If You Build It, Will They Come?
Response of Waterbirds to Salt Pond Enhancements in Pond SF2
Stacy Moskal

The sight of spherical and saw-toothed islands that are clearly man-made in a former salt pond can be a bit disconcerting as you head westbound on the Dumbarton Bridge in South San Francisco Bay. Large billboard signs that publicize that wetland restoration is in progress greet commuters and visitors alike as they approach the site. Unbeknownst to the casual observer, these strange-looking islands are part of an ongoing experiment to determine how best to provide habitat for migratory and wintering shorebirds.

The San Francisco Bay-Delta is the largest estuary on the west coast of the Americas and it is surrounded by a large urban center with a population of 7.5 million people. Healthy estuaries provide important ecosystem services such as water purification and flood protection. All living organisms also benefit from estuaries because of the food, open space, recreation, transportation of people and goods, and diversity of wildlife they offer.

There are multiple habitat types within the estuary: tidal marshes, mud flats, salt ponds, dry salt flats, and subtidal zones. These habitat types are home to several federally-listed endangered species like the California Clapper Rail, California Least Tern, Salt Marsh Harvest Mouse and Soft Bird's-Beak. Federally-listed threatened species like the Western Snowy Plover and California Red-Legged Frog depend on the estuary for their survival. Several “species of special concern” that have been designated by the State of California also reside in the San Francisco Bay Estuary and include the Salt Marsh Common Yellowthroat and the Salt Marsh Song Sparrow.

Historically, 200,000 acres of tidal marshes and shallow pans were used by the indigenous people for waterfowl hunting and salt production. By 1930, much of the wetlands were filled to support population booms due to the gold rush and urbanization, and for salt evaporation ponds. The area was diked and drained to allow for more grazing and agriculture in the North and Central Bays and for salt production in the South Bay. Today, roughly 95% of the historical wetlands have been lost to development and conversion. Since wetlands provide critical habitat for endangered species and have important ecosystem functions, which have become relevant as a result of climate change and sea level rise, the San Francisco Bay ecosystem is currently undergoing the largest wetland restoration project on the Pacific Coast.

In 2003, Cargill, Inc. sold 15,100 acres of salt ponds in South San Francisco Bay to federal and state governments. The South Bay Salt Pond Restoration Project was formed to oversee the large scale restoration.

Salt ponds and dry salt flats have become extremely important habitat for one million migratory and wintering shorebirds. The California Least Tern and Western Snowy Plover have also come to depend on these types of habitat for their survival. The San Francisco Bay is such an important location for shorebirds it has been designated as a Western Hemisphere Shorebird Reserve Network Site of Hemispheric Importance. These habitats are used as roosting and breeding sites as well as foraging areas at high tide.

The current management plan of the Restoration Project is to convert 50-90% of the existing 30,000 acres of former salt ponds to tidal salt marsh. Because migratory birds have come to rely on these ponds for food and roosting sites, the South Bay Salt Pond Restoration Project has agreed that there should be no net loss of waterbirds in the bay when the ponds are restored back to tidal salt marsh.

So how do we maintain the abundance of migratory and wintering shorebirds in a smaller footprint of managed ponds? The most logical way is to increase the number of shorebirds using the remaining managed ponds. To increase the number of shorebirds using a pond, we need to know and understand what characteristics these birds are looking for as their ideal habitat and how to maximize them. The South Bay Salt Pond Restoration Project is conducting adaptive management using sound science: 1) design an experiment; 2) test the hypothesis; 3) change management or restoration design based on experimental results. An experiment designed with adaptive management in mind was created at a former salt pond on the Don Edwards San Francisco Bay National Wildlife Refuge named SF2.

Pond SF2 is on the west side of bay, just south of the Dumbarton Bridge (Hwy 84). Prior to the pond's enhancements, SF2 was a gypsum-covered dry pond, home to a couple thousand roosting sandpipers, a handful of nesting snowy plovers, and a few dozen foraging bufflehead and common goldeneye. In 2009-2010, the Restoration Project enhanced SF2 by constructing 30 nesting and roosting islands and installing water control structures to allow tidal flow into the 155-acre pond.

These islands were designed with the purpose of conducting an experiment. All of the islands have a north facing slope that provides protection from winds that come from the northwest which is the Bay Area's typical wind pattern. Half of the islands are round in shape, therefore having a low shoreline edge to area ratio. The other islands are longer and linear with a saw-tooth south edge, thus having a high shoreline edge to area ratio. The round versus linear islands were designed to test whether birds prefer to be clumped in distribution (thereby choosing the round islands) or more spread out (thereby choosing the linear islands). These islands are also spatially distributed strategically throughout the pond. Some islands were constructed closer to exposed mud flat when water levels are low, and some were placed closer to the trail or road to see if human disturbance is a factor. To allow for the ebb and flow of bay waters into the pond, The Restoration Project placed two sets of tide gates in the bay-front levee. By opening or closing various screw gates, water levels within the pond can be manipulated up or down.

In order to determine whether salt pond enhancements are an effective way to increase migratory shorebird abundance in ponds, the Restoration Project, the Resources Legacy Fund, Council on Ocean Affairs, Science and Technology, and U.S. Geological Survey (USGS) funded a graduate research project to conduct the study. We surveyed the pond weekly at three locations from October to May for two seasons (2010-2011 and 2011-2012). Using a map with a 50 m grid superimposed on an aerial photo of the pond, we recorded all birds observed within the pond, noting grid location, bird behavior (foraging or roosting), and habitat type (island #, levee, water, pond bottom, or man-made structure). All avian predators observed were also recorded. We defined a survey as two counts: the daily high and the daily low tide within one 24-hour period.

Enough data have been collected over the last two seasons to begin our data analysis. Some of our preliminary results are presented here. Of the 265,546 non-breeding waterbirds that were observed on the pond over the two seasons, 36,884 (14%) were using islands. Since it is common for waterbirds to forage at adjacent mud flats at low tide and roost in ponds at high tide, more birds were observed using the pond at high tide (212,369) than low tide (53,177) but the proportion of birds using islands remained similar.

To help see general trends and simplify analysis, data is often examined at guild level. In this case, a guild is a group of bird species that forage similarly. Of the observed fish eaters (like cormorants and pelicans), 50% were seen on islands. Other guilds like dabbling ducks (like Mallard, Northern Shoveler, Northern Pintail, and American Wigeon) and gulls were seen on islands 30% of the time. All other foraging guilds were observed on islands 25% of the time or less.

Regardless of the guild, birds observed on islands were typically roosting, not foraging. With the exception of small shorebirds (Western Sandpiper, Least Sandpiper, and Dunlin), all guilds had a higher number of birds observed on linear islands as opposed to the round type. It is still uncertain as to why this is. In general, waterbirds have a tendency to choose roost sites that are extremely close or slightly in the water. The linear islands provide more interface between land and water which may be preferred. A second reason may be the linear islands offer multiple sections of shelter from the wind due to the saw-tooth edge as opposed to the round ones that only offer one section thereby forcing all the birds into one section. Further analysis may be able to help explain the island use discrepancy. It also appears that shorebirds are choosing to use the islands closest to the bay, perhaps awaiting the next low tide when their foraging habitat would once again become available.

From this experiment, we hope to better understand how waterbirds use islands. Not only does shape and proximity to mud flats likely impact island use, but other characteristics may also be important such as island morphology (height, size, slope, texture, vegetation), distance to disturbance (trails, power lines, roads), and water quality (temperature, pH, dissolved oxygen, salinity). Most of these features were measured at SF2; the goal is to combine the measured characteristics with the bird observations to determine which island and water attributes are most influential in waterbird use.

Since the SF2 enhancements, The Restoration Project has begun two other island projects: pond A16 in the Alviso Unit of the Don Edwards San Francisco Bay National Wildlife Refuge and ponds E12/13 in California Department of Fish and Game's Eden Landing Ecological Reserve. Construction of the islands in pond A16 will most likely be completed by the end of 2012 while ponds E12/13 is still in the design phase. As part of the adaptive management process, the USGS will continue to analyze the data to help land managers improve island designs in future enhancement efforts. For further information on the USGS San Francisco Bay Estuary Field Station and their research, visit http://www.werc.usgs.gov.

Stacy Moskal is a South Bay native and a wildlife biologist with the U.S. Geological Survey. Stacy has worked in the San Francisco Bay Estuary since 2005. She is currently working on her master's degree at San Jose State University.