Distribution and Abundance of the Aquatic Oligochaete Host Tubifex tubifex for the Salmonid Whirling Disease Parasite Myxobolus cerebralis in the Upper Colorado River Basin

Benthic invertebrate samples were collected from six sites in the upper Colorado River basin in north‐central Colorado to determine the distribution, abundance, and infection rates of Tubifex tubifex, the aquatic oligochaete host for the salmonid whirling disease parasite Myxobolus cerebralis. The disease has been implicated as a factor in severe recruitment declines in wild rainbow trout Oncorhynchus mykiss in this drainage. In Windy Gap Reservoir, T. tubifex constituted 73% of the oligochaete community, compared with 53% in the Colorado River upstream from the reservoir and 33% in the river downstream. Total oligochaete densities were slightly higher in the reservoir than above or below the reservoir. About 1.2% of the worm population was infected with M. cerebralis in the reservoir; infection rates were similar in the river. In Willow Creek, a tributary to the upper Colorado River, 73% of the oligochaetes at two sites downstream of cattle grazing and riparian‐area gravel mining were T. tubifex, compared with 54% at an upstream site. Total oligochaete densities were also significantly higher at the midstation site, just downstream of the area of heaviest riparian use. Samples from all three sites on Willow Creek also tested positive for M. cerebralis, with an overall infection rate of 4.3%. Infection rates in both Windy Gap Reservoir and Willow Creek declined over the summer. Careful management of riparian habitats and watersheds may be useful in mitigating the effects of whirling disease in salmonids by eliminating conditions that result in high T. tubifex abundance (such as siltation and nutrient enrichment in streams). Monitoring infection rates in worm populations can be useful in identifying sources of M. cerebralis spores for worms and sources of infection for fish.