Understanding how larval habitat influences nutritional sources for Pacific lamprey is critical for successful rearing and conservation efforts. The proposed study will provide new insight into the nutritional pathways larval lamprey utilize to obtain nutrients when reared under differing conditions and how freshwater mussels may alter and enhance their nutritional uptake. These insights will benefit conservation management in multiple RMUs by improving lamprey artificial propagation outcomes for repopulation efforts and informing the selection of habitat for restoration, reintroduction, supplementation. Cultured larval lamprey primarily consume organic matter in the sediment resulting from bacterial conversion of feeds and other organic materials added to rearing containers. However, not much is known about this nutritional pathway and how other organisms within the environment contribute to and/or influence the diet of larval lamprey. Freshwater mussels such as the Western Pearlshell (Margaritifera falcata) enhance nutrient availability to the substrate and are often found in association with larval lamprey. Abernathy Fish Technology Center (AFTC) has observed that source water type (creek water vs well water) influences the variety of microorganisms colonizing rearing container sediment, which could lead to different organic matter pathways in rearing tanks. This study will examine changes in growth and stable isotope signatures to identify and measure the efficiency of nutrient sources utilized by larval lamprey under differing growing conditions. Larval Pacific Lamprey larvae will be reared in well or creek water, and in a polyculture with adult Western Pearlshell mussels to assess how symbiotic organisms influence the larvae food intake.