Feeding Ecology of Rough Scad Trachurus lathami in the South West Atlantic Ocean: Seasonal Diet Variability in Response to the Río de la Plata Plume Salinity Shifts

This research examines Trachurus lathami diet composition and ontogenetic shifts (stomach content analysis and multiple hypothesis modeling) in the Northern Argentinean Continental Shelf (NACS, 34°–41° S, Southwest Atlantic Ocean), related to season (warm: October–March; cold: April–September) and water salinity (low < 33.5, ls; high ≥ 33.5, hs), influenced by the Río de la Plata plume. A diet shift with fish size and a preference for larger prey during the cold season, linked to offshore distribution in hs waters, are hypothesized. Copepods dominated the diet, with larger species (> 1.5 mm, e.g., Calanoides carinatus) prevalent in hs waters and smaller species (< 1.5 mm, e.g., Oncaea spp.) in ls waters. Amphipods (Themisto gaudichaudii), decapod, and barnacle larvae were significant in the warm season, and the former was absent in the cold period. Ontogenetic shifts were minimal, being copepods the primary prey across sizes. Results indicate T. lathami is a generalist feeder, with diet changes driven by seasonal copepod availability linked to the Río de la Plata plume dynamics. These findings underscore the importance of long-­term monitoring amid NACS warming and prey shifts to support fisheries´ sustainability, given T. lathami rising biomass since the mid-­90s and interactions with commercial pelagic species.