Wildlife Spotlight: Delta Smelt (Hypomesus transpacificus)

Delta Smelt . Photo: USFWS

By Cameron Jaggard

The delta smelt may only grow to 2-3 inches in length, but it serves as an important barometer for the overall health of the Sacramento-San Joaquin Estuary, as well as upstream waters.  Once commercially harvested in the 19th and early 20th century, populations of delta smelt have rapidly declined in recent decades. 

The species was first listed by the U.S. Fish and Wildlife Service (FWS) in 1993 as threatened under the national and state Endangered Species Acts.  Continued declines in the delta smelt population pushed the FWS to raise the protection status of the delta smelt to endangered in March 2009.

Life Cycle

To understand why the delta smelt is in such dire straights, it is important to know about its habitat and life cycle.  Unlike salmon and steelhead, the smelt does not travel much.  It is only found in the brackish and freshwater habitats of the northeastern San Francisco Estuary. 

Delta smelt gather in schools to feed on tiny animals that drift with the current, called zooplankton.  Spawning takes place between late winter to early fall, when delta smelt migrate further inland to lay eggs in freshwater.  The eggs attach to submerged aquatic vegetation and they usually hatch in about two weeks. 

Once the smelts are liberated from their egg case, the hatched fish drift passively as zooplankton to the mouth of the delta.  The river’s exhalations push the smelt into an area of freshwater and saltwater mixing known as the entrapment zone.  Smelt will utilize this area as a rich source of food and shelter until it is time to reproduce.  Most delta smelt live an average of one year to a maximum of two years.

The delta smelt shares the San Francisco Estuary with a number of other fish species.  Pacific salmon, longfin smelt, and steelhead comprise just a few of the hundreds of species that rely on this habitat for survival.  Since the delta smelt is small and a somewhat passive swimmer, it provides a vital prey base to the larger fish species utilizing the bay-delta ecosystem.  Fewer delta smelt don’t just signify poor environmental conditions in the delta, but they also indicate a weaker food web for larger predators, like the wild salmon.

Reasons for Decline

Water has quickly become one of the most sought after resources in all of California.  Urban residents, farmers, and power plants suck freshwater from the Sacramento and San Joaquin Rivers.  These two rivers are the life blood of the San Francisco Estuary. 

Freshwater still flows from the rivers into the estuary, but not in the way that the delta smelt and other species are adapted to.  Outflows of water can be extremely reduced or extremely increased by pumps and other water management structures.  Weak swimming delta smelt are forced out of their required habitat by these unnatural water flows.  Pumps can easily suck in smelt and kill them.  Besides the unnatural water regime, exotic fish species, water pollution and disease also present enduring hazards to the survival of the delta smelt. 

Delta smelt, with their metallic blue sheen and short, slender bodies, are a vital link in the food chain that connects the SF Bay to the Gulf of Farallones Marine Sanctuary.  To maintain balance in our greater ecosystem, it is crucial that we protect this tiny species.