Remarkable monkey goby expansion through a new invasion route into submountain rivers

The monkey goby, an invasive Ponto-Caspian species, recently extended its range to the west and north in the Vistula River system. Dispersal of the monkey goby upstream in this river system to its Carpathian tributaries was not expected to occur due to its habitat preferences. Preferring slow- to medium-running rivers with sandy substrates, the conditions in submountain rivers with fast water currents and coarse-grained bottom substrates probably prevented the monkey goby from expanding. However, the long periods of low discharge, which recently became frequent in the Carpathians, caused a significant part of the riverbeds to have shallow sections with slow-flowing water. This could have been an impulse to launch a “second wave of invasion” of the monkey goby in the Vistula River system. Catch data from 2022, indicated that the range of the monkey goby expanded by ~ 80 km upstream of the Vistula River and ~ 260 km upstream in the San River compared to previous catch data. The findings in the San River are the first known locations of this species in the Carpathians. Under the assumption that low water levels in the Carpathian rivers will continue to occur in the coming years, the monkey goby may increase its numbers and become dominant in the sections near the banks of these submountain rivers, which are also the habitats of native fish fry.