Fish reproduction is influenced by various external environmental factors. Among these factors, changes of photic conditions are thought be most closely related to the reproduction of many fish species. In recent studies, melatonin that mediates the changes of photic condition outside has been used in an attempt to get a similar effect to photoperiod manipulation without changing photoperiod. However, the effects of exogenous melatonin vary depending on life stages. We have previously investigated the effect of melatonin on reproductive activity in mature Nile tilapia (Oreochromis niloticus). In this study, effects of exogenous melatonin on the spawning activity, the rate of mouth brooding success rate and early survival rate of lavae produced from melatonin treated Nile tilapia were investigated. Nile tilapia at the time of puberty were randomly divided into three groups and fed on control (0 ㎍/g), low-dose melatonin (2.5 ㎍/g BW) or high-dose melatonin (25 ㎍/g BW) diet. Fish were reared indoor tanks at 27～28℃ under natural-photoperiod and fed twice a day at 10:00 and16:00 during the experiment (120 days). Fish were sampled to examine growth and gonadal development on Day 30, 60, 90 and 120. There were no significant differences of somatic growth between control (no melatonin) and melatonin-treated groups, indicating that melatonin administration did not cause any negative effects on the growth of experimental fish. The patterns of gonadal development were almost the same between control and melatonin-treated groups. In juvenile tilapia, treatment with low-dose melatonin resulted in the increase of spawning rate, the rate of mouth brooding success and early survival rate of laval tilapia. However, high-dose melatonin resulted in the decrease of spawning rate, the rate of mouth brooding success and early survival rate of laval tilapia, although the earliest spawning occurred in one of the high-dose group fish. Fish in high-dose melatonin group failed to recover from reproductively suppressed state even after the cease of melatonin treatment, indicating that too much melatonin might act negatively on spawning activities and gonadal maturation in Nile tilapia. In conclusion, the results from this study suggest the possibility that a pertinent dose of melatonin treatment can effectively accelerate the spawning rate and the rate of mouth brooding success, and bring positive effects on early survival rate of tilapia lavae.

Photoperiod is known to be the most potent environmental stimulus for reproduction in many lower vertebrates, and it exerts an endogenous effect by causing a rhythmic change in melatonin level, which increases during the dark phase and decreases during the light phase. In recent studies, melatonin administration was capable of changing seasonal reproductive activities in several fish species, and also resulted in modulating the timing of smoltification in a salmonid fish (Amano et al., 2004; Ligo et al., 2005). In this study, we have investigated the changes of spawning activities (number of eggs, frequency of spawning) by oral administration of melatonin in the Nile tilapia, Oreochromis niloticus. Fish were obtained from a local fish farm (Chungju, Chung Buk) and acclimatized in fish rearing facilities in Sunmoon University (Asan, Chung Nam). Randomly selected 24 fishes (12 males, 12 females; 200～400 g in body weight, BW) were divided into three groups and each fish was accommodated into glass aquarium individually. Fish were fed either with control diet (0 ㎎ melatonin) or low-dose melatonin diet (0.3 ㎎ melatonin/㎏, BW) or high-dose melatonin diet (3 ㎎/㎏, BW). Fish were individually checked everyday for the presence of eggs in their mouths, and the eggs were removed and counted when found. Fish were fed twice a day, but melatonin treated diet was given only once a day at 10:00a.m.. The daily feeding ration was set at 2% of the body weight so that the fish ate all the pellets provided. Fish were sampled for blood parameters (Hb, Glu, GOT, GPT) on March 2 (Day 30), April 2 (Day 60), May 3 (Day 90), June 3 (Day 120) and July 14 (Day 160). The sampling was done between 14:00 and 16:00 p.m.. For the sampling, fish were anesthetized in 50 ppm benzocaine, and body length (BL) and BW were measured. Blood sample was collected using a heparinized syringe from the caudal blood vessel. Melatonin affects several photoperiodic signals including reproductive activities in fish. Low-dose melatonin administration appeared to disturb spawning and sexual maturation in Nile tilapia in this study. Fish treated with low-dose melatonin spawned less numbers of egg than the fish treated with high-dose. In contrast, high-dose melatonin administration caused more frequent spawning than low-dose melatonin. On the other hand, somatic growth and blood parameter were not affected by melatonin treatments. These results suggest that pertinent melatonin could be involved in providing photoperiodic information as short day photoperiod. Fish treated with over dose melatonin seemed to have no melatonin-free interval. In that circumstance the melatonin message might have not been conveyed as a proper signal.