| dc.description.abstract | Abstract In many countries around the world, health problems related to food access are an increasing threat, and many people today still don t have access to sufficient protein and energy in their diet. In poor countries with growing populations fisheries will become important because of lower levels of investments and running costs. Nepal, a country rich in freshwater and high fish diversity should be able to develop the aquaculture sector, but the major bottleneck blocking fish availability seems to be an inadequate fingerling supply.Photoperiod is the main cue synchronizing the reproduction and the pineal organ is the mediator that converts the information of photoperiod into melatonin. Melatonin is secreted during dark, and because of this rhythmic manner it is found to be a time-keeping hormone, giving information about day length and season. Seasonally changing photoperiod is found to be an important cue initiating reproduction in temperate species. The understanding of the circadian axis and photoperiodic entrainment of reproduction have been used with success to manipulate the natural rhythm of spawning in many temperate species. Little is known about such biorhythm control in low latitude species, but a few recent studies have shown that tropical fish also can respond to photoperiodic changes by the use of the pineal system. This master thesis was initiated to investigate the plasma melatonin profile in copper mahseer (Neolissochilus hexagonolepis). Fish kept under natural photoperiod in the present study showed a rapid rise in melatonin at beginning of the dark period, after which the values gradually drop and reach low daytime levels at the beginning of light phase. For fish exposed to extended dark period, the melatonin levels showed a significant increase at the time for natural dark and a significant decrease at the time for natural light. A shift was observed in the melatonin pattern, with a peak in the melatonin level four hours after the peak was seen under natural photoperiod. The results presented in this study might indicate an intra-pineal oscillator capable of self sustain the melatonin pattern. Melatonin half- life was estimated to 10 minutes. Plasma cortisol levels were high during the experiments, but it is thought not to have had an influence on the plasma melatonin levels. | nb_NO |
