Effect of Gyrodactylus derjavini infections on cortisol production in rainbow trout fry.

Cortisol production in fry of rainbow trout (367-456 mg body weight) infected with the ectoparasitic monogenean Gyrodactylus derjavini (mean intensities 4.7 and 4.9 parasites per fish) was studied at two temperature levels, 4-6 degrees C and 11-12 degrees C, respectively. Due to difficulties in obtaining an adequate amount of plasma from the small sized fish, the corticosteroid concentration was measured in the body fluid recovered (as supernatant) after sonication and centrifugation of whole fry. Infected fry at 11-12 degrees C showed an elevated level of cortisol compared to uninfected fry. However, the cortisol concentration was lower than in fish exposed to handling stress. At 4 degrees C, the cortisol level in infected fish compared to uninfected was insignificantly increased. The findings are discussed in relation to the role of monogeneans as inducers of secondary infections.