EFFECTS OF CORTISOL IMPLANTS ON THE SUSCEPTIBILITY AND THE HISTOPATHOLOGY OF THE RESPONSES OF NAIVE COHO SALMON ONCORHYNCHUS-KISUTCH TO EXPERIMENTAL-INFECTION WITH LEPEOPHTHEIRUS-SALMONIS (COPEPODA, CALIGIDAE)

The effects of hydrocortisol implants on the susceptibility of naive coho salmon Oncorhynchus kisutch to infection with the economically important marine ectoparasitic copepod Lepeophtheirus salmonis was investigated under laboratory conditions. Cortisol-implanted coho salmon were more susceptible to infection than the control coho salmon. Copepods were lost from the gills of the control coho salmon by 10 d post-infection, and only a few remained on the body and fins at 20 d post-infection. Copepods were retained on the gills, fins and body of the cortisol-implanted coho salmon over the 20 d studied. Histological sections of control coho gills and fins revealed well developed epithelial hyperplasias and inflammatory responses to the presence of L. salmonis. The magnitude of the inflammatory response and the development of epithelial hyperplasia was suppressed in the cortisol-implanted coho. These data support the hypothesis that non-specific host defence mechanisms are important in resistance of coho salmon to infection with L. salmonis.