||The problem of early sexual maturation among farmed Arctic charr and other salmonids can be effectively reduced by 24h light overwinter, provided it is bright enough to over-ride interference from the natural daylength cycle. To determine the threshold light intensity to suppress the nocturnal elevation of plasma melatonin, three groups of individually tagged fish (n=26-28/group ca. 1040g) were reared on 12h light: 12h dark (LD 12:12) and subjected to nighttime light intensities of either 50-65, 0.1-0.3 or 0 (control) lux for five months (November to April). Daytime light intensity was 720-750lx. Diel plasma melatonin profiles in both November and April were similar; mean daytime levels ranged from 20 to 100pg/ml, and nighttime levels were inversely proportional to light intensity. In the control group at 0lx, plasma melatonin increased about four-fold after lights-off, ranging between 320 and 430pg/ml. Nighttime light intensity of 0.1-0.3lx halved plasma melatonin levels to 140-220pg/ml, and 50-65lx further reduced the levels to one quarter of the control group, 68-108pg/ml. Among the lit groups, daytime plasma melatonin levels were about 20-30pg/ml, significantly lower than the nocturnal levels suggesting the diel hormonal rhythm was not completely abolished. Fish grew steadily from about 1100g to 1600g between November and April, independent of light intensity (P=.67). Overall, the study demonstrated the sensitivity of pineal melatonin hormone to different light intensities in Arctic charr.