Myxosporean Parasite (Ceratonova shasta and Parvicapsula minibicornis) Prevalence of Infection in Klamath River Basin Juvenile Chinook Salmon, March – August 2018

Juvenile Klamath River Chinook salmon (Oncorhynchus tshawytscha) were assayed from late March to August 2018 by quantitative polymerase chain reaction (QPCR) and histology for myxosporean parasite infection of Ceratonova shasta and Parvicapsula minibicornis. During the first 8 weeks of the season, juvenile Chinook salmon were assayed in real-time for C. shasta. Fish were collected early in the week and processed for necropsy, DNA extraction, and QPCR, in order to provide timely data to fishery managers regarding flow management. Ceratonova shasta prevalence of infection (POI) exceeded the emergency dilution flow criteria of 20% in the Shasta to Scott (K4) reach on April 30th, the 6th week of the monitoring program.

Ceratonova shasta prevalence of infection by QPCR in Chinook salmon collected above the Trinity River confluence during the peak out-migration period (May-July) was 20%, lower than 26% observed in 2017, and 48% in 2016. Parvicapsula minibicornis prevalence of infection in Chinook salmon above the Trinity River confluence for the same time period was 92%, compared to 82% and 89% in 2017 and 2016, respectively.

Among the fish groups tested, naturally produced Chinook salmon had a 11% prevalence of C. shasta infection by QPCR, which was higher than the 5% observed in 2017 but lower than the 27% in 2016. The onset of infection (first detection) in 2018 occurred on April 23 when mean daily river temperature below Iron Gate Dam was 12.5°C. By histology, natural fish sampled from the Shasta to Scott (K4) and Scott to Salmon (K3) reaches from mid-April through the end of May had very low C. shasta POI (0-3%). Ceratonova shasta was not detected in six of the seven sample sets from the two reaches. Additionally, pathology scores were zero for six of seven sample dates, indicating infection levels were well below clinical disease levels in natural Chinook juvenile salmon in the two upper reaches through late spring.

In coded-wire tagged (CWT) juvenile Chinook salmon released from Iron Gate Hatchery from June 8-27, C. shasta was detected in 35% of fish screened by QPCR. The highest C. shasta prevalence of infection in marked juvenile Chinook salmon ranged from 42-51% in fish residing 2-4 Weeks At Large (WAL) at time of recapture. The infection pattern (infection within 2-3 weeks post exposure) is commonly observed for juvenile Chinook salmon entering the main stem Klamath River, under typical temperature regimes (15-18°C) at time of release (mid-May to early June).

In 2018, the annual C. shasta prevalence of infection, historical comparison, and POI in Iron Gate Hatchery CWTs were all lower in both QPCR and histology samples, compared to 2017. The only increase in C. shasta POI was observed in natural fish by QPCR. Parvicapsula minibicornis infection increased in almost every sample type by both QPCR and histology testing. In 2018, 61% of all fish sampled were infected with P. minibicornis only. Clinical pathology scores were zero in natural fish by histology, however clinical signs of P. minibicornis were observed during necropsy later in the sampling season, especially in fish collected in mid-July in both the Salmon to Trinity (K2) and the Trinity to Estuary (K1) reaches.

In 2018, the annual C. shasta prevalence of infection, historical comparison, and POI in Iron Gate Hatchery CWTs were all lower in both QPCR and histology samples, compared to 2017. The only increase in C. shasta POI was observed in natural fish by QPCR. Parvicapsula minibicornis infection increased in almost every sample type by both QPCR and histology testing. In 2018, 61% of all fish sampled were infected with P. minibicornis only. Clinical pathology scores were zero in natural fish by histology, however clinical signs of P. minibicornis were observed during necropsy later in the sampling season, especially in fish collected in mid-July in both the Salmon to Trinity (K2) and the Trinity to Estuary (K1) reaches.