Non-invasive measurement of cortisol and melatonin in tanks stocked with seawater Atlantic salmon

This study examined the potential for non-invasive assessment of cortisol and melatonin status of seawater Atlantic salmon, by measurement of hormone release into the water using methodology developed recently for freshwater rainbow trout. Validation experiments demonstrated good recoveries of spiked cortisol (ca. 95%) and melatonin (ca. 85%) from seawater, that oxygenation of seawater did not affect recovery, and that only small losses occurred during freeze storage of samples. When groups of seawater Atlantic salmon were transferred to experimental tanks, water cortisol concentrations reduced over the first few days - indicating an acclimation response. Subsequent exposure to a handling stress caused a rapid 50-fold increase in water cortisol concentration, followed by a decrease. In a separate experiment, water cortisol levels were elevated in tanks of fish previously exposed to infectious pancreatic necrosis virus, and cortisol release rate was correlated to plasma cortisol level. Water sampling from tanks holding seawater salmon showed that water melatonin concentrations were elevated at night, conforming to expectation. These experiments demonstrate that the cortisol and melatonin status of seawater Atlantic salmon in tanks can be monitored non-invasively by water sampling, with the particular advantage that hormonal status can be tracked over time as the fish are not disturbed by sampling.