Spring-run Chinook salmon, Oncorhynchus tshawytscha, of the Feather River in California, is a threatened species of economic, cultural, environmental, and recreational importance whose wild population has been rapidly declining. A major source of juvenile mortality is disease caused by the parasite Ceratonova shasta, a myxosporean with a complex life cycle and two mandatory hosts; salmonids, and the freshwater annelid worm Manayunkia occidentalis in which the parasite sexually reproduces. Salmonids and C. shasta have been extensively studied on the Feather River, yet the annelids have not, a crucial knowledge gap preventing informed hatchery spring-run Chinook release strategies. Previous studies have identified an ‘infectious zone’, right below an influx of water from a warm and shallow source, the Thermalito Afterbay Outlet (TAO) where parasite spores/L are significantly higher than elsewhere in the river. We hypothesize that there is a positive association between chlorophyll a and annelid population density. To investigate this claim, we will conduct a field collection study, water chlorophyll analysis, and estimate prevalence of C. shasta infection in the annelids through qPCR. Data will be analyzed using PCA to identify which environmental factors are most likely to explain M. occidentalis population density and prevalence of infections with C. shasta.