The latest Paper of the Month for Parasitology is Parasite assemblages in volatile host stocks: inter- and intra-cohort variability restrict their value as biological tags for squid stock assessment and is freely available for one month. 

The Argentine shortfin squid, Illex argentinus, represents one of the largest fisheries in Argentina due to its commercial importance. According to the Food and Agriculture Organization, this species represented 16.4% of global squid catches in 2020. A common intersection between fisheries and parasitology lies in the use of parasites as biological tags to identify the population structure or stock composition of exploited resources. ‘Stock units’ refers to a group or population of specimens of the same species that occupies a particular geographic area and interbreeds. This concept is often used for assessing and regulating the exploitation of resources. Beyond being an important global food resource, I. argentinus occupies a prominent role as both predator and prey in marine ecosystems, so the assessment of their stocks is critical for both fisheries management and ecosystem conservation.

Parasites are ubiquitous components of biological systems and represent a wide diversity of taxa, necessarily establishing interactions with their host. Most of them have complex life cycles, with larvae of many species parasitising fishes and squids and adults living in top predators such as mammals, sharks, and birds.

So far, most of the studies using parasite tags to discriminate stocks deal with fish, with a comparatively lower number of studies considering invertebrates of commercial interest. The criteria for parasite tags have been established and redefined over the years, but finding parasites that exhibit all suitable features is rare; however, the residence time in the host is one of the crucial features to be considered. Thus, a validation of this methodology for squids, which exhibit short life cycles (~1 year), and high growth rates, is necessary. In fact, for regional fisheries of I. argentinus, knowledge about their parasites, and their value to discriminate stocks, is still limited.

Therefore, as a first step to evaluate the efficacy of parasites as indicators of stocks, we investigate the variability of parasite guilds within and between squid cohorts, considering the influence of both host and environmental factors. Our study focuses on squids belonging to the summer-spawning stock inhabiting the Patagonian continental shelf, between 43°S and 55°S, in the Southern Western Atlantic. An examination of 318 squids, from four samples, which represented three consecutive cohorts captured between 2020 and 2022, were examined for metazoan parasites.

We observed significant heterogeneity in the composition and structure of parasite communities, characterized by the dominance of short-lived parasites inhabiting the digestive tract, mainly larval cestodes. The influence of host size was evident, while there was no discernible effect of squid sex. We conclude that these variations are associated with recent changes in the squid habitats and diets, which occur during the different stages of development (ontogeny) and migrations. Consequently, interpreting patterns from samples that are spatially or temporally separated requires caution.Since many squid species exhibit similar biological characteristics, we recommend that the use of parasites as biological tags should be restricted to studies based on simultaneous sampling, while size or age must be considered for deriving proper conclusions.

The paper Parasite assemblages in volatile host stocks: inter- and intra-cohort variability restrict their value as biological tags for squid stock assessment by María Paz Gutiérrez, Delfina Canel, Paola E. Braicovich, Ana L. Lanfranchi, Manuel M. Irigoitia, Marcela L. Ivanovic, Nicolás I. Prandoni, Beatriz Elena and Juan T. Timi, published in Parasitology, is available free for a month.