Distribuzione e abbondanza di Padogobius nigricans (Canestrini,1867) e P. bonelli (Bonaparte, 1846) nel bacino umbro del fiume Tevere

Padogobius nigricans is an endemic species to Tyrrhenian drainages of Central Italy. Its distribution range is highly fragmented due to habitat modification and competition with P. bonelli, an endemic specie to North Italy introduced in central Italy since ‘80s. The first occurrence of P. bonelli in Umbria Region dates back in 1996, when the species was found in two tributaries of River Tiber. The aim of the present study is to investigate the distribution and abundance of the two species in the River Tiber basin, in order to assess the current distribution of P. bonelli and the possible impact of its expansion on the P. nigricans populations. Samplings were conducted by electrofishing in 77 sampling stations located on 36 watercourses. P. bonelli resulted more widespread in the upper and middle part of the River Tiber basin. Several watercourses were entirely colonized by the non-native goby and the first local extinctions of P. nigricans were observed. Intact populations of P. nigricans were only found in some river stretches were the weirs prevented the colonization of P. bonelli upstream. Where the two species co-occur, P. bonelli is numerically dominant on P. nigricans and a low proportion of the juveniles of P. nigricans was recorded in the presence of P. bonelli.