In the present study, the melatonin levels were measured in pearl mullets (also known as Van fish) and trouts. Pearl mullets grow naturally in Lake Van in Turkey, and trouts, raised by private fish farms, were used as study materials. These fish are known to secrete melatonin, a hormone associated with development and stress, as occurs in human beings. Melatonin was measured in the plasma taken from living pearl mullets and trouts by an ELISA method using commercial kits. The study was conducted in accordance with the ELISA kit procedure. In the spring, the mean melatonin levels of trouts were 160.8 ± 4.090 pg/mL in light and 198.3 ± 3.694 pg/mL in dark, whereas in winter 86.69 ± 3.203 pg/mL and 128.9 ± 1.999 pg/mL in light and dark, respectively. In pearl mullets, the mean melatonin levels were 84.73 ± 2.281 pg/mL in light and 118.1 ± 2.560 pg/mL in dark in spring, and in winter 36.32 ± 1.627 pg/mL in light and 70.53 ± 2.919 pg/mL in darks (p < 0.001).The results emphasized the significance of the photoperiod.