Myxosporean Parasite (Ceratonova shasta) Prevalence of Infection in Klamath River Basin Juvenile Chinook Salmon, March – August 2023

Juvenile Klamath River Chinook Salmon (Oncorhynchus tshawytscha) were assayed for myxosporean parasite Ceratonova shasta from late March through mid-August 2023 by quantitative polymerase chain reaction (QPCR). Histology testing was not conducted in 2023.
Annual C. shasta prevalence of infection (POI) increased in 2023, compared to 2022. The annual C. shasta POI in 2023 by QPCR was 42%, compared to 39% in 2022.
Natural fish were monitored in real-time for the first 11 weeks of the season to provide timely data to water resource managers and basin cooperators. Ceratonova shasta was first detected by QPCR in fish sampled the week of April 30 (37% POI) from the Kinsman rotary screw trap (~ river mile 147).
In late May, natural fish collected from Kinsman had high C. shasta POI (93-95%), and 27% of those fish had severe infections based on the DNA copy number (> 3 logs of C. shasta DNA. The majority of fish collected in 2023 were of natural origin.
During real-time monitoring, gill tissue was also collected and tested at a later date. Ceratonova shasta detection in gill occurred at the same time as infection in the intestine. Gill tissue, in conjunction with Oregon State University water sampling data, may be a more informative real-time metric to assess disease risk in the Klamath River.
Iron Gate Hatchery released Chinook Salmon smolts into the Klamath River starting on June 8, 2023. Coded-wire tagged (CWT) Chinook Salmon originating from Iron Gate Hatchery were collected in the Klamath River from June 13 to August 16. The highest C. shasta prevalence of infection occurred in fish 3 weeks at large (WAL), while the mean DNA copy number was highest at 2 WAL.
In 2023, C. shasta was undetected in March and April, however infection did occur later in the year, especially during June and July. Infection occurred later and at a higher magnitude, compared to 2022. This could be related to cooler spring water temperatures, flow, or exposure dose which are all connected factors that influence infection and severity in salmonid.