| Summary: | The partitioning of trophic niches is central for understanding the ecosystem processes associated with biological invasions. The recent successful establishment of Neogobius melanostomus in the Baltic Sea posed questions whether this invader found its own trophic niche, to what extent it competes with native benthivores, and if commercially important fish could be affected. In this study conducted along the Lithuanian Baltic coast, we used the stable isotope approach to identify potential trophic competitors of N. melanostomus. We also employed gut content analysis to quantify the incorporation of N. melanostomus into the diet of native piscivores. Furthermore, pre-invasion gut content data enabled us to check if there were any changes in the piscivorous fish diet after the invasion. We found N. melanostomus to be a subdominant benthivorous fish during the warm season of 2012–2013. It occupied a higher trophic position in spring (3.9) than in autumn (2.9), but spring specimens were also larger. The invader exploited a narrow (standard ellipse area 0.3–0.5‰2) and distinct core isotopic niche, but total niche area overlaps indicated potential trophic competition with Pleuronectes platessa, Platichthys flesus, Pomatoschistus minutus and Gasterosteus aculeatus in spring, and with P. minutus in autumn. The invader dominated the warm season (spring–autumn) diet of piscivorous fish (Gadus morhua, Myoxocephalus scorpius, Perca fluviatilis, Scophthalmus maximus) and its invasion was generally associated with decreased crustacean and increased fish-prey content. Therefore, it is plausible that N. melanostomus has created novel trophic links within the food web.
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