Four trials were performed to evaluate the effects of water temperature on critical aspects of L. salmonae development in rainbow trout (Oncorhynchus mykiss). The permissive water temperature range in which xenomas developed was between 9 degrees C and 20 degrees C. Parasite development was arrested at temperatures outside this range, as indicated by the absence of visible xenomas among exposed fish. In addition, when these trout were subsequently moved to temperatures within the permissive range, xenomas failed to develop. Water temperature, within the permissive range, had no significant effect on either the number of xenomas that formed or the proportion of fish that developed xenomas following gastric intubation with a standard dose of spores. The relationship between water temperature and xenoma onset-time was best described by polynomial regression analysis: onset = 320 - 33.4T + 0.9547Tsuperscript 2, where T is temperature ( degrees C). Xenoma onset rate was also described through a modified degree-days model, yielding a predictive equation appropriate for use under conditions of fluctuating temperature. The thermal units, expressed as days x ( degrees C above 7 degrees C) necessary for xenoma onset were 298.6 on average. Xenoma dissolution rates, from the time of onset, also appeared to have a trend; more rapid dissolution occurred as temperatures increased. However, this trend correlated minimally with regression models.
