A tropical fish community in thermal waters of southern Tuscany

In a small stream of southern Tuscany (Fossa Calda), fed by hot springs, we discovered a fish community dominated by tropical species, some of which have never previously been reported in Euro-Mediterranean natural freshwater environments. The aim of our research was to ascertain whether the three most abundant and widespread species (Amatitlania nigrofasciata, Hemichromis sp. and Oreochromis niloticus) have become established. Analysis of size class distribution and growth curves showed that the populations of these species are quite large and made up of juveniles, subadults and adults, as is the case in self-sustaining populations. These fishes were probably released intentionally, since they are widely used in aquariums and aquaculture and their survival in Fossa Calda was possible because of the constant high temperature of thermal waters. Spread to other streams seems unlikely. In fact, at some distance from the hot springs, where water temperatures are lower, populations of the tropical species were small.