Occurrence and distribution of exotic fishes in the River basin (Umbria-Italy) with special reference to new species recently introduced

aquatic ecosystems which leads to the qualitative decline of fish communities. In the river Tiber basin (Umbria-Italy) many exotic species were introduced in the last 25 years; currently 65% of fish species present in the watercourses of the Umbria region are exotic (Carosi et al., 2015). The purpose of the research was to analyze the occurrence and distribution of exotic fishes in the Tiber river basin with special reference to four fish species recently introduced: Gobio gobio (Linnaeus, 1758), Luciobarbus graellsii (Steindachner, 1866), Rutilus rutilus (Linnaeus, 1758) and Rhodeus sericeus (Pallas, 1776). Another aim was to analyze the ecological preferences of these species and their relations with the environmental characteristics in the investigated area. The study area comprised 92 watercourses of the Umbrian portion of the river Tiber basin; our analyzes utilized data collected during the periods between the 1990−1996, 2000−2006 and 2007−2014, in 171 sampling stations. A census of the fish fauna by elettrofishing was carried out at each sampling stations. Fishes were identified and for all specimens total length and weight were measured. 22 environmental parameters were used to characterize the river sectors (Figure 2). To analyze the relationships among environmental and fish species data matrices, the CCA analysis (ter Braak, 1986) was performed. 40 fish species were found. Only 14 were native; of the 26 alien species, 9 were translocated while 17 were transplanted (Table 1). Results showed that in the Tiber river basin there was a progressive increase over time of the average number of exotic species for each sampling station (Figure 1); the differences between the three periods considered were highly statistically significant at the ANOVA test (F=15.727, p=0.001). Gobio gobio, Rhodeus sericeus, Rutilus rutilus and Luciobarbus graellsii were present, respectively, in 14.05%, 8.78%, 0.05% and 0.01% of the total sites examined. Gobio gobio was introduced in the study area in 1999 and it was quickly established with a wide distribution in the middle and lower reaches of Tiber river and its main tributaries, namely Chiascio, Topino, Paglia and Nestore rivers. Also Rhodeus sericeus and Rutilus rutilus were quickly established in the Tiber river basin since their introduction occurred in 2003 and 2004 respectively, although their distribution is currently more fragmented than the gudgeon. The presence of Luciobarbus graellsii in the study area was detected for the first time in 2010 but it is currently present in the Tiber and in the Nestore rivers with abundant and well structured populations. The results of CCA analysis showed that fish assemblage composition varied along the longitudinal gradient according to environmental changes in a sequence in which the exotic species being located downstream in the respect of native ones; that areas were characterized by warmer waters, slow current rate and low water quality (Figures 2-3)(Lorenzoni et al., 2006). One of the main conservation threats to the native fish fauna is represented by the introduction of exotic species, that can determine the extinction of endemic ones (Simberloff, 2010; Smith et al., 2015) and can influence the functional composition of the fish communities. Recent studies, based on the use of relative weight to assess the impact of the presence of exotic species on the condition of native species in the Tiber river basin (Giannetto et al., 2012), showed that non native species seems to be a real threat to endemic fish species, as Barbus tyberinus Bonaparte, 1839 and Telestes muticellus (Bonaparte, 1837). All the four exotic species analyzed in the present study were quickly established in the Tiber river basin with self-sustaining populations. In a few years these species have significantly extended their distribution in the study area (Carosi et al., 2015) and are still expanding their ranges.