Restoration Approaches

HOW CAN WE BRING BACK OYSTER BEDS?

Olympia oyster beds have disappeared in areas where they once thrived for many reasons. To bring them back, we first have to do some detective work to figure out what the biggest obstacles are to returning healthy oyster beds in a particular estuary site. For saving a human life, medics have a sequence of A-B-C questions, first checking if the airway is open, then whether there is breathing, and finally circulation. Those life threats have to be addressed in that particular order. In the same way, oyster restoration practitioners ask a sequence of questions that have to be addressed in a logical order.

Question 1: is the water quality good?

If yes, if water quality is sufficient to support oysters, with no extended low oxygen periods or contaminants that pose threats to them, you can go on to the next question.
If no, you need to address this issue. Many areas that formerly hosted oyster beds do not any longer because water quality is too poor. Oysters like all animals need oxygen. In portions of estuaries that are stagnant due to tide gates or dikes, or in areas that receive excessive nutrient runoff from fertilizer or sewage, oxygen levels can drop too low for oysters. Bringing back healthy water quality is the first step for restoration not only of oyster beds but also other invertebrates and fish in the estuary.

Question 2: are there enough places for baby oysters to attach on the mudflat?

If yes, if there are enough naturally occurring shells, cobbles, rocks or gravel bars such that oysters have plenty of places to form a bed, you can go on to the next question.
If no, you need to address this issue. In some places, there is only deep mud. To bring back an oyster bed to such a location, a base layer of shells or cobble must be provided as a foundation for the bed.

Question 3: do oysters live at least a few years on this bed?

If yes, if many oysters grow to a good size and stick around for at least a few years, you can go on to the next question.
If no, you need to address this issue. Figure out what is killing oysters. If it is competition with a non-native species, or predation, consider focusing restoration either higher or lower on the shore, to be outside the tolerance of the species posing the threat. Alternatively, it may be necessary to choose a different site where survival is higher, or to provide fresh shell yearly until oyster settlement is sufficiently high to outpace mortality.

Question 4: are there enough baby oysters?

If yes, if bare surfaces put out at the right water depth become covered with native oysters within a year or two, and you answered yes to all the previous questions, conditions should be favorable for restoring a thriving, persistent oyster bed to this location.
If no, you need to address this issue. Oyster beds can only persist if there are lots of new baby oysters settling regularly. On average, there need to be at least as many new oysters getting established each year as are dying from old age or predation. And in areas with high sedimentation rates or where legacy shells degrade quickly, oyster beds need to keep building up their elevation to stay above the mud or beat the pace of shell bed degradation. Many estuaries today no longer have enough adult oysters to spawn and generate new generations. Adult oyster numbers first have to be increased before restored oyster beds will be sustainable. Ideally, there will be high adult numbers at multiple nearby sites, so that at least one of them experiences appropriate environmental conditions for high reproductive output in any given year – a portfolio effect to increase the odds of high, regular settlement on the restored bed.

Conservation aquaculture is a tool for increasing local adult oyster numbers. Local oysters are brought to a hatchery and pampered. They generate baby oysters, which also can be pampered with good algal food and protected from predators for a while, and then outplanted to the field, either on beds or on higher profile temporary structures that keep them out of the mud. Once enough adults are established from aquaculture, ideally at many sites with varying conditions, there should be plenty of baby oysters.

Adult numbers can also be increased by moving adults or spat from more distant locations that have plenty to parts of the estuary that have fewer. For example restoration practitioners can deploy attractive oyster habitat (e.g., strings of shells or aggregates of shells in bags) around the estuary of interest at multiple locations, allow the local oysters to settle onto the far-flung “collectors”, and then concentrate the recruited oysters near or onto the restoration site. This approach is an excellent and low-cost option in estuaries where oyster settling and recruitment varies across space and time, with good years and bad years in any particular location. When a restored oyster bed needs a little boost from time to time during a bad year, then remote setting can help the restored bed keep pace with legacy shell loss and sedimentation by relying upon settlement across a broader spatial scale in order to capture the nearby “good year” locations.

Schematic flow for restoring native oyster beds (Source: powerpoint file Kerstin made, jpg version in this folder)

TWO DIFFERENT APPROACHES: RESTORING BEDS VS. PROTECTING SHORELINES

1) Restoring oyster beds

Because Olympia oyster beds were once present in our estuaries and now have almost disappeared, efforts are underway all along the coast to bring back this vital habitat type. In the past, new juvenile oysters would settle on the shells of hundreds of years of dead oysters that built up habitat on the mudflats. Those old shell layers are gone from many estuaries, in part because they were scooped out and used to build roads and other structures after colonization. Mudflats have also gotten muddier, with sedimentation rates increased by human activities such as clearing trees or mining in the watershed. On top of that, settlement rates by new juveniles have decreased, as adult population sizes in the estuary shrunk due to factors such as poor water quality. So to restore an oyster bed, all those factors must be addressed.

Swinomish Indian Tribal Community oyster bed restoration with shell addition in Skagit Bay (source: Julie Barber)

Oyster bed restoration using spat settled on shells anchored on wooden stakes in Hood Canal, Puget Sound (source: Stephanie Valdez)

Conservation organizations and Tribes that are working to restore natural beds typically add layers or shell or small rocks to serve as the base layer, mimicking the natural shell beds that would have been present providing foundation structure in the past. This layer must be thick in areas with deep mud.

Then, if settlement rates are too low for oysters to initially generate sustainable beds, conservation aquaculture may be used to support them. For example, spat (baby oysters) on shells may be added to the foundation layer, or hatchery-raised adults may be placed in oyster baskets nearby to serve as a source of new babies.

Monitoring a restored bed in Alamitos Bay, southern California

2) Shoreline protection

Waves can eat away at the edges of salt marshes, and also of human infrastructure such as hiking trails on levees or roads built adjacent to the estuary. This will become an increasing threat as sea levels rise as a part of a warming climate. In some cases, where shoreline edges must be protected, structures are added in the water to buffer these edges from wind-driven waves. On the Atlantic coast of North America, salt marshes extend low into the intertidal zone, and oysters extend high into the intertidal zone. Indeed, the most landward oysters and the most seaward marsh plants co-occur. Because of this similarity in elevation, and because Atlantic oyster beds grow quite tall, restoration of natural oyster beds can effectively protect salt marshes from wind waves.

Tall stacks of reef balls in San Francisco Bay (Source: Stephanie Kiriakopolos)

On the Pacific coast of North America, salt marshes are mostly higher in the intertidal zone, and the native oyster is limited to the low intertidal zone. Oyster beds are typically found more than two meters (six feet) below the marsh-upland boundary where many trails and roads are located on levees.

Monitoring large reef balls in San Diego Bay (Source: Danielle Zacherl)

Oyster beds are not very tall, and today often have fairly sparse oyster numbers. So on this coast, oyster beds on mudflats do not provide much dampening of the erosive power of wind waves on the shoreline above, especially during high tides. In order to buffer wind waves that cause erosion of shorelines, where protecting the static location of that shoreline is deemed important, artificial structures can be placed in the water in front of the valued edge that is being defended. These structures are typically tall, extending much higher upward than oysters can survive, thus providing protection to the adjacent shoreline from wind waves. The lower portions of the structures occur within the tolerance range of native oysters, and where settlement rates are high, there may be many oysters living on them. Other native coastal species also attach to these structures, which therefore often have much greater representation of species that settle on hard substrates than the surrounding mudflats.

SCHEMATIC OF THE TWO CONCEPTUALLY DIFFERENT APPRAOCHES

Tidal datums on the Y axis are meters above Mean Lower Low Water and are based on Elkhorn Slough, California.