Inter-stock congruence of long-term changes in macroparasite communities of a small pelagic fish ¿An effect of climate change?

The impact of climate change on the distribution and sustainability of pelagic fish remains poorly understood, even though it can cause drastic shifts in species distribution, reproduction, migratory behavior, and recruitment. In turn, parasites can reveal important details about the biology of their hosts and the changes they undergo. The aim of this study was to evaluate whether changes occurred in the parasite community structure of the Argentine anchovy, Engraulis anchoita, after three decades, during which environmental changes have been documented in the Argentine Sea. Parasite assemblages of anchovies captured throughout their distribution in the Argentine Sea in 2022 were examined and compared with those of anchovies collected in the same regions between 1993 and 1995. Multivariate analyses were performed by assigning fish to three spatial groups (belonging to two different stocks), identified from the first sampling period (Northern Buenos Aires, Southern Buenos Aires, and Patagonian). The dataset included 42,290 metazoan parasites belonging to 13 species. Samples differed significantly among groups and between periods. A decrease in parasite abundances was observed for most species across all three groups, resulting in similar patterns of dissimilarity between periods. The congruence of these changes across three groups over a broad geographic scale suggests that such variations are likely driven by large-scale processes, possibly climate change, rather than by random, short-term, cyclical, or local variability.