Abstract
Seagrasses are highly productive, but human nearshore activities have reduced their global distribution by >29% since the twentieth century. In the United States and Canada, the native seagrass Zostera marina (eelgrass) provides habitat for many species and multiple ecosystem services. By supplying spawning surface for fish and substrate for invertebrates, eelgrass creates foraging areas for high densities of migratory birds. Eelgrass beds stabilize sediment, protect adjacent shorelines, improve water quality, and sequester carbon in their underlying substrate. San Francisco Bay (California, USA) is a significant estuary for eelgrass, and recent surveys show that eelgrass beds are in decline. Protecting eelgrass is a conservation priority for federal, state, and local agencies, yet few studies have documented the extent of eelgrass loss due to human impacts such as boat anchoring. The purpose of our study was to provide factual evidence for policy makers by quantifying damage to eelgrass caused by illegal anchor-outs in San Francisco Bay, an issue that has been disputed for decades. Using aerial imagery and GIS analyses, we determined the amount of direct damage to eelgrass caused by anchor-outs. We found that boats damage up to 41% of the eelgrass bed, and each boat may cause up to 0.3 ha of damage. These results can be used to inform decisions about anchor-outs by stakeholders and government agencies. Furthermore, our efficient analytical approach could be implemented in other coastal regions.
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Acknowledgements
This research was funded by the Gordon and Betty Moore Foundation. We thank Pat and Julie Belanger of 111th Aerial Photography Group for taking the aerial images. Dr. Katharyn Boyer and Dr. Ellen Hines gave valuable input regarding the identification of anchor scour and eelgrass conservation in San Francisco Bay. We appreciate Dr. Tendai Chitewere, Heather Richard, Keith Merkel, Barbara Salzman, and representatives from Marin County, the City of Sausalito, and the RBRA for discussions on the anchor-out issue that informed this work. Conversations with Alden Bevington and others from the Richardson Bay Special Anchorage Association also provided insight into the anchor-out community. We also thank Andrea Jones, Anna Weinstein, Rebecca Schwartz Lesberg, and our peer reviewers for their helpful suggestions.
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Kelly, J.J., Orr, D. & Takekawa, J.Y. Quantification of damage to eelgrass (Zostera marina) beds and evidence-based management strategies for boats anchoring in San Francisco Bay. Environmental Management 64, 20–26 (2019). https://doi.org/10.1007/s00267-019-01169-4
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DOI: https://doi.org/10.1007/s00267-019-01169-4