To understand why longfin smelt have been in decline in recent decades we must familiarize ourselves with the life history of this unique fish. Longfin smelt are an anadromous fish species, meaning that they migrate to and from oceanic (or high salinity) waters to freshwater during different life stages. Sexually mature longfin smelt adults migrate from high salinity waters in the Pacific Ocean or San Francisco Bay up the San Francisco Estuary during the late fall. Depending on prevailing water conditions, they tend to migrate to or past the confluence of the Sacramento and San Joaquin rivers. Beginning in December longfin smelt begin to spawn in these upper portions of the estuary and may continue to spawn until May. Longfin smelt are thought to be semelparous, meaning they only reproduce once in a lifetime and die shortly after spawning. Once the fertilized eggs incubate and hatch, larval longfin smelt (unable to swim at this stage) begin to be pushed downstream by the outflow of the rivers. This outflow will generally transport the larvae to Suisun Bay or San Pablo Bay but can push them as far down as Upper San Francisco Bay. It is in these locations That larval longfin smelt feed and grow until becoming juveniles or adults. Adults then appear to migrate oceanward to feed until the next fall rolls around and it is time to migrate upstream and spawn.

               This is a complex life history that makes the species more difficult to study and understand than non-migratory species. Further adding to the complexity of this species is that they can spawn in a geographically broad swath of the estuary, and where they tend to center their spawning can change year to year depending on where their ideal salinity concentration is located in the estuary.

                In the San Francisco estuary outflow of the Sacramento and San Joaquin rivers are tightly tied to salinity and water temperature. Salinity and temperature can affect growth and survival of longfin larvae and adults. High outflows introduce colder and fresher (low salinity) water into the estuary, both of which are thought to be beneficial to longfin smelt, particularly during the spawning season.

                A strong positive relationship has been shown between average freshwater outflow from the rivers during spawning season and abundance of longfin smelt (Kimmerer 2002). Years of high outflow seem to result in high recruitment of larvae into the population. Is this because the temperature, salinity, or food resources are more favorable under these conditions, is there a greater amount of habitat available, or do longfin simply like large inputs of freshwater? Additionally, when and how much freshwater flow is needed for good spawning and high survival.

                To answer these questions, we have utilized the Dayflow dataset provided by the California Department of Water Resources. The Dayflow dataset contains daily estimations of a number of water related parameters in the estuary, including estimated daily estuary outflow going back to the 1920’s. Using this dataset, we will examine how magnitude and/or timing of outflows are important to this species. Summarizing outflows on an annual basis looking at the importance of different months and the importance of flow volumes we hope to answer numerous questions. Additionally, we aim to include these explanatory variables into longfin growth, spatial, and life history models to enhance our understanding of the species and potentially inform future management of the estuary.

Kimmerer, W. J. (2002). Physical, biological, and management responses to variable freshwater flow into the San Francisco Estuary. Estuaries 25: 1275-1290.

Agency: U.S. Fish and Wildlife Service

Program: US Fish & Wildlife Service - DFP

Location: San Francisco Bay National Wildlife Refuge Complex