Longfin smelt (Spirinchus thaleichthys) is a fish species that is found along the Pacific coast from Alaska to California. A distinct population segment (DPS) exists in the San Francisco Bay-Delta, and this population has undergone significant declines over the past 20-30 years. Similar to the federally endangered delta smelt (Hypomesus transpacificus) which is also native to the San Francisco estuary, longfin smelt are a euryhaline pelagic fish species that appears to utilize different portions of the estuary at different life stages. The San Francisco Estuary experienced a pelagic organism decline (POD) in the early 2000’s when multiple fish species including the longfin smelt declined in abundance. This resulted in the longfin smelt being designated a threatened species by the state of California, and the U.S. Fish and Wildlife Service similarly recognized the longfin smelt DPS warranted protective measures.

                We know that longfin smelt abundance has been significantly declining over the past few decades, but the question is, why? What is causing the sudden decline of this species, and what can be done to reverse this decline? To answer this question, we first need to look at the San Francisco Estuary and the water therein, after all, fish like water.

                The San Francisco Estuary has undergone substantial changes over the last century. The estuary used to be primarily floodplain and marsh land that was tidally and seasonally inundated with water. When farming came to the watershed many of these marshes were diked, preventing inundation, and were used for agriculture. This resulted in changes to the carbon cycle in the estuary, ultimately reducing terrestrial organic carbon inputs into the food web. Such changes can result in dramatic changes to basal groups in the food web that can have a trophic effect that works its way up the food web. This can have negative impacts on fish communities.

                In addition to the conversion of marshland to agriculture, the water demand in California has increased over the last century. The San Francisco Estuary is essential to the state of California as it comprises approximately half of the surface water supply to the state and provides water to more than 25 million people. Water is pumped from the estuary to many locations throughout the state. The water is needed for residential consumption, industry, agriculture, and ecological services, however there is a finite amount of water to go around and this can result in competing interests. In addition to the amount of water in the estuary being finite, the amount of water that is available is unpredictable seasonally and yearly. Climate change is predicted to further reduce the amount of water available annually as well as reduce the predictability of timing and amounts of water.

                So, we know that everybody needs water, including the longfin smelt. But how much water do longfin smelt need, and when do they need it? If we can answer these questions we may be able to help the species recover while still allowing for needed water diversions to the state of California.

                The next number of blog posts will further explore the relationship between longfin smelt and water in the San Francisco Estuary.

Agency: U.S. Fish and Wildlife Service

Program: US Fish & Wildlife Service - DFP

Location: Sacramento Fish and Wildlife Office