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Biological Invasions (the establishment of species beyond their historical range) are a major force of ecological and evolutionary change. Invasions are fundamentally changing the structure and function of most ecosystems around the world and are impacting many dimensions of human society. Moreover, the observed rates and impacts of new invasions have increased dramatically in recent time. ...more
Feature Story February 2013
A Necklace of Clam Shells May Save Oysters in Elkhorn Slough
By Dr. Chela Zabin
Last summer researchers and volunteers teamed up to bring oysters back to Elkhorn Slough. Elkhorn Slough is on Monterey Bay near the town of Moss Landing, CA. The slough is home to Olympia oysters (Ostrea lurida), a native species that was historically abundant in many locations along the west coast of North America but is now in decline.
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| Volunteers collected thousands of clam shells for use in the oyster restoration effort in Elkhorn Slough. |
Oyster reefs are a key marine habitat. Unfortunately oyster populations in many regions of the world have been devastated by a combination of overharvesting, habitat destruction, water pollution, increased sedimentation, and more recently, disease, introduced predators, competitors, and parasites. The result has been a steep decline in oyster reefs and the loss of many of their key benefits including water filtration, habitat provision, shoreline protection, and fisheries support.
Oyster restoration efforts have been high for a number of decades on the East and Gulf coasts of the US as this valuable food source became scarce and people became aware of the ecosystem benefits provided by oysters. Oyster restoration efforts on the West Coast are newer in large part because the small Olympia oysters have not been an important fishery species since the Gold Rush days. Olympia oysters were historically present and abundant in many locations along the west coast of North America, and are thought to have once provided a range of ecosystem benefits presently in short supply in this highly impacted region. Olympia oyster populations today are drastically smaller than they were historically, and oyster habitat is considered functionally extinct in several West Coast estuaries, including San Francisco Bay and Elkhorn Slough.
Native oysters were “very plentiful in all parts of the [Elkhorn] Slough” in the 1930s when naturalist George MacGinitie (1935) surveyed the estuary. At last estimate, oysters numbered 5,000-10,000, a level that places the population in danger of local extinction (Wasson 2010). Elkhorn Slough oysters may play a critical role in connecting northern and southern populations such as those in San Francisco to those in Mugu Lagoon near Oxnard, CA. Mugu Lagoon has the next known Olympia oyster population to the south and is hundreds of miles away. Thus, a population crash at Elkhorn might have significant region-wide effects.
Researchers at Elkhorn Slough rigorously evaluated several factors that might limit oysters there and concluded that lack of appropriate substrate was the single-most import factor (Wasson et al. 2010). Oysters need hard surfaces on which to settle and grow. Pilot studies indicated that oysters settled readily to shell “necklaces” made from the shells of the large native gaper clams strung on rope and suspended above the mud. The necklaces outperformed the other types of restoration structures tested. In 2012, Dr. Kerstin Wasson at the Elkhorn Slough National Estuarine Research Reserve and Dr. Chela Zabin from the Smithsonian Environmental Research Center's (SERC) Tiburon laboratory received funding for two years from the California Department of Fish and Game to scale up oyster restoration using the shell necklaces.
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| Six clamshell necklaces were places at 10 sites at the Elkhorn Slough Reserve. At each site, three necklaces were set up in the higher intertidal and three in the low. |
Gaper clams live in the mud near the mouth of the estuary, where native oysters do not occur. Resident sea otters prey on these clams, leaving the shores littered with empty shells. In the summer, Dr. Zabin, Reserve staff, and community volunteers collected shells at low tide and assembled 60 necklaces of 15 shells each. To learn more about the best locations for oyster restoration within the slough, they selected 10 sites at different distances from existing populations including rocky and mudflat sites. At each site they placed six necklaces at two tidal heights, three in the higher intertidal and three in the low (0 and -1 ft.).
By December an average of 50 oysters had settled on each necklace, with more than 3,000 across all the necklaces! Necklaces far from existing oyster populations did just as well as those near adult oysters. Muddy sites did just as well as rocky sites in terms of numbers of live oysters, but necklaces placed adjacent to rip-rap lost many newly settled oysters probably to predation by crabs hiding in the rocky crevices. Necklaces set lower in the intertidal zone had more oysters than those higher up, but also more non-native tunicates, bryozoans, sponges, and hydroids. Over the long-term, oysters in the low intertidal may suffer from overgrowth by these species.
This coming summer the team plans to triple the number of necklaces at Elkhorn Slough and fine-tune placement using lessons learned in 2012 such as avoiding rip-rapped sites. In addition, some of the necklaces from the low intertidal will be moved up out of the comfort zone of many of the non-native fouling species to test whether oyster growth and survival might be improved if these other species die back.
If you live near Elkhorn Slough and would like to volunteer with this summer’s effort contact Dr. Zabin at zabinc@si.edu. The effort will be getting underway in June. More information on the native oysters in Elkhorn Slough is available on their website.
References
MacGinitie GE 1935. Ecological aspects of a California marine estuary. The American Midland Naturalist 16 (5): 630-765.
Wasson K 2010. Informing Olympia oyster restoration: evaluation of factors that limit populations in a California estuary. Wetlands 30: 449-459.
All photos are by Chela Zabin
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