The non-native goby Padogobius bonelli in the River Tiber, Italy and its effect on the reproductive potential of the native goby, P-nigricans

The goby Padogobius bonelli is naturally endemic to northern Italy, but was introduced to several rivers in central Italy outside of its native range in the early 1980' s. In these watercourses, the rapid expansion of P. bonelli coincided with the decline of the endemic (for central Italy) P. nigricans. The two species share the same ecological niche and in laboratory studies it was observed that P. bonelli successfully and consistently outcompeted P. nigricans for breeding sites, preventing its reproduction. However, no studies about the effects of the introduction of P. bonelli on the biology of P. nigricans have been conducted in the field. The aim of this study was to investigate some life history traits of P. nigricans, including size at maturity, gonado-somatic index, fecundity and egg size and compare these datasets in terms of presence and absence of P. bonelli. Specimens were captured in the River Aggia, a tributary of the River Tiber. The introduced goby is present only in the downstream portion of the River Aggia due to the presence of a weir; thus, below the weir P. nigricans and P. bonelli live sympatrically, while upstream of the weir only the native goby is found. A total of 780 specimens of P. nigricans were collected, 547 upstream and 233 downstream of the weir. Females downstream of the weir produced a significantly higher number of eggs than females upstream, in both abundance and in relation to size. Several juveniles of P. nigricans (0+) were recorded upstream, yet downstream no juveniles were found in the months after the breeding period, with only a few small specimens captured during the winter season. These results highlight the difficulties that P. nigricans encounters in reproduction, possibly as a result of the competition with P. bonelli which poses a serious concern for the survival of the P. nigricans populations.