In the vast labyrinth of the West Coast’s largest freshwater tidal estuary, one native fish species has never been so rare. Once uncountably numerous, the Delta smelt since 2016 has largely vanished from most annual sampling surveys. But in December, state and federal biologists began for the first time ever releasing captively bred adult Delta smelt into the Sacramento-San Joaquin Delta as part of a three-year effort to draw the species away from the brink of extinction.