Effects of dietary lipid and light source on steatitis in steelhead, Oncorhynchus mykiss

The primary lipid source in feeds used at Pacific salmon (Oncorhynchus spp.) and steelhead (anadromous O. mykiss) conservation hatcheries in California and the Pacific Northwest United States is marine fish oil which is characterized by high levels of long-chain polyunsaturated fatty acids (LC-PUFAs). As these fatty acids are prone to peroxidation, the high marine fish oil content of salmonid feeds is likely a contributing factor in a disease called steatitis which is an ongoing issue at some hatcheries. Dermal lesions associated with steatitis have been observed when affected fish were transferred from enclosed hatchery buildings to outdoor rearing units exposed to sunlight. Therefore, a two-factor study examining possible interactive effects of dietary lipid source and ultraviolet (UV) radiation on growth responses, histology and tissue fatty acid profiles of juvenile steelhead was conducted. For 10 weeks, fish were fed diets containing canola oil (CO), fish oil (FO) or oxidized fish oil (OFO) while exposed to fluorescent or UV light. Each treatment was randomly assigned to triplicate groups of 50 fish initially weighing 1.6 g/fish. The main effects of lipid and light source on growth responses and survival were not statistically significant, but percent weight gain was significantly affected by interaction of the main effects. Whole body histopathology revealed significantly higher (more severe) steatitis scores in fish fed FO or OFO compared with fish fed CO and in fish exposed to UV light compared with fish exposed to fluorescent light. Whole body lipid concentration and tissue fatty acid profile were significantly affected by lipid and light source but not their interaction. These results suggest reducing dietary LC-PUFAs and minimizing UV light exposure may reduce steatitis in steelhead.