Delayed expansion of Ponto-Caspian gobies (Pisces, Gobiidae, Benthophilinae) in the Nemunas River drainage basin, the northern branch of the central European invasion corridor

During the past few decades, rapid expansion of Ponto-Caspian gobies has been observed in the rivers connected to the central European invasion corridor. The Nemunas River catchment has been connected to this corridor for more than two hundred years, yet there is no evidence of Ponto-Caspian gobies in the Nemunas River. This study aimed to assess the presence of Ponto-Caspian gobies within the Nemunas River and its main tributaries, specifically in the Lithuanian part of the Nemunas and Neris Rivers, which may serve as possible routes for these fishes' expansion. Of all the presumptive PontoCaspian goby species only Neogobius fluviatilis (Pallas, 1814) and Neogobius melanostomus (Pallas, 1814) were caught. The relative abundance and biomass of both species was very low. The pilot analysis of piscivorous fish diet revealed that N. fluviatilis is already selectively consumed by Esox lucius. Although both species can reproduce and sustain viable populations in the Nemunas River, different scenarios for their future establishment within the Nemunas River drainage basin are predicted. The hypothesized distribution of N. melanostomus will remain restricted only to the outlets of the Nemunas River; while the distribution of N. fluviatilis in the future will be wider. We presume that the area colonised by N. fluviatilis will expand until the species establishes in the entire Nemunas River drainage system.