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16 pages, 7 figures, 3 tables, supplementary material https://doi.org/10.3354/meps12962 ; The recent northward expansion of the round sardinella Sardinella aurita in the Mediterranean Sea has been documented as a consequence of rising sea temperature. At the same time, declines in sardine and anchovy biomass have been observed in the NW Mediterranean Sea, necessitating an assessment of whether the expansion of round sardinella may affect sardine and anchovy populations. Here, we combined stomach content and isotopic analyses to describe the trophic habits of round sardinella in the NW Mediterranean Sea and its trophic relationships with 2 sympatric small pelagic fish, European anchovy Engraulis encrasicolus and European pilchard Sardina pilchardus. Results revealed changes in the diet of round sardinella during the year. In summer, the most important prey were copepods (Acartia spp.) and cladocerans (Penilia avirostris). During winter, the diet was composed mainly of copepods and tunicates (mainly appendicularians), but microplankton was also numerically important in adult diets. In contrast to previous studies, during spring, round sardinella principally fed on salps (Thaliacea). To our knowledge, this is the first time that salps have been identified as an important prey for round sardinella. When compared to coexisting small pelagic fish, we found that round sardinella adults had a different trophic niche than anchovy and sardine. In contrast, round sardinella juveniles partially overlapped the trophic niche with the juveniles of the other 2 species. Therefore, the range expansion of round sardinella probably would not affect sardine and anchovy populations. Only in a situation of food limitation could juveniles of round sardinella compete with and affect both sympatric species. Our results provide new insights into the ecological role of this range-expanding species in the NW Mediterranean Sea, and highlight the importance of gelatinous zooplankton as prey ; We also thank the General Fisheries Commission for the Mediterranean Sea (GFCM) for their financial support to present this work at the International Symposium on drivers of dynamics o small pelagic fish resources. This study was funded by ECO-TRANS (CTM2011-26333) and PELWEB (ES-PN-2017-CTM2017-88939-R) projects. M.A.P. was funded by an FPI grant and a Short-Stay FPI grant (BES-2012-054267; Spanish Government). J.N. was funded by a Ramon y Cajal contract (RYC-2015-17809) of the Spanish Government ; Peer reviewed