Breeding Waterbird Populations Have Declined in South San Francisco Bay: An Assessment Over Two Decades

C Alex Hartman,Cheryl Strong,Carley Schacter,Mark Herzog,Rachel Tertes,Yiwei Wang,Nils Warnock,L Max Tarjan,Joshua Ackerman

doi:10.15447/sfews.2021v19iss3art4

Abstract

Availability of wetlands with low salinities during the breeding season can influence waterfowl reproductive success and population recruitment. Salinities as low as 2 ppt (3.6 mScm–1) can impair duckling growth and influence behavior, with mortality occurring above 9 ppt (14.8 mScm–1). We used satellite imagery to quantify the amount of available water, and sampled surface water salinity at Grizzly Island, in the brackish Suisun Marsh, at three time-periods during waterfowl breeding (April, May, July) over 4 years (2016–2019). More water was available and salinity was lower during wetter years (2017, 2019) than during drier years (2016, 2018), and the amount of water in wetlands decreased 73%–86% from April to July. Across all time-periods and years, the majority (64%–100%) of wetland habitat area had salinities above what has been shown to negatively affect ducklings (> 2 ppt), and up to 42% of wetland area had salinities associated with duckling mortality (> 9 ppt). During peak duckling production in May, 81%–95% of available water had salinity above 2 ppt, and 5%–21% was above 9 ppt. In May of the driest year (2016), only 0.5 km2 of low-salinity water (< 2 ppt) was available to ducklings in the study area, compared to 2.6 km2 in May of the wettest year (2017). Private duck clubs own the majority of wetland habitat at Grizzly Island and consistently had a greater percentage of land flooded during summer than did publicly owned wetlands, but private wetlands generally had higher salinities than public wetlands, likely because they draw from higher-salinity water sources. By July, few wetlands remained flooded, and most had salinities high enough to impair duckling growth and survival. Local waterfowl populations would benefit from management practices that provide fresher water during peak duckling production in May and retain more water through July.

Highlights

San Francisco Bay is one of the most important sites for waterbirds along the Pacific Flyway, supporting more than 1 million waterbirds annually (Page et al 1999; Takekawa et al 2001; Stenzel et al 2002; Warnock et al 2002)
Forster’s Tern, and especially American Avocet nest abundance, began to decline after 2010 (Figure 8), coinciding with the loss of highly productive island nesting habitat in Alviso (Ponds A7 and A8) as a result of the opening of these managed ponds to muted tidal action to begin the process of tidal marsh restoration
In an effort to maintain breeding waterbird populations, the South Bay Salt Pond (SBSP) Restoration Project constructed new nesting islands to enhance four managed ponds (SF2, A16, E12, and E13) that will not be converted to tidal marsh

Summary

Introduction

San Francisco Bay is one of the most important sites for waterbirds along the Pacific Flyway, supporting more than 1 million waterbirds annually (Page et al 1999; Takekawa et al 2001; Stenzel et al 2002; Warnock et al 2002). Over the last few decades, San Francisco Bay has supported the largest breeding populations of American Avocets (Recurvirostra americana) and Black-necked Stilts (Himantopus mexicanus) and nearly 30% of the breeding population of Forster’s Terns (Sterna forsteri) along the Pacific coast (Stenzel et al 2002; Rintoul et al 2003; Strong et al 2004; McNicholl et al 2020). More than 90% of Forster’s Tern nests, >70% of American Avocet nests, and ~20% of Black-necked Stilt nests in south San Francisco Bay occur on islands within managed ponds (Strong et al 2004; Ackerman and Herzog 2012; Hartman et al 2016a; Ackerman et al 2020)

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Discussion

Conclusion