REVISED FINAL REPORT
Suggested citation: Van Schmidt, Nathan D. 2023. South Bay Salt Pond Waterbird Surveys: September 2022–May 2023. Report prepared for the South Bay Salt Pond Restoration Project.
Executive Summary
This report serves as a data summary and coarse-scale assessment of waterbird and water quality monitoring efforts at six pond complexes in the South San Francisco Bay. Coyote Hills, Dumbarton, and Mowry salt ponds are owned by Don Edwards San Francisco Bay National Wildlife Refuge and managed for salt production by Cargill Salt. Alviso and Ravenswood complexes are owned and managed by the U.S. Fish & Wildlife Service as part of the Don Edwards San Francisco Bay National Wildlife Refuge. Eden Landing Ecological Reserve (Eden Landing) ponds are owned and managed by California Department of Fish and Wildlife, with the exception pond CP3C, which is owned by Cargill Salt. This report is based primarily on data collected by the San Francisco Bay Bird Observatory between September 2022 and May 2023.
The purpose of this ongoing study is to describe avian use of ponds to guide regional waterbird conservation, management, and habitat restoration efforts. The South Bay Salt Pond Restoration Project (SBSPRP) is restoring 15,000 acres of former salt evaporation ponds to a mix of tidal marsh and enhanced managed pond habitats. Restoration and monitoring is entering its third decade, and long-term population analyses have shown declines in some species and guilds, but evidence of cyclical trends (regular increases and decreases) in others, which may be driven by climate patterns or density-dependent regulation. It is therefore important to determine the drivers of these long-term trends, and to contextualize local trends with population trends elsewhere in the range, to assess the likelihood that observed declines are driven by SBSPRP actions rather than other factors. Understanding how waterbirds use ponds, identifying key habitat associations, and incorporating features essential to pond-dependent species into restoration plans is important for helping maintain baseline numbers of waterbirds in the South Bay and recovering populations that have experienced long-term declines.
From September 2022 – May 2023, we conducted waterbird surveys and water quality sampling at 82 ponds (22 Cargill-managed salt production ponds and 60 SBSPRP-managed ponds). We examined species richness, abundance, and behavior of waterbird assemblages within and among pond complexes. We grouped species into guilds (e.g., dabbling ducks, diving ducks, gulls) based on foraging methods and prey requirements to understand waterbird use of these ponds. We used these data to assess long-term changes in waterbird numbers relative to baseline counts from before marsh restoration. We recorded 1,246,072 waterbird observations of 82 species (all sites combined). The Alviso and Eden Landing pond complexes supported the greatest species diversity and Alviso had the highest abundances of all complexes. The abundance of 5 out of 7 currently tracked species/guilds has increased in SBSPRP ponds since prior to restoration activities in 2005–2007 (6 of 8, if counting small shorebirds in both the fall and spring). Exceptions comprise Bonaparte's gulls, dabbling ducks, and medium shorebirds; plus phalaropes, which supplemental summer surveys show have declined. For most of the species/guilds that increased in abundance across all ponds, the increases are largely due to higher counts within the SBSPRP area. This indicates long-term benefits of the project for many waterbird populations within South San Francisco Bay. However, 6 of the currently tracked fall/winter/spring taxa have declined in recent years, which is cause for caution. Only small shorebirds are currently stable (slightly increasing in fall and slightly declining in spring).
Two species/guilds have declined below trigger values defined in the Adaptive Management Plan: Bonaparte’s gulls and phalaropes. In the previous year, Bonaparte’s gull had declined by 84% relative to their baseline abundance, crossing the adaptive management plan’s trigger threshold of >50% decline below the baseline in a single year, or >25% in three consecutive years. This winter, detections of Bonaparte’s gull had increased from the previous year to an average of 763, but this still represents a decline of -40% and the running average two-year decline is -62% (winter 2021 and 2022 seasons; winter 2020 was not surveyed due to COVID-19 restrictions).
Given that two other saline specialist guilds also show signs of decline within the SBSPRP area–phalaropes have passed the trigger level, and eared grebes remain in salt production ponds but have dwindled in the SBSPRP footprint–the loss of high-salinity habitats is likely one of the causative factors. We recommend special consideration should be given to habitat needs of phalaropes and Bonaparte’s gulls, but note that these are poorly understood within the South San Francisco Bay ecosystem. More focused field studies on their habitat use and selection are needed to identify their habitat requirements and understand how to manage them effectively to recover these populations. In the meantime, we recommend cautiously maintaining a variety of water quality parameter levels in order to support guilds with different habitat requirements.
