Aquaculture North America

Tiny smelt fights for life in West Coast waters

April 4, 2023
By Julia Hollister

The Longfin smelt – living in bays and estuaries from Northern California through Alaska – is in trouble and loss of freshwater flows is the reason.

Environmentalists say the tiny Longfin Smelt is vital to the ecosystem but is endangered due to loss of freshwater. Photo: Jacob Stagg

Environmentalists say the tiny fish is vital to the ecosystem.            

“The smelt occupies the San Francisco Bay Estuary and areas of the Pacific Ocean out to the Farallon Islands,” said Tamara N. Ward, assistant field supervisor, and spokesperson for External Affairs, U.S. Fish and Wildlife Service. “It requires appropriate water temperatures and salinity levels to carry out their life functions. The amount and duration of freshwater flowing into the estuary greatly influences the location and extent of where those appropriate conditions exist.” 

In November 2022, the U.S. Fish and Wildlife Service announced a proposal to list the San Francisco population of the little-known fish as an endangered special. If finalized, a process that could take about a year, conservationists hope it can help bring the population back.

Bay-Delta longfin are anadromous, meaning older juveniles and adults can migrate to the ocean, but must return to fresh water to spawn.  

 Longfin smelt can be distinguished from other smelts mainly by their long pectoral fins. Habitat for longfin and Delta smelt largely overlap, but adult longfin habitat extends further west, including the coastal ocean. Longfin smelt generally also spawn earlier in the season, and longfin usually live two years while Delta smelt usually only live one year. 

This fish species is 3.5 to 4.3 inches long with a translucent silver appearance on its sides and gut, while its back has an olive to iridescent pinkish hue. The fish lives about two to three years in bays and estuaries from Northern California north along the coast through Alaska. Reproduction begins in late fall/early winter and extends into the spring as water temperatures and salinity levels allow.

Why is it in trouble?
Ward says its habitat loss and primary threat is mainly due to the long-term reduction and alteration of freshwater flows into the estuary. This threat is exacerbated by limited food resources and impacts associated with climate change. 

Changes to the San Francisco estuary and the introduction of competing non-native species like the overbite clam have altered the food web that supports longfin smelt and reduced its natural food base. 

The Fish and Wildlife Service is proposing to list the longfin smelt as an endangered species.

(There are 140 species –threatened and endangered – on the list.) Since 2018, The FWS added 45 species (domestic and foreign) to the Lists of Endangered and Threatened Wildlife and Plants and delisted 25 species. 


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Overall, the lists have increased over the last five years.

The recovery planning process identifies actions to halt and reverse the species’ decline, and identifies criteria for when a species may be downlisted or delisted. Implementation of recovery actions generally requires the participation of a broad range of partners, including other federal agencies, states, tribes, nongovernmental organizations, businesses and private landowners. Examples of recovery actions include habitat restoration, research, captive propagation and reintroduction, and outreach and education. 

If this species is listed, funding for recovery actions will be available from a variety of sources, including federal budgets, state programs and cost-share grants for non-federal landowners, the academic community and nongovernmental organizations. Listing will also require federal agencies to consult with the Service when proposing projects within the DPS’ range and will enable the designation of critical habitat. 

The federal Endangered Species Act allows the Service an additional year to publish a critical habitat designation. Longfin smelt are currently listed as a threatened species under California Endangered Species Act, which prohibits unpermitted possession, purchase, sale or take of listed species.

“The diet of Bay-Delta longfin smelt is very specific and varies by age class and location within the estuary,” Ward said. “Bay-Delta longfin smelt larvae select strongly for the copepod as their food resource. All other prey types combined account for only about 10 percent of their diet.”

When Bay-Delta longfin smelt reach about one inch in length, their diet switches and is nearly all mysids, such as opossum shrimp. The overbite clam is not a prey item for longfin smelt, which are a pelagic foraging fish. Rather, it is a competitor, as the clam filter feeds on small plankton at the bottom of the water column, and effectively depletes the food web and energy available to longfin and other pelagic fish in the Bay/Estuary.

As a very selective predator, the fate of the longfin smelt follows productivity and availability of its specific prey, and the environmental conditions leading to their respective densities in the water column.

Longfin smelt play a key role in the food web, and its disappearance could affect species both higher and lower in the food chain. Species decline is also an indicator of overall environmental quality. The longfin smelt decline is one of numerous indicators of environmental problems in the estuary that should be considered when choices about resource use are made. Others species that rely on the Delta that have seen steep declines in recent years include Chinook salmon, steelhead, green sturgeon and Delta smelt.

Is climate change the culprit for Logfin smelt’s demise? Ward and other scientists at Fish and Wildlife have some hard facts.

Numerous decades of declining abundance indicate the species’ inability to sustain itself and respond to the threats it’s facing. After reviewing the best scientific and commercial information available and seeking input from experts as well as federal and state agencies and academic, local and private entities with experience in longfin smelt, estuary ecosystems and water-management activities, field surveys show the Bay-Delta longfin smelt abundance, density and distribution throughout the San Francisco Bay estuary have substantially declined over time. 

Data from the Fall Midwater Trawl from 1967 until 1986 showed abundance was highly variable but steadily declined over time. Since 1987, when the overbite clam was introduced into the SF Bay-Delta, the median longfin abundance has been less than 1/10th the median abundance of the pre-clam period. The maximum value of the index during the last 10 years is only about 2 percent of the maximum value from the pre-clam years. The decline has also most likely led to reductions in the DPS’ genetic diversity, thereby reducing its adaptive capacity. 

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