Temperature dependent population growth of Gyrodactylus derjavini on rainbow trout, Oncorhynchus mykiss.

Rainbow trout were experimentally infected with the ectoparasitic monogenean Gyrodactylus derjavini at three temperatures, 5.5 degrees C, 11.6 degrees C and 18.7 degrees C. Subsequently, the infection level was monitored during the following 6 weeks. The population growth was positively correlated with temperature in the initial part of the investigation. At 18.7 degrees C growth of the parasite population was fast reaching a peak abundance within 3 weeks, whereafter the infection level decreased significantly. Peak abundance was recorded in week 5 at 11.6 degrees C. A very slow but steady parasite reproduction was noted at 5.5 degrees C, but no peak abundance was reached within 6 weeks. The decline in parasite population after reaching peak levels is likely to be caused by a host response. The host response is also indicated by the parasites' active selection of particular microhabitats. This anti-monogenean response developed fastest at 18.7 degrees C, more slowly at 11.6 degrees C and was not recorded within 6 weeks at 5.5 degrees C.