Life History and Status of Delta Smelt in the Sacramento-San Joaquin Estuary, California

Abstract

The delta smelt Hypomesus transpaciftcus is endemic to the upper Sacramento-San Joaquin estuary. It is closely associated with the freshwater-saltwater mixing zone except when it spawns in fresh water, primarily during March, April, and May. The delta smelt feeds on zooplankton, principally copepods. Its dominant prey was the native copepod Eurytemora affinis in 1972-1974 but the exotic copepod Pseudodiaptomus forbesi in 1988. Because the delta smelt has a 1 -year life cycle and low fecundity (mean, 1,907 eggs/female), it is particularly sensitive to changes in estuarine conditions. Tow-net and midwater trawl samples taken from 1959 through 1981 throughout the delta smelt's range showed wide year-to-year fluctuations in population densities. Surveys encompassing different areas showed declines in different years between 1980 and 1983. After 1983, however, all studies have shown that the populations remained at very low densities throughout most of the range. The recent decline ofdelta smelt coincides with an increase in the diversion of inflowing water during a period of extended drought. These conditions have restricted the mixing zone to a relatively small area of deep river channels and, presumably, have increased the entrainment ofdelta smelt into water diversions. Restoration of the delta smelt to a sustainable population size is likely to require maintenance of the mixing zone in Suisun Bay and maintenance of net seaward flows in the lower San Joaquin River during the period when larvae are present.

A Conceptual Model of the Aquatic Food Web of the Upper San Francisco Estuary

“…As most fish grow, they progress to larger foods, such as mysids, amphipods and other epibenthic invertebrates. Filter feeders such as American (Alosa sapidissima) and threadfin (Dorosoma petenense) shad, and planktivores such as delta and longfin (Spirinchus thaleichthys), smelt remain largely tied to the pelagic food web, although they use epibenthic organisms such as mysids at least periodically (Moyle et al 1992;Feyrer et al 2003;Feyrer et al 2007;Slater and Baxter 2014).…”

Abundance Trends, Distribution, and Habitat Associations of the Invasive Mississippi Silverside (Menidia audens) in the Sacramento–San Joaquin Delta, California USA

“…To gain a better understanding of the extent of this ecological interaction between Delta Smelt and Mississippi Silversides, knowledge of the life history of both species is crucial. However, though multiple studies have described the biology and population dynamics of Delta Smelt (Moyle et al 1992;Bennett 2005;Feyrer et al 2007;Merz et al 2011;Sommer et al 2011;Sommer and Mejia 2013;Rose et al 2013), little is known about the life history and habitat associations of Mississippi Silversides within the Delta. Characterizing the abundance and distribution of this invasive species with environmental factors may provide insight into what has allowed Mississippi Silverside to become so widespread and abundant in this system.…”

Plasticity in vertical migration by native and exotic estuarine fishes in a dynamic low‐salinity zone

“…In the San Francisco Estuary, previous studies indicated that some zooplankton and young (larval and juvenile) fishes maintain peak abundance in or near the LSZ Moyle et al 1992;Kimmerer et al 1998). Furthermore, the degree of retention in this low-salinity and nursery habitat is considered a potentially important mechanism regulating recruitment success and population dynamics.…”

“…In this paper, we investigate the degree of flexibility in behavioral responses by native and exotic species to changing hydrodynamics to shed light on mechanisms that may contribute to the nursery habitat potential of the LSZ in the San Francisco Estuary Moyle et al 1992;Jassby et al 1995;Bennett and Moyle 1996). This work was part of an interdisciplinary investigation to evaluate the potential relationships between the hydrodynamics of the LSZ and the abundance and vertical distribution of zooplankton and young fishes under a range of freshwater river-flow conditions.…”