jats:italic>Engraulis anchoita</jats:italic> in the south‐west Atlantic, south of 34°S were identified by the use of parasites as biological tags. A total of 671 fish were examined for parasites and 19 parasite species were found in 664 Argentine anchovies. The parasite populations and both infra‐ and component communities showed marked latitudinal gradients. Cluster analyses allowed the identification of four groups, one composed of autumn samples (35–37°S) and three of spring ones. Among the latter, a Patagonian group (43–46°S) was clearly separated from a Bonaerense group, which was divided into two groups: North Bonaerense (34–40°S) and South Bonaerense (40–43°S). Autumn samples were characterized by low values of prevalence for most parasite species. Among spring samples <jats:italic>Pseudanthocotyloides heterocotyle</jats:italic>, <jats:italic>Lecithochirium microstomum</jats:italic>, <jats:italic>Parahemiurus merus</jats:italic>, <jats:italic>Scolex polymorphus</jats:italic> and <jats:italic>Corynosoma australe</jats:italic> were characteristic of the northern region, whereas <jats:italic>Cardiocephaloides</jats:italic> sp., <jats:italic>Anisakis simplex</jats:italic>, <jats:italic>Contracaecum</jats:italic> sp. and <jats:italic>Hysterothylacium aduncum</jats:italic> were more prevalent in southern waters. Latitudinal environmental gradients and their influence on the distribution of both zooplankton and definitive hosts, could be determinants of the clines observed.</jats:p>

<jats:p>The occurrence of seven species of the larval parasitic nematode <jats:italic>Anisakis</jats:italic>, which can be used as a biological tag for hake <jats:italic>Merluccius merluccius</jats:italic> stocks throughout their geographical range, is reported. Hake were collected from 14 localities in the Mediterranean Sea and the Atlantic Ocean. <jats:italic>Anisakis</jats:italic> larvae (<jats:italic>n</jats:italic> = 1950), which were recovered, were identified to species by means of genetic markers (allozymes). Within <jats:italic>Anisakis</jats:italic> type I, the larvae of <jats:italic>A. pegreffii</jats:italic>, <jats:italic>A. simplex s.s</jats:italic>., <jats:italic>A. typica</jats:italic> and <jats:italic>A. ziphidarum</jats:italic> were detected, while within <jats:italic>Anisakis</jats:italic> type II, <jats:italic>A. physeteris</jats:italic>, <jats:italic>A. brevispiculata</jats:italic> and <jats:italic>Anisakis</jats:italic> sp. were identified. There were significant differences in the relative proportions of the various <jats:italic>Anisakis</jats:italic> species identified in hake samples from the Mediterranean Sea and Atlantic Ocean, suggesting the existence of different stocks of <jats:italic>M. merluccius</jats:italic> in European waters.</jats:p>

<jats:p>The use of parasites as biological tags allowed the identification of three stocks of Argentine hake <jats:italic>Merluccius hubbsi</jats:italic> 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 <jats:italic>Corynosoma australe</jats:italic> and <jats:italic>Grillotia</jats:italic> sp. 1. Among Patagonian hake, <jats:italic>Anisakis simplex</jats:italic> showed similar values of prevalence in all zones, fish from San Matías Gulf were characterized by <jats:italic>Kudoa rosenbuschi</jats:italic>, <jats:italic>Grillotia</jats:italic> sp. 1 and <jats:italic>Contracaecum</jats:italic> sp. Among all compared areas, San Jorge Gulf and the Patagonian Shelf were the most similar, sharing most of their biological indicators (<jats:italic>Grillotia</jats:italic> sp. 2, Pseudophyllidea sp. 1 plerocerci and <jats:italic>Hysterothylacium aduncum</jats:italic>); 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.</jats:p>

<jats:p>Three stocks of the Brazilian flathead <jats:italic>Percophis brasiliensis</jats:italic> were identified on the coast of Argentina and Uruguay using parasites as biological tags. A total of 177 fish were examined and 23 parasite species were found. Fish were caught in four zones: north of the Argentine–Uruguayan Common Fishing Zone (34°30′–36°30′ S; 53°30′–56°00′ W), south of the Argentine–Uruguayan Common Fishing Zone (38°08′ S 57°32′ W), El Rincón zone (39–41° S; 60–62° W) and San Matías Gulf (41°40′–42°10′ S; 63°50′–65°00′ W). Discriminant analyses allowed the identification of three discrete stocks in the four zones (86·44% of classified samples were correctly identified), with <jats:italic>P. brasiliensis</jats:italic> from both north and south of the Argentine–Uruguayan Common Fishing Zone clumping together, as a single stock. Some species were important in discriminating among groups, <jats:italic>Anisakis simplex</jats:italic> was related to southern areas, while <jats:italic>Grillotia</jats:italic> sp., <jats:italic>Corynosoma australe</jats:italic> and <jats:italic>Hysterothylacium</jats:italic> sp. were important in determining the position of fish from the Argentine–Uruguayan Common Fishing Zone. These results were corroborated by comparing parasite prevalence and abundance among zones. The main differences were observed in those comparisons involving fish from the San Matías Gulf. Fish from the Argentine–Uruguayan Common Fishing Zone were characterized by higher infections of <jats:italic>Grillotia</jats:italic> sp., <jats:italic>C. australe</jats:italic>, <jats:italic>Corynosoma cetaceum</jats:italic> and <jats:italic>Hysterothylacium</jats:italic> sp., while the samples from El Rincón and San Matías Gulf showed higher infections of <jats:italic>A. simplex</jats:italic>. Samples from San Matías Gulf were characterized by lower levels of parasitism for all other species. Differences in environmental factors and their influence on the distribution of zooplankton and other hosts in the food web may be differentially shaping the parasite community structure in each zone, resulting in identifiable stocks of the <jats:italic>P. brasiliensis</jats:italic>. The present study confirmed the existence of regional biological tags that delineated fish assemblages.</jats:p>