Nonrelationship between plasma melatonin and background adaptation in the rainbow trout ( Salmo gairdneri)

Adult female rainbow trout ( Salmo gairdneri) were maintained outdoors on black, white, and neutral colored backgrounds. Reflectometry readings were used to assess the color adaptation of the fish. Plasma samples were collected from the same individuals both before and after exposure to the backgrounds, as well as at both 1200 and 2400 hr. Using a sensitive radioimmunoassay, no significant differences in plasma melatonin levels could be detected for the fish kept on the three backgrounds. Since the fish did show significant color modification in response to 18–21.5 days on the backgrounds, it is concluded that melatonin may not participate in background adaptation in trout. A consistent nocturnal elevation in melatonin levels was noted on all backgrounds. The stress associated with taking multiple blood samples from the same fish may have caused a significant increase in post-treatment compared to pretreatment nocturnal melatonin titers.