Outbreaks ofIchthyophthirius multifiliisin Redtail BarbsBarbus haasiin a Mediterranean Stream during Drought

In 2008, inland waterways in Catalonia (northeast Iberian Peninsula, Spain) experienced one of the worst droughts recorded in this region in recent decades. During this period, an epizootic of Ichthyophthirius multifiliis was detected for the first time in a Mediterranean stream, with 21% prevalence in a population of redtail barbs Barbus haasi. Environmental features and the fish population in this stream were compared during 2007–2009. Fish density and the average fish size were reduced significantly after the outbreak of I. multifiliis in this population. During 2008, parasitized fish were significantly larger than nonparasitized fish. In addition, a significant, positive correlation was found between parasite load and fish size. The origin of I. multifiliis is unknown, but an introduced species detected in April 2007 may have carried it. The combination of stress to the redtail barbs due to suboptimal conditions and favorable environmental conditions for parasite multiplication (e.g., suitable water temperature and low water flow) could have enhanced fish susceptibility to the parasite in April 2008. Further studies are needed to establish the incidence of freshwater fish diseases in Mediterranean watersheds, and water management policies should be reviewed to improve the conservation of native fish fauna