Water‐based measurement of rainbow trout melatonin

Melatonin (MLT) was isolated from water samples using solid phase extraction cartridges and measured by radioimmunoassay. In tanks of rainbow trout Oncorhynchus mykiss , concentrations of MLT in water increased rapidly from <0·1 ng l −1 in the light phase to 0·7 ng l −1 in the dark giving calculated release rates of <1 ng kg −1  h −1 and 15 ng kg −1  h −1 , respectively. This cycle in water MLT values corresponds to reported changes in plasma MLT concentrations. The colour of the tanks and a daytime acute handling stress did not affect concentrations of MLT in either the light or dark phase.