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

The sea louse, Lepeophtheirus salmonis, is an ectoparasitic copepod of Atla ntic salmon, Salmo salar L., capable of causing severe damage. This study w as conducted to examine the physiological response of salmon to the stress of sea lice infestation. Smoltified salmon were acclimatized in 30% saltwat er and exposed to high levels of lice infestation. The number of copepods p er 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 alt erations in the primary and secondary stress indicators, including plasma c oncentrations of cortisol, glucose, electrolytes, thyroid hormones T-3 and T-4, as well as the haematocrit level. The results were examined for correl ations between the stress indicators, the number of copepods per fish and t he 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 cort isol (mean 63.1 nmol L-1 controls: 179.8 nmol L-1 for parasitized) and gluc ose (mean 3.545 mmol L-1 controls: 4.567 mmol L-1 for parasitized) levels w ere significantly increased due to the presence of the lice. This was belie ved to be a direct result of the sea lice development into the larger life stages, thus increasing the level of host damage.