EFFECTS OF DIETARY LIPID SOURCE AND ULTRAVIOLET RADIATION ON GROWTH AND FATTY ACID PROFILE OF STEELHEAD, Oncorhynchus mykiss

Steatitis is a nutritionally induced disease characterized by inflammation of adipose tissue observed in terrestrial and aquatic species.  This disease is generally attributed to peroxidation of long chain polyunsaturated fatty acids (LCPUFAs) in animals consuming diets rich in fish and/or fish oil.  Both diet and ultraviolet (UV) light are thought to contribute to recurring steatitis outbreaks in salmonid hatcheries as the disease is often detected following transfer of juveniles from indoor hatchery buildings to outdoor rearing systems exposed to sunlight.  Diets used in these hatcheries contain fish meal and fish oil as primary sources of protein and lipid, respectively.  To characterize the effects of dietary lipid and UV radiation on steelhead (Oncorhynchus mykiss) growth, composition and steatitis, fish were fed for 10 weeks diets containing canola oil (CO), fish oil (FO) or moderately oxidized fish oil (OFO) and subjected to fluorescent or artificial UV light.  Utilizing a completely randomized factorial design, each treatment was assigned to triplicate groups of 100 fish initially weighing 1.6 g/fish.  Final growth responses and survival were unaffected by either lipid or light source individually, but weight gain was significantly affected by their interaction.  Whole body histopathology revealed significantly higher (more severe) steatitis scores in fish fed FO or OFO compared with fish fed CO.  Steatitis scores also were significantly higher in fish exposed to UV light compared with fish exposed to fluorescent light.  Whole body lipid concentration and tissue fatty acid profiles were significantly affected by dietary lipid and light source, but not by their interaction.  Results of this study suggest feeding diets low in LCPUFAs and minimizing exposure to UV light may reduce steatitis in steelhead.