Subtidal – Waterfowl
What is Subtidal Habitat?
Updated: 20250808
Subtidal: This submerged habitat remains underwater even during the lowest tidal cycles. Shellfish beds including oyster reefs and submerged aquatic vegetation including eelgrass beds are critical nurseries for invertebrates and fish and provide important foraging for diving ducks and other waterbirds. These habitats also protect and buffer local shorelines in the face of sea level rise and other climate stressors.
Subtidal habitat in the Bay is struggling to provide quality habitat for the diving duck species indicator.
Diving Ducks use the subtidal habitat of the open bays in the winter, mainly foraging near shellfish reefs and eelgrass beds. These species breed elsewhere and rely on SF Bay as migratory stopovers and wintering grounds.
Diving Duck Indicators
Diving Duck Indicator Species
Scaup
Ruddy Duck
Surf Scoter
Canvasback
Bufflehead
Common Goldeneye
Diving ducks declined in open waters of the North and Central Bay and were relatively stable in the South Bay.
Methods
Data on diving ducks in subtidal, open water habitats were collected during the Midwinter Waterfowl Survey which occurs once annually during January and covers North and South San Francisco Bay.
Diving ducks are the most abundant waterfowl in San Francisco Bay, using open bay and pond habitats for resting and feeding. Historically, nearly half of the lesser and greater scaup and surf scoters in the lower Pacific Flyway have overwintered in the SF Bay. However, these species have declined significantly across the Bay over the last 30 years, with scaup populations now below the targets set by the San Francisco Bay Joint Venture.
Conversely, ruddy duck numbers, often found in former salt ponds, have risen over the last decade. Other species like canvasback, bufflehead, and goldeneye have shown fluctuating but overall stable trends across the Bay.
The reasons for the decline in scoter and scaup are unclear, likely stemming from changes in both their breeding and wintering grounds. Improving food availability and creating undisturbed resting areas in the Bay’s open water and pond habitats could improve conditions for these species in SF Bay.
Diving ducks declined in open waters of the North and Central Bay and were relatively stable in the South Bay.
Losses-smoothed long-term trends of diving duck abundances in the (A) North Bay Open Bay, (B) Central Bay Open Bay,( C) South Bay Open Bay regions of the San Francisco Bay from the Midwinter Waterfowl Surveys. Counts were adjusted for differences in survey effort among years.
Diving Duck Trends Across All Habitats
[PLACEHOLDER IMAGE]
Loess-smoothed long-term trends of diving duck species in San Francisco Bay from the Midwinter Waterfowl Survey conducted in January from 1989 to 2024. Counts were summarized from the North Bay Open Bay, North Bay Salt Ponds, Central Bay Open Bay, Shoreline, South Bay Open Bay and South Bay Salt Ponds and were adjusted for differences in survey effort among years.
Diving canvasback
Short description. Add image or rich content here.
Barrow's Goldeneye
Details about the bird and its habitat.
Surf scoter
Notes about behavior and diet.
Black Oystercatcher
Info about rocky shore habitat.
Herring eggs in eelgrass
Spawning timing, predators, and protection.
Olympia oysters
Ecosystem services and restoration.