Federal wildlife officials formally listed the San Francisco Estuary longfin smelt as an endangered species in July. Whether the action is a first step toward recovery or just an administrative milestone on the path to extinction is too early to say, but one thing is already clear: The longfin smelt is ominously close to vanishing. Now, as its existential clock ticks, scientists are hustling to better understand the species’ biology and environmental requirements and, with luck, safeguard its future. The estuary’s population of longfin smelt—Spirinchus thaleichthys, a species that can live in saltwater and ranges as far north as Alaska—has been declining for several decades, with an accelerated dip starting around the turn of this century. Once plentiful enough to be a target for commercial fishers, it now shows a feeble presence in annual sampling programs.