Distribution, abundance and condition of invasive Ponto-Caspian gobies Ponticola kessleri (Günther, 1861), Neogobius fluviatilis (Pallas, 1814), and Neogobius melanostomus (Pallas, 1814) in the Sava River basin, Croatia

Abstract

Distributions, population densities, invasive potentials and adverse impacts of invasive Ponto-Caspian gobies on native fauna in the Sava River basin remain unknown, whereby 23 locations in the Sava basin were sampled during a three-year period (2011–2013). Among the five Ponto-Caspian goby species previously reported in Croatia, only Neogobius fluviatilis (291), Neogobius melanostomus (177) and Ponticola kessleri (21) specimens were collected. Proterorhinus semilunaris and Babka gymnotrachelus were not found. N. fluviatilis was dominant among the invasive gobies at the majority of locations and almost omnipresent (apart from the upper Sava reaches) in the Sava basin. N. melanostomus was found only in the navigable Sava reaches, but its distribution range still seems to be expanding. The status of P. kessleri invasion remains unclear, as it was only found in 2011 at three locations in the lower Sava reaches. No significant (P > 0.05) increase in average catch-per-unit-effort (CPUE) values was observed during the three-year period. There is some evidence that a decline in Gobio gobio populations might be expected in response to increasing N. fluviatilis population densities. The findings suggest that further range expansions by the Ponto-Caspian gobies can be expected in Croatia and the implementation of measures to limit their spreading should be considered.