which can result in an increased susceptibil- ity of other host species to parasites (Chubb, 1997; Galli, 2000). From recent data of ARPAT (2010-2013) the Massaciuccoli waters are dark, cloudy and eutrophic, with high concentrations of nitrogen and phosphorus from intensive farming around the lake and a high organic load. Similar adaptability of the goldfish has been observed in the water reservoir of Corbara (Umbria, Italy) (Pedicello et al., 2010).