Investigation on the shelter occupancy by two Italian gobies, Padogobius bonelli and P.nigricans in streams of the upper River Tiber basin

Padogobius bonelli, an endemic goby to northern Italy, has established abundant populations in the River Tiber basin (Central Italy) and its expansion coincided with the decrease of the endemic P. nigricans. P. bonelli exhibits a more aggressive behavior than P. nigricans, and laboratory studies suggested that P. bonelli can outcompete P. nigricans for breeding sites, preventing its reproduction. The main aim of this study was to analyze the actual occupation of shelter sites with respect to the densities of the two species in 6 tributaries of the upper River Tiber in order to confirm on the field the hypothesis that P. bonelli would interfere with P. nigricans nesting. Fish of both species were collected by electrofishing to estimate the density and then the areas were accurately searched for nest sites. The stones containing batch of eggs were photographed and a sample of eggs from each nest was collected for genetic analysis in order to recognize to which species belongs the nest by a PCR-RFLP method. The preliminary results of the present research substantially confirm the superiority of P. bonelli on P. nigricans in the competition for the shelters. Depending on the adult’s densities, P. bonelli may come to occupy 97% of the nest site, and can gain a significant percentage of shelter despite the small abundance of individuals respect to P. nigricans. These findings arise serious concern on the conservation of P. nigricans increasing the alert about a potential further reduction of the populations of this species.