About halfway between Santa Cruz and Monterey is Elkhorn Slough. This estuary, with its meandering waterways and abundant wildlife, is a destination for kayakers, bird watchers and researchers. About a third of the slough is a National Estuarine Research Reserve. “So you’re seeing a little sample of the classic estuarine habitat types here. We are standing on a salt marsh beneath our feet. You see channels with permanently standing water and creeks. And then the third major habitat type of estuaries is mud flats. And that’s what we are about to venture out on is the mud,” said Kerstin Wasson, Research Coordinator for the Elkhorn Slough National Estuarine Research Reserve.
Wearing wader overalls, Wasson and UC Davis Biologist Dr. Chela Zabin walk into the thick, sticky mud. They’re headed toward a cluster of five different man-made oyster reefs that rest on top of the mud. “What you see are a number of different reef designs that we’ve tried over the years, biodegradable mesh tubes filled with clam shells. These are shell necklaces where we’ve strung clamshells onto strings,” said Wasson as she points out the different reefs. The reefs are a pilot project in search of the best habitat for California’s only native oyster, the Olympia oyster. Until Wasson rediscovered it in Elkhorn Slough about five years ago, the population was thought to be locally extinct. Now local extinction is a real possibility. “We don’t have many adults left, in the whole complex here, we think there’s maybe 500 adults alive,” said Wasson. That’s 500 in the reserve and maybe 5000 in the whole Elkhorn Slough. At birth oyster larvae can swim around, but to grow into a full size adult it must attach to something hard. When oyster populations are abundant, the oysters make their own reefs by attaching to each other and old oyster shells. But when the populations are as low as they are at Elkhorn, the oysters need some help. The three foot long clam shell necklace reef suspended above the mud is proving to be most effective. “After two years they’re adult size oysters growing on clam shells,” said Wasson.
Beyond Elkhorn Slough the nearest Olympia oyster populations are to the south near Malibu at Mugu Lagoon and to the north in San Francisco Bay. That’s where Dr. Cheyla Zabin does much of her oyster restoration work. She says the point of local restoration is not to create oysters that could be harvested. California’s Olympia oyster population isn’t big enough. “So that’s a huge challenge. You really need to bring numbers up so people can harvest, but there’s still enough oyster shell around for new oysters to settle on. That’s not an issue for us here. Maybe that will be a problem that we’d love to have in the future, but that’s not really the point of oyster restoration in California,” said Zabin. The point is in part to regain the ecosystem benefits provided by oysters like improved water quality through their filter feeding. Those are benefits not yet seen at Elkhorn. “In my mind you don’t even need to invoke those kinds of what will they do for us arguments. I mean, I think we care about there being condors and polar bears and native oysters, just for the sake of preserving the legacy of the native biodiversity that existed in our habitats on earth,” said Wasson. A recent grant from the California Department of Fish and Game will help preserve that legacy. With the addition of 180 oyster reefs in the Reserve, Wasson hopes to double the Olympia oyster population.
Much of the work at the Elkhorn Slough Reserve is done by volunteer citizen scientists. For more details visit elkhornslough dot org