Little Fish in Big Trouble: The Bay Delta’s Longfin Smelt

Longfin Smelt, is a small pelagic forage “baitfish” that was once one of the most abundant fishes in the San Francisco Estuary.  Our work on Longfin Smelt was recently featured by KCET’s Alistair Bland.  Here is the link to the news piece.

The extreme wet weather we’ve had during winter 16-17 turned the Lower South Bay Fresh for several months. This has resulted in the first successful reproductive event for Longfin Smelt in Lower South Bay since the 1982-1983 El Nino. Longfin Smelt have been encountered in our fish surveys of the Alviso Marsh during the winter months since we began our surveys in 2010.

Longfin Smelt typically arrive in late fall when temperatures begin to cool.  We see them every month during the winter through early spring.  This is typically the spawning season for Longfin Smelt.  We’ve been seeing them in large numbers inside the tidally restored salt ponds along Coyote Creek. They specialize on feeding on mysid shrimp, “estuarine krill”, which also typically increase in abundance in the winter and spring when Longfin Smelt are in the marsh.

Longfin Smelt seek out low-salinity or tidal freshwater habitats to spawn in the winter months. However, the Alviso Marsh in winter is typically too salty for Longfin to successfully reproduce.  In March of 2017 we found our first larval (baby) Longfin Smelt in the marsh. In our April Survey we captured over 200 of this little Longfin Smelt up to about 35mm about an inch and a half in length.  Our largest single catch of 59 fish occurred inside Pond A21, a tidally restored salt pond that was breached in summer of 2006.


Longfin Smelt were listed as threatened under the California Endangered Species Act in 2010 and as a result 8,000 acres of tidal wetland restoration was prescribed by the state to help mitigate the losses of fish in the giant pumps located in the South Delta.  These pumps are the key to California’s agricultural success.  Our work is demonstrating tidal restoration may provide additional habitat for the Longfin Smelt, however, it is unclear whether additional rearing habitat will bring Longfin Smelt abundance back to where it was before it was listed.  We will continue to monitor the abundance and growth of these fish in the coming months. Stay Tuned.



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