Olympia oysters have lived in Elkhorn Slough for many thousands of years, but are now very rare. In many years, no baby oysters are produced anywhere in the estuary. The Elkhorn Slough Reserve and Foundation hope to support the recovery of native oysters in the estuary through science-based restoration projects.
Background on Olympia oysters
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.
Native oysters at the Slough:
once common, now rare
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. Today the native Olympia oyster is absent or extremely rare in most parts of the estuary, including areas where it once thrived.
Factors limiting Olympia oysters at the Slough
Studies by Elkhorn Slough Reserve’s research coordinator, Kerstin Wasson (see http://library.elkhornslough.org/attachments/Wasson_2010_Informing_Olympia_Oyster_Restoration.pdf) 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, most available space is taken up by non-native sponges, tunicates and tubeworms.
Olympia oyster restoration goals
Because Olympia oysters are so rare at Elkhorn Slough, and undergo many consecutive years with no recruitment (no new baby oysters join the adult populations), they are in real danger of local extinction. Olympia oysters have gone extinct in the next major estuary to the south, Morro Bay, making the next closest oyster population to the south of Elkhorn Slough very far away, in Mugu Lagoon. To the north, the nearest substantial population is in San Francisco Bay.
The goal of oyster restoration at Elkhorn Slough is thus to prevent local extinction, to maintain a population that has lived in this estuary for thousands of years, and to provide a stepping stone connecting northern and southern oysters along our coast. We hope to double the numbers of oysters in the estuary (currently around 5000-10,000) to buffer the population against local extinction.
Natural restoration designs
Unlike Asian or Atlantic oysters, which often form extensive reefs, Olympia oysters tend to grow in small clusters. Our restoration efforts attempt to mimic this natural growth form. We have experimented with various small, modular reefs that provide habitat for oysters. Most of our designs use native gaper clam shells generated by sea otter foraging as the hard substrate, combined with stakes to keep the clusters out of the mud.
Learning from restoration experiments
Elkhorn Slough Reserve staff members Kerstin Wasson and Susie Fork, together with Smithsonian Environmental Research Center scientist Chela Zabin, spent two years conducting large replicated restoration experiments on the Elkhorn Slough Reserve, with funding from CDFW’s Environmental Enhancement Fund. About 200 modular reefs were deployed, including clam shell “necklaces”, Reef Balls, and stakes of various types of wood. The team compared oyster success under different habitat conditions, and summarized lessons learned in a scientific paper (http://library.elkhornslough.org/attachments/Zabin_2016_Restoration_Of_Native_Oysters.pdf)
They examined effects of bottom type (gravel or mud), distance existing adults (near/far), and tidal elevation on oysters that settled on clam-shell “necklaces”. The most striking effect was tidal elevation. More oysters settled lower down, but more non-native sponges, tunicates, and bryozoans did too. So where to put the reefs depends on the restoration goals: to maximize oyster numbers, put them low, but to maximize oyster dominance, and have virtually pure native cover, put them high. They also did some adaptive management with this concept, initially putting the reefs low to accumulate lots of newly settled oysters, then moving them high to kill off the non-natives. While using stressful conditions to enhance natives over non-natives is common in terrestrial landscapes (grazing, fire, etc.), this is perhaps the first application of this concept to marine systems, so it has some general value to practitioners working on restoration beneath the tides in other systems.
Non-native oyster aquaculture
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 mud snail, happily reproduced and became widespread in the estuary. Non-native oysters are farmed at various estuaries along the Pacific coast, including Tomales Bay and Puget Sound. Native oysters are farmed at a very small scale in Puget Sound.
Olympia oysters up and down the coast
Elkhorn Slough researchers have collaborated with others to determine optimal conditions that favor Olympia oyster restoration in central California (http://library.elkhornslough.org/attachments/Wasson_2014_A_Guide_To_Olympia.pdf
), and along the entire West Coastv(http://library.elkhornslough.org/attachments/Wasson_2015_A_Guide_To_Olympia.pdf
). More about these collaborative assessments and a “do-it-yourself” tool to site selection are available at www.oysters-and-climate.org. A team of researchers led by Wasson from Elkhorn Slough has also recently examined recruitment of oysters along 2500 km of Pacific coast, determining that there is little synchrony, and identifying that marine-influenced estuaries with only small networks of oyster sites are more prone to recruitment failure.