Melatonin is a hormone synthesized and secreted by the pineal gland. Its production is regulated by day length with increasing amounts of the hormone being produced in the autumn and winter, as daylight hours decrease. Melatonin has been shown to have many effects in vertebrates, from schooling behavior in fish to enhancing immune function in ground squirrels. Field and laboratory studies of mammals have shown that seasonal changes can affect various immune parameters. It has been suggested that melatonin serves to up-regulate the immune system in temperate climates as a natural defense against higher energetic demands and seasonal hardships associated with the winter months. There has been little research to date investigating whether melatonin could effectuate other physiologic and behavioral parameters in vertebrates. Research investigating the effects of melatonin enhancing growth or metamorphoses in vertebrates has provided inconclusive data. This study showed increased weight gain, length, and condition factor for melatonin-implanted fish compared to control fish at ten weeks post-implant. If melatonin is shown to enhance the growth rate and condition factor of fish, this hormone may also prove beneficial for the aquaculture industry as a food supplement, decreasing the time to market for several commercially viable species.