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Elkhorn Slough Research: Native Oysters at Elkhorn Slough
Olympia Oyster in Elkhorn Slough
Olympia oyster

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Olympia oysters are native to the Pacific coast of North America. These are elegant little oysters, with orange and purple striping on young individuals, and are very tasty. Studies have shown that native oyster reefs improve water quality by their filter feeding activity, and provide habitat that increases estuarine fish and invertebrate diversity.

At Elkhorn Slough, Native American middens reveal that native oysters were present in the estuary from sites near the mouth (e.g., Struve Pond, Moro Cojo) to the upper estuary (e.g., South Marsh), and eaten by humans, for the past seven thousand years. When George MacGinitie first studied the invertebrates of the estuary in the 1920s, he also reported oysters as being highly abundant in many sites from the mouth to the head of the estuary. But within a decade, they had been overharvested by oystermen from San Francisco Bay, and became so rare that they were not detected in surveys for many decades.

Olympia Oyster Elkhorn Slough Research
Olympia oyster (tagged to measure growth rates) surrounded by non-native
tubeworm at Kirby Park.

Meanwhile, non-native (Asian and Atlantic) oysters were brought to Elkhorn Slough and cultured on racks, from the 1930s to 1980s. These non-native oysters never reproduced in the estuary because the water was too cold. So when aquaculture stopped, they disappeared from the estuary. But unfortunately, many non-native species that were accidentally introduced with them, such as a bright orange sponge and a little horn snail, happily reproduced and became widespread in the estuary.

Today the native Olympia oyster is absent or extremely rare in most parts of the estuary, including areas where it once thrived. The only sites with more than 100 live oysters present are at Kirby Park, North Azevedo Pond (managed by the Elkhorn Slough Foundation), and various sites on the Elkhorn Slough Reserve (North Marsh outlet, South Marsh, Whistlestop Lagoon, and Parsons Railroad bridge).

Studies by Elkhorn Slough Reserve’s research coordinator, Kerstin Wasson, have revealed three major threats to native oysters at Elkhorn Slough. The first is poor water quality: oysters are absent from sites with indicators of extreme eutrophication (high nutrient concentrations, high turbidity, and low night-time oxygen levels). The second is burial by sediments: in many areas, oysters are smothered by mud if they grow on the tiny bits of natural hard substrate that are available (such as shells); they only survive burial by growing on artificial hard substrates such as rip rap. The third major threat is non-native fouling species: at lower tidal heights, oysters are overgrown by non-native sponges, tunicates and tubeworms.

Oyster Research at Elkhorn Slough
Volunteers assemble mesh tubes with clam shells to build mini-reefs.

Oyster Research at Elkhorn Slough
Mini-reefs of clam shells deployed by summer intern on Elkhorn Slough Reserve mudflats.

From 2009-2011, Wasson piloted small-scale restoration projects with the help of interns, community volunteers and staff. Since sediment burial poses a threat, addition of larger hard substrates can help increase oyster populations in areas where conditions are appropriate. The team collected the shells of large butter and gaper clams to use for restoration. These large clams do not occur in the parts of the estuary where oysters are abundant. However, they are abundant near the mouth of the estuary, and shells are generated in large numbers by sea otter foraging. The team arranged these shells into various modular reef designs: putting them in biodegradable mesh tubes, stringing them onto "necklaces", and placing in wooden box frames, or embedding them in thin concrete platforms. These clam shell reefs were placed at various sites managed by the Elkhorn Slough Reserve and Elkhorn Slough Foundation and monitored for oyster recruitment and growth. The shell necklaces performed the best: they had the most oysters and the least amounts of sediment and non-native species.

Wasson, staff member Susie Fork, and Smithsonian collaborator Chela Zabin, with support from CDFG’s Environmental Enhancement Fund, are scaling up restoration efforts and refining methods for oyster restoration at the slough. Over the next two years, the team will deploy 180 oyster reefs at Elkhorn Slough, beginning with 60 shell necklaces placed at 10 sites and 24 “Reef Balls” at 3 sites in summer 2012. The Reef Balls are dome-shaped cement structures widely used in marine and estuarine restoration, and are being simultaneously tested in San Francisco Bay for oyster substrate as part of a living shorelines project.  Along with continuing to refine oyster reef design, the team at Elkhorn will compare oyster performance at two tidal elevations, at sites with soft vs. firmer mud and at sites near and far away from existing oyster populations. All reef structures are mobile and modular, so they can be relocated as the team determines best sites and tidal heights.

Zabin, Wasson and Fork deploy the shell necklaces.


Oyster Research at Elkhorn Slough
Staff use a PVC  pole to move Reef Balls from the boat into place on the mudflat.

The Elkhorn Slough Reserve and Foundation hope to support the recovery of native oysters in the estuary where they have grown for thousands of years. Populations now are low enough that there is a real danger of local extinction. The next closest oyster population to the south is in Mugu Lagoon; the next to the north is in San Francisco Bay. The Elkhorn Slough population is thus important to restore as a part of local ecosystem integrity, and also as a stepping stone connecting northern and southern oysters along our coast.

Oyster Research in Elkhorn Slough
Researcher surveying oysters at North Azevedo Pond, a restoration site
managed by the Elkhorn Slough Foundation with some of the highest
native oyster recruitment rates in the estuary.


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