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

The sea louse, Lepeophtheirus salmonis, is an ectoparasitic copepod of Atlantic salmon, Salmo salar L., capable of causing severe damage. 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 six times over the 29-d experimental duration and examined for alterations in the primary and secondary stress indicators, including plasma concentrations of cortisol, glucose, electrolytes, thyroid hormones T-3 and T-4, as well as the haematocrit level. The results were examined for correlations between the stress indicators, the number of copepods per fish and the 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 L-1 controls: 179.8 nmol L-1 for parasitized) and glucose (mean 3.545 mmol L-1 controls: 4.567 mmol L-1 for parasitized) levels were significantly increased due to the presence of the lice. This was believed to be a direct result of the sea lice development into the larger life stages, thus increasing the level of host damage.