One major challenge in Asian aquaculture is the limited availability of carp fries. In order to establish out of season supply of fries, knowledge about the pineal melatonin clock and calendar function, and its influence on the reproductive system, is needed. Three experiments were carried out to examine pineal melatonin dynamics of mrigal carp (Cirrhinus mrigala), under natural outdoor, and manipulated photoperiods. Plasma melatonin profile of the mrigal carp kept under natural outdoor photoperiod reached a peak early in the dark phase, and then showed a tendency of falling plasma melatonin levels until daytime levels were reached at first light. The second experiment intended to induce production of melatonin at different times during the light phase, and gave production of melatonin only in late afternoon. This indicates a refractory period of the pineal gland during daytime, and that production of melatonin might be controlled by a circadian clock. In the third experiment, animals were exposed to continuous darkness. The resulting plasma melatonin profile was in accordance with subjective darkness and light, with a peak late in the dark phase. This further supports the assumption that a circadian clock controls pineal melatonin production and secretion in the mrigal carp.