Abstract
In the Columbia River Basin (CRB), USA, anthropogenic factors ranging from dam construction to land use changes have modified riverine flow and temperature regimes and degraded salmon habitat. These factors are directly implicated in native salmon and steelhead (Oncorhynchus species) population declines and also indirectly cause mortality by altering outcomes of ecological interactions. For example, attenuated flows and warmer water temperatures drive increased parasite densities and in turn, overwhelm salmonid resistance thresholds, resulting in high disease and mortality. Outcomes of interactions between the freshwater myxozoan parasite, Ceratonova shasta, and its salmonid hosts (e.g., coho O. kisutch and Chinook salmon O. tshawytscha) are well-described, but less is known about effects on chum salmon O. keta, which have a comparatively brief freshwater residency. The goal of this study was to describe the distribution of C. shasta relative to chum salmon habitat in the CRB and assess its potential to cause mortality in juvenile chum salmon (listed as threatened in the CRB under the U.S. Endangered Species Act). We measured C. shasta densities in water samples collected from chum salmon habitat throughout the lower CRB during the period of juvenile chum salmon outmigration, 2018-2020. In 2019, we exposed caged chum salmon fry from two hatchery stocks at three C. shasta-positive sites to assess infection prevalence and survival. Results demonstrated: (1) C. shasta was detected in spawning streams from which chum salmon have been extirpated but was not detected in contemporary spawning habitat while juvenile chum salmon were present, (2) spatiotemporal overlap occurs between C. shasta and juvenile chum salmon in the Columbia River mainstem, and (3) low densities of C. shasta caused lethal infection in chum salmon fry from both hatchery stocks. Collectively, our results suggest C. shasta may limit recovery of chum salmon now and in the future.