Prevalence of Myxobolus cerebralis at Juvenile Salmonid Acclimation Sites in Northeastern Oregon

Abstract

Fry of rainbow trout Oncorhynchus mykiss held as sentinel fish became infected with Myxobolus cerebralis when held at all juvenile acclimation sites for spring Chinook salmon O. tshawytscha and steelhead O. mykiss (anadromous rainbow trout) and thus expand the known range of M. cerebralis in northeastern Oregon. In spring 2001, replicate cages of rainbow trout fry were placed at the intake site of each facility and sampled at approximately 14, 28, and 50 d. Infection prevalence, as determined by the presence of M. cerebralis DNA following polymerase chain reaction (PCR) amplification, was relatively low (<17.5%) at most acclimation sites. However, a significantly higher prevalence of infection in sentinel fry held at the Wallowa facility (65%) identified this site as presenting a higher risk for juvenile steelhead to M. cerebralis exposure than other sites. Further, PCR analysis of ossified cranial elements from steelhead juveniles held at the Wallowa facility for 6 weeks demonstrated a comparable prevalence of infection, 52%. Detection of similar infection prevalence in fry and juveniles at the same site demonstrates the usefulness of sentinel exposures as a surrogate for lethally sampling valuable salmonid juveniles. In addition, this study suggests that infection of anadromous salmonids may complicate management efforts to contain the parasite within northeastern Oregon.