Diseases encountered in rainbow trout cultured in recirculating systems

Recirculating systems create unique environments for fish culture which may provide favorable conditions for disease occurrence or the reproduction of opportunistic microorganisms. Stressful conditions in recirculating systems, such as poor water quality or high stocking densities in the culture tanks, may contribute to disease outbreaks. Non-infectious problems, including high levels of ammonia, nitrites, carbon dioxide, suspended solids, or ozone residual levels have also caused mortalities in recirculating systems. The diseases encountered in rainbow trout (O. mykiss) cultured in recirculating systems include: those caused by bacteria (bacterial gill disease, furunculosis, bacterial kidney disease, fin rot), parasites (Gyrodactylus, Chilodonella, Trichodina, Epistylis, Trichophrya, Ichthyopthirius, Ichtyobodo, proliferative kidney disease, amoebic gill infestation, Coleps), fungi (Saprolegnia), and viruses (infectious pancreatic necrosis, viral hemorrhagic septicemia, and infectious hematopoietic necrosis). Treatments with chemotherapeutants in the water or feed in a recirculating system present special considerations; the main one is whether the biofilter will be treated and how the chemicals could affect its function. Management practices designed to prevent the occurrence of diseases or the degradation of water quality are critical to a successful recirculating facility. The introduction of known pathogens with infected fish should be prevented either by hatching eggs at the facility from disease-free broodstock, or by purchasing fingerlings from disease-free certified broodstock and by creating a quarantine period. Each recirculating facility should design a protocol for prevention of and control of fish diseases with the aid of a fish health professional, based on the generally accepted principles of fish health management.