Parasite infections in recirculated rainbow trout (Oncorhynchus mykiss) farms

Parasite infections in recirculated rainbow trout farms were monitored through 22 months. Parameters such as temperature, mortality, pH, nitrite and ammonia-concentrations, use of formalin, mortality and feed conversion ratio were also monitored. Due to introduction of rainbow trout from traditional earth ponds into the new systems, all farms were found to be infected with a number of parasitic organisms known from traditional farming. In some farms white spot disease caused by Ichthyophthirius multifiliis was associated with high fish mortality. Furthermore, Trichodina spp., Apiosoma sp., Ambiphrya sp., Epistylis sp., Chilodonella piscicola, Icthyobodo necator, Spironucleus salmonis, Gyrodactylus derjavinoides and the eye fluke Displostomum spathaceum were recorded. Findings indicated that seasonal fluctuations of parasite populations are partly masked by the effects of chemical treatment and a less variable temperature level throughout the year. Thus, stabile and low temperature of supplying well water resulted in lower fluctuations of parasite infections compared to traditional farms using river water. The study underlines the problems caused by introduction of infected fish from traditional earth ponds into recirculated systems. The study frames production advantages of using pathogen free fish for stocking in modern farms aiming to achieve pathogen-free production.