The physiological response of Atlantic salmon, Salmo salar L., to a single experimental challenge with sea lice, Lepeophtheirus salmonis

Lepeophtheirus salmonis is capable of causing severe damage to Atlantic salmon and this study was conducted to examine the physiological response of salmon to the stress of sea lice infestation. Smoltified salmon were acclimatized in 30% saltwater and exposed to high levels of lice infestation. The number of copepods per fish ranged from 15 to 285, with a mean of 106. The infested salmon were sampled 6 times over the 29-day experimental period and examined for alterations in primary and secondary stress indicators, including plasma concentrations of cortisol, glucose, electrolytes, thyroid hormones T3 and T4, and haematocrit level. The results were examined for correlations between stress indicators, number of copepods per fish and life stage of the copepods. The presence of L. salmonis elevated stress indicators in relation to the specific sea lice stage. By day 21, both cortisol (mean 63.1 nmol/litre for controls; 179.8 nmol/litre for parasitized) and glucose (mean 3.545 mmol/litre for controls; 4.567 mmol/litre for parasitized) levels were significantly increased due to the presence of the lice. This is believed to be a direct result of sea lice development into larger life stages, which increasing the level of host damage..