A simple technique used to filter and quantify the actinospore of Myxobolus cerebralis and determine its seasonal abundance in the Colorado River

We used a simple technique for filtering the actinospores of Myxobolus cere bralis from natural waters to observe seasonal periodicity at eight sites i n the upper Colorado River drainage. We used a tub lined with 20-mum-mesh P ecap screen to concentrate actinospores from 1,900-L samples and estimate d ensity by microscope count. Identity of the observed actinospores as those of M. cerebralis was confirmed in 86 samples by the use of a polymerase cha in reaction test. The 42-ha Windy Gap Reservoir appeared to be a point sour ce of actinospores; the highest densities observed were consistently from s amples taken at sites just below the reservoir. Both densities and the freq uency of detection were much lower 26 km below the reservoir. The actinospo res first appeared in abundance after the runoff in both years of the study . Actinospore densities tended to be greatest during summer and early fall and lowest during spring. In August 1997, a series of significant how fluct uations and attendant water temperature swings appeared to alternately inhi bit and stimulate the release of actinospores. Populations of rainbow trout Oncorhynchus mykiss continue to suffer recruitment failures throughout the study reach, apparently because of the effects of whirling disease in age- 0 fish. This suggests that the detection of low numbers of actinospores by this technique at some sampling locations may indicate a level of infectivi ty that is destructive for the susceptible rainbow trout.