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

Cortisol production in fry of rainbow trout (367-456 mg body weight) infect ed with the ectoparasitic monogenean Gyrodactylus derjavini (mean intensiti es 4.7 and 4.9 parasites per fish) was studied at two temperature levels, 4 -6 degreesC and 11-12 degreesC, respectively. Due to difficulties in obtain ing an adequate amount of plasma from the small sized fish, the corticoster oid concentration was measured in the body fluid recovered (as supernatant) after sonication and centrifugation of whole fry. Infected fry at 11-12 de greesC showed an elevated level of cortisol compared to uninfected fry. How ever, the cortisol concentration was lower than in fish exposed to handling stress. At 4 degreesC, the cortisol level in infected fish compared to uni nfected was insignificantly increased. The findings are discussed in relati on to the role of monogeneans as inducers of secondary infections.