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Physical Processes Influencing Spawning Migrations of Delta Smelt

Locations of turbidity monitoring stations in California Delta.

Locations of turbidity monitoring stations in California Delta.

The delta smelt (Hypomesus transpacificus) is a small pelagic fish endemic to the San Francisco Estuary (SFE). Although it was once abundant, the numbers of delta smelt have declined dramatically over the last two decades such that it is now protected under the State and Federal Endangered Species Acts. As a result, delta smelt have become a major focus of environmental and water management. However, even with accelerated efforts to understand the ecology of this species, several key gaps remain in the knowledge of delta smelt life history, habitat preferences, and the factors limiting abundance. In particular, little is known about its spawning ecology, including annual spawning migration from the brackish Suisun Bay region into the freshwater Delta. Also urgently needed are innovative management strategies that can effectively reduce entrainment of delta smelt in the major state and federal water export facilities located in the south Delta. Although the role of such mortality in the decline of the species is highly controversial, entrainment of delta smelt often restricts export operations and interferes with allocations of fresh water throughout California.

The goal of our proposed 4-year study is to provide information on the extent to which water of low transparency, i.e., high turbidity, triggers the spawning migration of delta smelt. This information is critically needed for devising management strategies that would manipulate water clarity at key locations and times to reduce entrainment mortality, as well as promote habitat for this endangered species. Delta smelt appear to undergo a sudden shift in distribution, i.e., migrate, from Suisun Bay into the Delta after they encounter pulses of turbid water from the first major land-runoff produced by a winter rainstorm, the so-called “first-flush”. We propose field sampling during the period immediately before and after the first-flush markedly increases turbidity in the Delta. Field sampling would integrate monitoring of hydrodynamics, water turbidity, salinity, temperature, fish, and their zooplankton prey at a two locations (near Decker Island in the Sacramento River and Jersey Point in the San Joaquin River).

Project Chief: Jon R. Burau
Phone: 916-371-2582
Email: jrburau@usgs.gov

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