The endangered Delta smelt is a 3-inch fish found only in the Sacramento-San Joaquin Delta. It is considered especially sensitive because it lives just one year, has a limited diet and exists primarily in brackish waters (a mix of river-fed fresh and salty ocean water that is typically found in coastal estuaries).

Along with other native fish species, the smelt population has declined because of a combination of entrainment in pumping facilities, poor water quality, increasing water temperatures, loss of habitat and invasive species that compete for food.

The smelt’s tendency to swim near the State Water Project and Central Valley Project export pumps in the South Delta has complicated water deliveries to farmers and cities. Because the fish is a state-listed endangered species, strict limits on how many smelt can be harmed by the Delta pumps apply and can force water managers to temporarily curtail water withdrawals from the estuary. 

Very few, if any, Delta smelt remain in the wild. The California Department of Fish and Wildlife’s annual Fall Midwater Trawl Survey uses nets dragged through Delta’s water to capture, count and make population assessments for several Delta fishes. This survey has caught fewer and fewer Delta smelt most years for two decades, and since 2018 it has not caught a single one. Counts of other fish species, including longfin smelt (a native fish) and striped bass (an introduced species), have declined precipitously through the history of the same sampling program.

The Fall Midwater Trawl Survey is used as a gauge for measuring the health of the Delta ecosystem. The survey began in the 1960s, about when the large Delta pumps began operating, and its results suggest a relationship between increased water exports and declining fish numbers.

However, other factors have also contributed to the collapse of the estuary’s biodiversity. Warmer river water, a result of declining snowpack, hotter summers and reduced flows have proven unfavorable to native species. In addition, invasive clams have proliferated in the Delta and, in the process, outcompeted native species such as the Delta smelt for nutrients in the water. Mississippi silversides, a nonnative fish, prey on Delta smelt eggs and larvae, and have been identified as a culprit in the species decline.

The future of the Delta smelt

In 2009, the Delta smelt was designated as endangered by California, while its status under the federal Endangered Species Act remains threatened. Because the Delta smelt lives only within the Sacramento-San Joaquin Delta, and because of its one-year lifespan, the species has often been considered a reliable ecological indicator that reflected the health of the Delta estuary. However, in the past decade it has become too scarce to serve that purpose. Peter Moyle, a prominent UC Davis fish biologist called the species “functionally extinct” in 2015, and since the end of California’s 2012-2016 drought, it has teetered on the brink of extinction.  

The fate of the Delta smelt is part of a continuing controversy over the fate of the faltering Delta ecosystem—the hub of the state’s water supply. Over the years, several lawsuits have been filed in disputes over the amount of water that can be safely exported through the region, and increasingly severe droughts have heightened the tension between those who depend on Delta water exports and those trying to save the smelt and other species from extinction.

In 2008, federal biologists issued a “biological opinion” finding that continued operation of the Delta pumps threatened the smelt with extinction, and new protections were initiated. These restrictions on pumping water would ultimately not help the smelt’s population, and they also made the fish into a symbol of California’s water conflicts — what have often been characterized as fights between fish and farms.

In 2019, the federal government issued a new biological opinion that eased protections on Delta smelt while calling on agencies to boost the fish’s population using hatchery production. That program kicked off in December 2021, and over a period of weeks, more than 30,000 captively raised Delta smelt were released into the Delta near Rio Vista, along the Delta’s northwestern edge. The program is aimed at enhancing wild spawning success and boosting overall numbers. Scientists, however, have expressed doubt that the releases will provide lasting relief for the species unless they are coupled with large-scale habitat enhancement programs and increased flows of water through the Delta.