If freshwater resources are plentiful and stable, Oncorhynchus mykiss can choose to stay in freshwater as fully speckled rainbow trout; if food is scarce, water is too low or temperatures are too warm, the fish can opt to make an ardous journey to the ocean, transforming into their silvery anadromous forms. Unlike salmon, they return to natal streams in two runs —summer and winter-and can spawn more than once in their lifetime. But increasing temperatures, overfishing and numerous manmade obstacles threaten even the most resilient fish.