Longfin smelt were once one of the most abundant open-water fishes in the San Francisco Bay Estuary — commercially important fish, key to the Bay food web. Today the species' numbers have plummeted to record lows in the Bay-Delta, and it's nearing extinction in other Northern California estuaries. Due to poor management of California's largest estuary ecosystem, which has allowed excessive water diversions and reduced freshwater flow into the Bay, the longfin smelt has undergone two catastrophic declines in just 20 years.

Formerly so common that it supported a commercial fishery in San Francisco Bay, the longfin smelt was long assumed by government agencies to be undeserving of Endangered Species Act protection. But this fish is far from immune to the devastation of its habitat, and its rapidly falling numbers are a bright-red flag. To ensure that the smelt receives the protection it needs, in 2007 the Center, Bay Institute and Natural Resources Defense Council petitioned for state endangered species protection for the longfin smelt in California and federal endangered species protection for the San Francisco estuary population. Eventually, both petitions were received positively, and in 2009, the California Fish and Game Commission voted to declare the smelt threatened under the state's Endangered Species Act. But the U.S. Fish and Wildlife Service responded to our petition and a notice of intent to sue by denying federal protection to the San Francisco Bay-Delta population while promising to look at the status of the species as a whole. . 

To ensure the fish earns federal protection in the Bay-Delta before it's too late, in 2009 the Center sued the Service for its failure to heed the longfin smelt's danger, and in 2011, the agency announced it would rethink its decision on the Bay-Delta population. But in 2012, while the Service determined this population warranted protection, the agency declined to provide it, adding the fish to the it the "candidate" list.