Evaluation of a management strategy to control the spread of Myxobolus cerebralis in a Lower Columbia river tributary

In October 2001, Myxobolus cerebralis, the myxozoan parasite that causes salmonid whirling disease, was detected in juvenile rainbow trout Oncorhynchus mykiss from a private hatchery that received water from Clear Creek, a tributary of the Clackamas River, Oregon. The Oregon Department of Fish and Wildlife closed the surface water portion of the hatchery in March 2003 and initiated a monitoring program to evaluate the success of the closure in containing further parasite spread. From 2002 to 2005, rainbow trout sentinels were held in live cages for 2 weeks at locations upstream, downstream, and within the area of the facility and then were tested for M. cerebralis infection. Infection prevalence in groups held in the hatchery pond, the outflow, and downstream of the facility was initially high; however, by May 2004 infection was no longer detected in Clear Creek, although the parasite continued to be detected in the hatchery pond. A single rainbow trout collected approximately IS river kilometers upstream of the facility in 2002 was infected with M. cerebralis. The parasite was not detected in fish collected from other portions of the Clackamas River drainage, indicating that the introduction was limited. Tubifex tubifex, the invertebrate host for the parasite, were abundant in the hatchery ponds, but only a single specimen was identified in the main stem of Clear Creek. Actinospores of M. cerebralis were only detected in the hatchery waters. The monitoring indicated that the parasite had not become widely established in Clear Creek and that partial closure of the hatchery removed the primary source of infection for fish