Oncorhynchus mykiss are a highly sought after recreational fish as well as an important species in their ecosystems and are endemic to much of the Northern United States. Due to their importance, O. mykiss is a key hatchery species and because of an unknown complication, many hatcheries experience low return rates. Otoliths are an important part of a fish’s body as they play a key role in hearing and sensing vibrations in the water around the fish. Hatchery raised O. mykiss often experienced large numbers of fish whose otoliths were found to be composed of a mineral called vaterite, which is less dense and may lead to issues with hearing and balance. The aim of our study is to figure out why large amounts of hatchery O. mykiss develop vaterite on their otolith. To test this hypothesis, we 160 fish, (4-5 days post-hatch) and placed into replicate mesh cages held in a flowthrough raceway at Castalia State Hatchery. Cages were randomly assigned to one of 3 commercial diet types (Low, Medium, or High protein/lipid) and one of two diet ration sizes (&fraq12; or full ration based on the hatcheries normal ration). Each combination was done in triplicate. Fish were sampled every two weeks for weight and length and fish had otoliths extracted and saved for analysis under light microscopy. After 3 months, we found that all groups had vaterite presence, however the highest was found in groups feed the full ration diets. Although this experiment did not give a definitive answer, it served as a great study to observe otolith configuration and vaterite presence in juvenile fish and can help lead us into the next step of finding the solution to this problem.