Seasonal patterns in fish species composition in the northern coastal zone of El Rincón.

El Rincón is a highly productive and oceanographically complex system on the inner shelf of the northern Argentine Sea, supporting key habitats for numerous fish species. Its coastal fringe includes ~150 km of exposed sandy shoreline, notably the beaches of Monte Hermoso and Pehuen‑Co, where this study was conducted. The regional climate is temperate, with cold winters (mean 8 °C), warm summers (25 °C), and mild spring and autumn conditions (14–15 °C). This study aimed to describe seasonal changes in the composition and relative importance of fish species inhabiting the coastal zone of El Rincón. Data was obtained in collaboration with artisanal fishers operating in the area. Seven seasonal expeditions of five consecutive fishing days were conducted between May 2022 and October 2023, spanning two winters, two springs, and two autumns. During each expedition, species composition and the number of individuals per species were recorded at landing. Catch per unit effort (CPUE) was standardized as individuals per 100 m of gillnet per day. For each season and year, an Olmstead–Tukey diagram (OT) was applied to classify species based on CPUE and frequency of occurrence. Across 38 fishing trips, 32 fish species were identified: 12 chondrichthyans and 20 osteichthyans. The OT indicated two dominant species, five frequent species, and 25 rare species. Mustelus schmitti (IUCN: Vulnerable) was dominant in all seasons in both years, with the lowest CPUE in spring 2023. Sciaenidae species also showed dominance. Notably, Squatina guggenheim  (IUCN: Endangered) was frequent, whereas Galeorhinus galeus   (IUCN: Cricially endangered)  was frequent only in spring, and Genidens barbus appeared as occasional in autumn 2023. Species turnover between years was evident, with several rare taxa appearing in 2023 and others absent relative to 2022. Sampling bias is possible when fisheries data is used as a proxy for species composition. For example, some species known to exist in the area were not caught, like the patagonian grouper (Acanthistius patachonicus) or the flounder (Paralichthys orbignianus).