Effects of season, temperature, and photoperiod on plasma melatonin rhythms in the goldfish, Carassius auratus

Abstract: Effects of season, environmental temperature, and photoperiod on plasma melatonin concentrations were studied in the goldfish, Carassius auratus. When goldfish were reared under natural conditions, melatonin levels at mid‐dark exhibited seasonal changes, with higher levels obtained in June and September than in December and March. When fish were kept under light: dark (LD) cycle of 12: 12 at 5, 15, or 25°C during March‐April, temperature‐dependent increases in melatonin levels at mid‐dark were observed. When animals were maintained under LD 16: 8 or LD 8: 16 in combination with temperature changes (5, 15, and 25°C) during January‐February, the duration of nocturnal elevation in melatonin was controlled by the length of the scotophase while the amplitude was influenced by environmental temperature. These results indicate that plasma melatonin profiles in the goldfish exhibit seasonal changes that are regulated by both photoperiod and temperature.