TEMPERATURE-DEPENDENT POPULATION-GROWTH OF GYRODACTYLUS-DERJAVINI ON RAINBOW-TROUT, ONCORHYNCHUS-MYKISS

Rainbow trout were experimentally infected with the ectoparasitic mono genean 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 pos itively correlated with temperature in the initial part of the investi gation. At 18.7 degrees C growth of the parasite population was fast r eaching a peak abundance within 3 weeks, whereafter the infection leve l decreased significantly. Peak abundance was recorded in week 5 at 11 .6 degrees C. A very slow but steady parasite reproduction was noted a t 5.5 degrees C, but no peak abundance was reached within 6 weeks. The decline in parasite population after reaching peak levels is likely t o be caused by a host response. The host response is also indicated by the parasites' active selection of particular microhabitats. This ant i-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.