Protecting and restoring freshwater habitats for steelhead in the face of potential climate change impacts is a major challenge. Steelhead, protected by the Endangered Species Act, are the sea-going form of Oncorhynchus mykiss. The same species, isk nown as rainbow trout, when it reside life-long in freshwaters. Rainbow trout are not protected. The offspring of either form may opt for the opposite life-history, but the mechanisms for this are not well understood. Five coordinated, independent studies addressed aspects of how life-history expression relates to habitat, water temperature, sex, metabolic rates, and lipid levels. Scientists found that steelhead spawn in areas with both suitable spawning gravels and juvenile rearing habitat in proximity. Lipid accumulation was related to metabolic rate and life history: fish with lower metabolic rates and higher lipid levels tended to be residents (rainbows), whereas those with lower lipid levels tended to be steelhead, and female. Overall, 70 percent of steelhead were female, and this was true for 65 to 70 percent of steelhead from northern California to southeast Alaska to central Idaho. Lipid levels decreased at higher temperatures, and more fish were steelhead. This work makes a major contribution to scientists' understanding about life-history expression in O. mykiss and other species that exhibit similar variation in life-histories. It provides insight for the variation in life-history expression between sexes and challenges the notion that faster-growing fish tend to be sea-going steelheads.