Triplicate groups of 75 rainbow trout, Salmo gairdneri Richardson, (mean weight 0–76 g) were fed a semipurified diet containing added niacinamide in amounts of 0.0, 1.25, 2.5. 5, 10, 20, 40, 80 or 160 mg/kg diet (ppm), for 16 weeks to determine the amount of niacinamide needed for optimum growth, feed conversion, and prevention of dermatopathy induced by UV light. At the end of the study, fish were exposed to a 275-W sun lamp limiting in the UV-B spectrum (280–360 UV) for 20h. Trout fed no supplemental niacinamide grew significantly slower than all other fish. Growth increased with each niacinamide increment through 10ppm. but not at higher increments. Feed conversion paralleled growth response to niacinamide. Exposure to UV light induced dermal lesions in trout fed less than 10ppm niacinamide. Initially, focal white spots appeared anterior to the dorsal fin, and caudal and pectoral fins became depigmented and translucent. Later, extensive erosion in caudal and pectoral fins was accompanied by desquamation of the epidermis and underlying tissues of the caudal peduncle, head and snout. Results indicated that rainbow trout need at least 10ppm niacinamide for optimum growth, feed conversion and protection against 'sunburn’caused by UV light.
