Delta Smelt: Life History and Decline of a Once-Abundant Species in the San Francisco Estuary

Abstract

This paper reviews what has been learned about Delta Smelt and its status since the publication of The State of Bay-Delta Science, 2008 (Healey et al. 2008). The Delta Smelt is endemic to the upper San Francisco Estuary. Much of its historic habitat is no longer available and remaining habitat is increasingly unable to sustain the population. As a listed species living in the central node of California's water supply system, Delta Smelt has been the focus of a large research effort to understand causes of decline and identify ways to recover the species. Since 2008, a remarkable record of innovative research on Delta Smelt has been achieved, which is summarized here. Unfortunately, research has not prevented the smelt's continued decline, which is the result of multiple, interacting factors. A major driver of decline is change to the Delta ecosystem from water exports, resulting in reduced outflows and high levels of entrainment in the large pumps of the South Delta. Invasions of alien species, encouraged by environmental change, have also played a contributing role in the decline. Severe drought effects have pushed Delta Smelt to record low levels in 2014-2015. The rapid decline of the species and failure of recovery efforts demonstrate an inability to manage the Delta for the "co-equal goals" of maintaining a healthy ecosystem and providing a reliable water supply for Californians. Diverse and substantial management actions are needed to preserve Delta Smelt.

Effects of freshwater flow and phytoplankton biomass on growth, reproduction, and spatial subsidies of the estuarine copepod Pseudodiaptomus forbesi

“…The modification of flows and diversion of fresh water have led to conflicts over environmental effects and particularly over protection of fishes that are affected by these practices, notably salmon and delta smelt, which has declined in abundance over the last * two decades and is now considered at imminent risk of extinction (Moyle et al, 2016). However, the decline in delta smelt is due at least in part to food limitation at various life stages (Bennett, 2005;Slater & Baxter, 2014;Hammock et al, 2015).…”

Is Extinction Inevitable for Delta Smelt and Longfin Smelt? An Opinion and Recommendations for Recovery

“…For the Delta Smelt, the time to make key decisions for its survival in the Delta may have already passed." (Moyle et al 2016a). …”

The State of Bay-Delta Science 2016: An Introduction

“…Most ecological studies of the Delta have focused on the aquatic ecosystem, because of the obvious conflict between water exports and conservation of this ecosystem. Delta Smelt (Hypomesus transpacificus), has been a focal species in conflicts over water exports and, despite major changes in water allocation intended to benefit them, smelt are nearing extinction (Moyle et al 2016). Four races of Chinook Salmon (Oncorhynchus tshawytscha) and Steelhead Trout (Oncorhynchus mykiss) move through the Delta on their seaward and spawning migrations.…”