Habitats: Managed Ponds
Commercial salt ponds were constructed around the edge of San Francisco Bay beginning in the mid 1800s. Many former salt production ponds in San Francisco Bay have recently transitioned to public ownership and are being restored and managed for wildlife. These shallow ponds now provide habitat for hundreds of nesting terns, gulls, and shorebirds, and roosting and feeding habitat for hundreds of thousands of migrating and wintering shorebirds and ducks.
The South Bay Salt Pond Restoration Project (15,100 acres) plans to restore 50–90% of the South Bay ponds to a mix of tidal marsh and shallow managed ponds. The Napa-Sonoma Marshes Wildlife Area in the North Bay is restoring 4,200 acres of salt ponds to tidal marsh. Cargill Salt still manages about 11,000 acres for salt production, all in the South Bay.
The species or groups of species depicted here currently use salt pond habitat.
As restoration progresses, the bird community may change: tidal marsh species will colonize newly created salt marsh habitat; some waterbirds, such as shorebird and duck species that use open water or tidal flat, may move out. Ongoing monitoring will track how bird species and numbers change over time.
Nesting Shorebirds (South Bay ponds) – Population trends unknown, underscoring the need for ongoing monitoring of breeding shorebirds. American Avocets and Black-necked Stilts are the most abundant nesting shorebirds; breeding American Avocets estimated at 1,380 pairs and 590 pairs estimated for Black-necked Stilts, as of 2003.
Note: The Western Snowy Plover also nests in the salt ponds; see the Endangered Species section.
*Data for medium-sized and small shorebirds and dabbling ducks come from peak counts for shorebirds (spring) and ducks (winter) in North and South Bay ponds.
Primary threat: Loss of shallow pond habitat for roosting, foraging, and nesting waterbirds. Wintering and migratory shorebirds roost and feed in salt ponds at high tide. Ducks utilize shallow, low salinity ponds to forage and roost. Terns and shorebirds nest on islands and levees in pond habitat.
Rising sea levels from global climate change may increase water depths or erode levees and nesting islands, impacting habitat for wintering, migrating, and nesting birds.
Nest predation and competition from a growing population of California Gulls, which prey upon eggs and chicks or displace nesting waterbirds. Forster’s and Caspian Terns have already been displaced from some of their historic nesting colonies by gulls.
Contaminants impairing bird reproduction. Mercury, a legacy of years of mercury mining and use of mercury in gold mining, is a prevalent contaminant throughout San Francisco Bay. Mercury is especially high in the South Bay, where runoff from a large mercury mine in the upper watershed has released, and continues to release, mercury-laden sediments. Mercury impacts waterbird reproduction, specifically for the Forster’s Tern, in which 48% of breeding adults are at or above high risk of impaired reproduction due to their present methylmercury concentrations.
Planning, Management, Restoration
Convert a large proportion of the salt ponds to managed ponds. Maintain ponds with appropriate depths – and habitat of varying salinities – to benefit nesting, migrating, and wintering shorebirds and ducks.
Practice adaptive management by monitoring waterbird responses to restoration and modifying restoration as needed (as in the South Bay Salt Pond Restoration Project).
Continue to create islands within managed ponds for breeding and roosting birds. Experiment with adding vegetation to some islands to create cover for chicks.
Improve dissolved oxygen within managed ponds by optimizing water flow between pond and Bay waters and reducing nutrient inputs from adjacent uplands.
Conduct an education campaign to highlight the connection between urban waters and the Bay.
Slow growth of the California Gull population by reducing gull access to trash at local landfills and other areas. Evaluate whether removal of target gulls helps reduce predation pressures on nesting waterbirds.
Identify, protect, or manage key existing waterbird nesting areas Bay-wide, given that waterbird populations may be affected by a reduction of pond habitat.
Monitor changes in abundance of breeding, migrating, and wintering waterbirds over time to evaluate the overall effects of restoration.
Determine current breeding population size of nesting American Avocets and Black-necked Stilts. Monitor their use, density, and reproductive success at created islands.
Assess habitat characteristics that enhance waterbird densities, (e.g. water quality, water depth, salinity, invertebrate biomass, island characteristics), and provide restoration project managers with habitat characteristics that could maximize densities of waterbirds in the remaining ponded habitat as tidal marsh restoration proceeds.
Suggest ways to reduce the population growth of California Gulls and their impact on other breeding waterbirds by identifying the causes of population growth and evaluating methods to control it.
Assess and track the changes in methylmercury concentration in nesting Forster's Terns, American Avocets, and Black-necked Stilts as tidal restoration proceeds. Determine reproductive threshold concentrations of methylmercury in waterbirds to assess changes in risk of contaminant exposure as a result of tidal restoration efforts.