The effect of prolonged cortisol administration on the serum haemolytic activity of Atlantic salmon

The spontaneous serum haemolytic activity against trinitrophenyl-labelled sheep red blood cells was found to decline in adult Atlantic salmon, Salmo salar, that were implanted with cortisol-containing vegetable shortening. Fish that were implanted for 3 days demonstrated no significant decline in mean serum haemolytic ability. A significant decrease in haemolysis titre occurred in fish that were implanted with cortisol for 10 days and strongly correlated with individual fish cortisol levels. A further significant decrease occurred in fish that were implanted with cortisol for 17 days of which 63% demonstrated fungal lesions. Fish with long term implants had a similar incidence of fungal infection as well as increased mortality. The haemolytic activity of fish sera may be an important indicator of immunocompetence involving opportunistic pathogens such as fungi. The potential use of serum haemolytic activity as a biological indicator of chronic stress is discussed.