Influence of a thermohaline front on the trophic ecology of the small pelagic fish Lycengraulis grossidens (Clupeiformes: Engraulidae)

Lycengraulis grossidens is economically important for artisanal fisheries and prey of top predators in the Río de la Plata (RP) (SW Atlantic). In particular, Samborombón Bay (SB)—RAMSAR site and key marine summer feeding area for the species—is strongly affected by climate variability. Diet composition, feeding strategy and dietary shifts related to fish size and to an existing thermohaline front between two SB zones were determined in late austral summer (April). A high incidence of brackish/marine prey was hypothesized, varying ontogenetically and in relation with the front. Trophic incidence exceeded 90%. Neomysis americana (Mysidacea) dominated the diet, followed by decapod larvae, gastropods and teleosts. The feeding strategy was specialist at the population level for N. americana but generalist for other prey. Both nMDS and GLM analyses supported the hypothesis, as N. americana , copepods and gastropods were more consumed in North SB, while teleosts South SB, linked to the thermohaline front affecting prey distribution. Smaller individuals (<100 mm) consumed copepods and decapod larvae, whereas larger individuals (>150 mm) increasingly fed on N. americana and gastropods. Unlike previous studies reporting piscivory at larger sizes, SB specimens maintained a zooplankton‐based diet, with a strong preference for N. americana . Results highlight the species' trophic plasticity and the influence of environmental and ontogenetic factors. This study provides valuable new information on the trophic ecology of L. grossidens in marine waters after a gap of over 60 years, contributing to a better understanding of its trophic role in highly dynamic estuarine and coastal ecosystems.