How does artificial light affect the growth of fish? To clarify this mechanism, it is important to mention the common endocrine mechanism in fish.  The environmental factors such as temperature, rainfall, photoperiod, water current stimulate the hypothalamus of fish to synthesis gonadotropin releasing hormone (GnRH), consequently triggers the pituitary to synthesis gonadotropin hormone (GTH). GTH then stimulates the gonad to mature by synthesizing gonadal hormone. This is also called brain-hypothalamus-pituitary-gonad axis (B-P-G). To do this mechanism, fish needs energy; ultimately energy is partitioned for maturation rather than growth. It is widely accepted technique in aquaculture to improve growth of fish by producing sterile fish such as hybrid, triploid etc. This sort of genetic manipulation is not only expensive but also species specific. Considering this point environmental manipulation is more useful to inhibit maturation by affecting on B-P-G axis mechanism, can be improved the growth of fish. Artificial lighting is widely used in industrial aquaculture to control maturation, improve growth of fish (Porter, et.al., 2003, Taylor, et.al., 2005 and Wilkinson, et.al., 2010). Reducing the incidence of sexual maturation by photoperiod manipulation improved growth of fish (Duston et. al., 2003)

There are some monoamines acts as neurotransmitters (endogenous chemicals that transmit signal from neuron to cell through synapse) in fish such as-

• Catecholamine
  • Dopamine,
  • Epinephrine (adrenaline),
  • Norepinephrine ( Noradrenaline)

• Melatonin
• Serotonin
• Histamine etc

Though all monoamines are important for their varieties of physiological function, only the function of dopamine and melatonin has been taken in to account for this small discussion. Dopamine controls the movement of animal, inhibits the function of GnRH, is related to unpleasant mood, cause Parkinson’s disease (if low level) and Schizophrenia (if high level). On the other hand, melatonin (also called, hormones of darkness) receives dark and light information, regulates circadian (day-night cycle in animal) rhythm, remove insomnia, improves immune system, positively regulates the activity of GnRH. However, serototin converts to melatonin and regulates sleep, improves pleasant mood. Available research found that dopamine level in the brain of animal is high at day time and both serotonin and melatonin level is high at night time, indicated that light has a direct effect on them. Delgado, et. al. (2001) described that highest level of dopamine is released at day time in clawed frog, Xenopus laevis, while lowest in darkness. On the other hand, melatonin rhythmicity in fish is inversely related with light intensity (Bromage, et.al., 2001).

After exposing the fish in the long artificial light condition, dopamine level of fish brain is increased. It is notable that Dopamine is a gonadotropin-release inhibitory factor which blocks spontaneous and GnRH-stimulated GtH release in fish (Lin, et.al., 1989). Similarly, long artificial lighting also reduces the level of melatonin in the fish brain. Gosh and Nath (2005) showed that melatonin controlled the reproduction of cat fish (Clarias batrachus). Melatonin level is affected by photoperiod, which controls the maturation of farm fish (Porter, et. al., 2000). Artificial lights suppressed melatonin circulation and significantly affected the daily variation of luteinizing hormone (LH- triggers ovulation) (Bayarri, et. al., 2004). By this way, constant photoperiod (artificial light) showed the improvement of growth of fish by inhibiting the sexual maturation (Davie, et.al., 2007). Expose the fish to shorter or longer artificial lights are used to advance or delay gonadal maturation and spawning time; when maturation can be delayed or inhibited until marketable size is reached, growth reductions and flesh quality deteriorations can be avoided (Bromage, et. al., 1995; Bromage, et. al., 2001; Bayarri, e.t al., 2004).
However, constant artificial light can cause stress for fish. Thus research needs to be done to confirm suitable photoperiodic length in targeted culture species in Bangladesh. Since most of aquaculture fish species in Bangladesh depend on the phytoplankton production of the pond, artificial light can also improve phytoplankton production to increase food availability in pond.