Steelhead trout (Oncorhynchus mykiss) inherited a complicated life from their ancestors. Small trout leave the streams of their birth and swim sometimes hundreds of miles to reach the ocean, where they may feed (and be fed upon) for several years. When the urge to reproduce hits, a mature fish strikes out for the spawning grounds of his or her birth. But at this point in the story, many of these trout run headfirst — literally and figuratively — into a major challenge: a dam.

In the San Francisco Bay, for example, dams and flood control measures have excluded steelhead from the upper reaches of Alameda Creek for almost 50 years. A project to restore steelhead to their native run along this creek will build a fish highway to bypass these obstacles — and will get a boost in population size from the long-lost relatives of the native steelhead: rainbow trout.

Evolution bestowed upon the steelhead lineage two alternate life history strategies: either migrating to the ocean, or remaining in freshwater streams without migrating — migrators are called steelhead and homebodies are called rainbow trout. The native steelhead trapped behind the newly built Alameda Creek dams in the 1950s and 60s became resident rainbow trout — and their descendents still survive there today.

Biologists predict that when the fish highways are up and running, steelhead and the native rainbow will be able to interbreed since they share recent common ancestry. And hopefully, once the steelhead and Alameda Creek’s rainbow trout are reunited, the lineage will sustain itself, with some fish remaining in residence and others venturing into the open ocean and returning to spawn.