Analysis of tagging data has revealed that survival to adulthood of steelhead released from one state hatchery is generally 1.5 to 3 times higher than that of the same stock of steelhead released from a nearby hatchery. The mechanisms for the differences in post-release performance between fish reared in the two hatcheries are unknown. Different hatchery rearing protocols (densities, pond rearing, feeding rates, etc.) and different acclimation protocols have been theorized as possible explanations for these differences. Physiologists at Abernathy have analyzed gill samples collected from steelhead at both hatcheries to determine whether these differences could be related to the fishes' ability to transition from a freshwater environment to a saltwater environment which is essential for successful migration to the ocean. This information can be useful to improve adult returns in hatchery-reared steelhead.