Biological and ecological data on an established rainbow trout (Oncorhynchus mykiss) population in an Italian stream

The rainbow trout Oncorhynchus mykiss (Walbaum, 1792) is probably the most widely introduced fi sh species in the world, but interestingly few populations have acclimated and reproduced outside the native area. This work provides biological and ecological data on one rare European self-sustaining population through three-years of data collection in the Lemme Creek (Northern Apennines, Italy). Regular reproductive events were recorded and analysed: gonad maturation, the presence of nests, juveniles and the rearing of alevins from naturally hatched eggs confi rmed that the reproductive period is in the fi rst months of the year. Biometric and population parameters are comparable to values recorded for the species, with a density of 0.60± 0.21 individuals/m2, a biomass of 5.18 ±2.75 g/m² and a weight/length relationship W = 5 E- 05L^2.69. We also provide the fi rst data on the diet of an acclimated European rainbow trout population (most studies regard hatchery-reared rainbow trout): gut content analysis revealed a high trophic spectrum, with evident seasonal differences. We suggest that interactions among several factors played a main role in the invasion and recruitment success. The lack of reproductive capacity has long been considered an interesting asset of this sport-fi shing species, so that introductions of rainbow trout in European waters have been encouraged for many years and intentional releases still occur. Our study demonstrated that, in some cases, O. mykiss can constitute self-sustaining, stable populations, able to survive over a long time period and to colonise areas that naturally lack other salmonids. The ability to create naturally reproducing populations may represent a local threat to the rich biological diversity of the area.