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Relatively little hard substrate occurs naturally in the estuary. Rock habitat encompasses boulders to bedrock; i.e., rock that is not normally moved by currents. Shellfish beds and some algal beds are a subset of rock habitat.
Rock habitat occurs mainly as scoured low-relief bedrock in the deep, narrow channels where the estuary passes through the Coast Range and as bedrock outcrops and boulders in the areas of the Central Bay where currents are strong. Many rock outcrops, especially those near the entrance to San Francisco Bay, have been lowered by blasting to reduce the hazards they present to ships, and these may be lowered further as ships with greater draft are built. Some rock outcrops are flat-topped and are surrounded by boulder fields, presumably a result of previous blasting.
This habitat is threatened by blasting to remove it for navigational safety, colonization by invasive species, possibly by sediment deposition, and for intertidal rock, from oil spills and trampling. While rock habitats support valued ecosystem services and are in short supply in the estuary, restoration seems impractical. The Subtidal Goals Project recommends protecting and managing rock habitat from being removed for vessel traffic and damaged by public access and enhancing it by removing invasive species and debris. It also recommends improving scientific understanding of the ecosystem services this habitat provides and the species that use it.
To read more, download the rock habitats section of our report.