Pathogen Risk to Bull Trout Salvelinus confluentus Populations and Reintroduction Efforts in the Upper Willamette Basin, Oregon

Bull trout recovery efforts in the Willamette River basin include a reintroduction program that began in 1993.  This involved direct translocation of bull trout fry to recipient streams in 1993–2005 and a head-start captive-rearing program in 2007–2013.  In 2013, high mortality (82%) and a high incidence of deformities occurred among fry in captivity.  We used radiography, necropsy, and histology to assess factors adversely affecting these fish.  Bull trout reared in the hatchery in 2013 and 2014 exhibited deformities such as severe kyphosis and fusion of vertebrae.  Histology revealed heavy infections by metacercariae of the trematode parasite Nanophyetus salmincola directly associated with lesions.  We found no infections or deformities in specimens brought directly from the donor tributary, suggesting that deformities stemmed from N. salmincola surviving passage through the UV disinfection system in the hatchery.  We conducted surveys for the snail host of N. salmincola in the basin and began an ongoing effort to conduct fish health examinations on bull trout incidental mortalities and surrogate fishes.  Bull trout spawn in colder stream reaches upstream of the extent of the snails, but parasites such as N. salmincola may contribute to higher mortality among juvenile bull trout that move downstream too early.