The effects of different daytime light intensities on the diel patterns of plasma melatonin in fingerling and broodstock rainbow trout (Oncorhynchus mykiss)

Diel plasma melatonin levels were measured to determine the effects of varied light intensities in juvenile and adult rainbow trout. This study showed that night-time melatonin plasma concentrations were significantly higher (P < 0.05) in juveniles (600±75 pg/mL to 492±29 pg/mL) than broodstock (314±3 pg/mL to 265±55 pg/mL) at 1250 lux. Light intensities of 700 lux were unresponsive in both groups. This amplitudinal difference between juvenile and adult fishes response to light may suggest a developmental importance of melatonin.