Branchial release of free cortisol and melatonin by rainbow trout

Individual rainbow trout Oncorhynchus mykiss were held in a specially constructed tank that enabled water to be collected separately from the anterior and posterior ends of the fish. Measurement by radioimmunoassay showed that > 95% of the cortisol and melatonin released into the water originated from the anterior end (dominated by the gills). High performance liquid chromatography confirmed the identity of both hormones. (c) 2005 Crown copyright.