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Shellfish Mariculture in Drakes Estero, Point Reyes National Seashore, California Summary When Drakes Estero, which lies within the Point Reyes National Seashore (PRNS) about 25 miles northwest of San Francisco, California, was designated by Congress in 1976 as Potential Wilderness, it contained a commercial shellfish (nonnative oyster and clam) mariculture operation. Oyster mariculture began in Drakes Estero with the introduction of the nonnative Pacific oyster (Crassotrea gigas) in 1932, after the beds of the native Olympia oyster (Ostrea lurida) had been depleted throughout the region by overharvest, and has been conducted continuously from that date forward. Hence, the cultural history of oyster farming predates the designation of Point Reyes as a National Seashore in 1962. Nevertheless, with the approach of the 2012 expiration date of the current National Park Service (NPS) Reservation of Use and Occupancy (RUO) and Special Use Permit (SUP) that allows Drakes Bay Oyster Company (DBOC) to operate within the estero,1 NPS has expressed concern over the scope and intensity of impacts of the shellfish culture operations on the estero’s ecosystem. Recent public documents produced by NPS characterizing the impacts of shellfish culturing in Drakes Estero have stimulated public debate over the pending expiration of the RUO and the presentation of scientific information that appeared to justify closing the oyster farm. The increasingly contentious nature of the debate led to the request for this 1 The term estero is used instead of estuary because Drakes Estero has more in common with coastal lagoons (low freshwater input and high salinity) than in a typical estuary.
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Shellfish Mariculture in Drakes Estero, Point Reyes National Seashore, California study to help clarify the scientific issues raised with regard to the shellfish mariculture activities in Drakes Estero (See Box 1 for Statement of Task). Oyster mariculture necessarily has ecological consequences in Drakes Estero as in other lagoons and estuaries, the magnitude and significance of which vary with the intensity of the culturing operations. These effects derive from two different sources: the presence of and biological activity of the oysters, and the activities of the culturists. Oysters provide many ecosystem services, so the return of oysters to Drakes Estero through commercial mariculture could enhance the ecosystem by restoring some historic baseline functions. The degree to which the presence and biogeochemical activities of the nonnative Pacific oysters in Drakes Estero reproduce the historic contributions of the original Olympia oysters to the ecological functioning of the estero depends on how closely historic oyster abundances, collective biomass, and thus filtering capacity plus hard-substrate habitat are matched by the cultured nonnative oysters. The hard BOX 1 Statement of Task—Part 1a An ad hoc committee will be formed to produce two reports on shellfish mariculture in coastal areas. In the first report, the committee will assess the scientific basis for the National Park Service (NPS) presentations and the report (including revisions), “Drakes Estero: A Sheltered Wilderness Estuary,” on the ecological effects of the Drakes Bay Oyster Company operations on Drakes Estero, Pt. Reyes National Seashore in California. In carrying out its task, the committee will address the following questions: What is the body of scientific studies on the impact of the oyster farm on Drakes Estero, and what have they shown? What effects can be directly demonstrated by research conducted in Drakes Estero itself? What effects can reasonably be inferred from research conducted in similar ecosystems? What conclusions can be drawn from the body of scientific studies, and how do they compare with what the NPS presented to the public? Have these conclusions affected NPS decision making? What are the most important subjects for future research to better understand the ecological consequences of anthropogenic influences on the estero, so as to inform NPS decision making? aThe committee will prepare a second report, published separately, that will address in more general terms the elements of best management practices for application to shellfish mariculture to enhance the benefits and minimize any negative ecological effects (See Appendix B).
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Shellfish Mariculture in Drakes Estero, Point Reyes National Seashore, California substrate habitat provided by oysters, elevated wooden racks, and plastic mesh bags on the bottom does not replicate the exact nature of structural habitat once offered by beds of native oysters on the bottom. Insufficient information is available to know how many oysters and how much biomass existed under historical baseline conditions, but Olympia oysters form extensive reefs covering the lower intertidal zone and extending into the subtidal of some relatively pristine lagoons, bays, and estuaries of British Columbia, a possible analog of the ecosystem in Drakes Estero prior to overharvesting of the native oyster. The activities of the oyster culturists can disturb wildlife such as harbor seals, which are of particular concern because they use the estero for resting, mating, pupping, suckling, molting, foraging, and sheltering from oceanic predators. NPS has recently released documents to inform the public about the impacts of oyster mariculture on the Drakes Estero ecosystem. Scientific conclusions presented in Drakes Estero: A Sheltered Wilderness Estuary change in successive versions from late 2006 through 2007, with some notable deletions of earlier material and a few additions. However, Drakes Estero: A Sheltered Wilderness Estuary never achieved a rigorous and balanced synthesis of the mariculture impacts. Overall, the report gave an interpretation of the science that exaggerated the negative and overlooked potentially beneficial effects of the oyster culture operation. Ultimately, the NPS “Acknowledgment of Corrections” (July 2007) and “Clarification of Law, Policy, and Science” (September 2007) retracted several misrepresentations of the Anima (1991) and (Elliot-Fisk, 2005) studies and presented descriptions of ecological impacts of the shellfish culture operations that closely approach the conclusions reached by this committee, with two major exceptions. First, NPS does not acknowledge the changing ecological baseline of Drakes Estero, in which native Olympia oysters probably played an important role in structuring the estuary’s ecosystem for millennia until human exploitation eliminated them in the period from the mid 1800s to the early 1900s. Second, NPS selectively presents harbor seal survey data in Drakes Estero and over-interprets the disturbance data which are incomplete and non-representative of the full spectrum of disturbance activities in the estero. The committee reached the following conclusions about how oyster and clam mariculture affects key aspects and ecological functions of Drakes Estero: Ecological impacts of enhancing benthic/pelagic coupling. Oysters have a prodigious filtering capacity that can provide resilience in the event of an algal bloom or increased sedimentation from stormwater runoff. As a by-product of this filtering activity, oysters deposit large quantities of pseudofeces and feces and thus transfer materials, including
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Shellfish Mariculture in Drakes Estero, Point Reyes National Seashore, California nutrients and organic carbon, from the water column to the sediments on the bottom. Oysters also release ammonium, thereby fertilizing and stimulating growth of phytoplankton, seagrasses, and macro- and micro-algae. Limited studies of Drakes Estero show the following: (1) relatively high flushing and exchange with the ocean in the areas where most of the oysters are grown, and thus low likelihood of development of sediment anoxia; (2) no empirical evidence of enhanced organic content or sediment hypoxia in eelgrass beds proximate to oyster culture racks; (3) only small increases in sand content of sediments under racks; (4) little change in benthic macro-invertebrate communities with proximity to racks—only enhanced amphipod abundances and an apparent negative effect on another crustacean, the tanaid (Leptochelia dubia), and; (5) relatively low dissolved nutrient concentrations. Based on studies of oysters in other estuaries, cultured oysters in Drakes Estero will contribute to water filtration, the transfer of nutrients and carbon to the sediments, and biogeochemical cycling, although the magnitude of these effects will depend on the stocking density and may be limited by the estero’s high rate of flushing through tidal exchange. Effects of the mariculture on eelgrass. Limited observations of eelgrass in Drakes Estero demonstrate absence of eelgrass directly under oyster culture racks and from propeller scar damage attributable to boats operated by the oyster farm. Mariculture activities had an impact on about 8% of the eelgrass habitat in Drakes Estero in 2007: 1% of eelgrass acreage was displaced by oyster racks and 7% was partially scarred by boat transit through the eelgrass beds. Research elsewhere demonstrates that damaged eelgrass blades have rapid regeneration capacity and that eelgrass productivity can be locally enhanced by the cultured oysters through a reduction in turbidity and fertilization via nutrient regeneration. Eelgrass habitat within Drakes Estero has doubled from 1991 to 2007, a trend seen in some other west coast estuaries. Effects of the mariculture on fishes. Only one study of the effects of oyster mariculture on fishes has been conducted in Drakes Estero. No statistically significant differences in species richness, abundance, or community composition of fish were detected among samples taken in eelgrass adjacent to oyster culture racks, 75 m away, or in neighboring Estero de Limantour. The guild of fishes known to associate with hard substrates exhibited a trend of higher abundances adjacent to the racks, driven largely by one species (kelp surfperch, Brachyistius frenatus). This observation is consistent with other research showing that fishes are attracted to structured habitat for protection and/or feeding. Effects of the mariculture on harbor seals. Drakes Estero is a significant breeding location for harbor seals, and about 20% of the mainland California population come ashore, or “haul-out”, on sandbanks within
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Shellfish Mariculture in Drakes Estero, Point Reyes National Seashore, California the estero during pupping season. Ongoing harbor seal surveys, conducted mainly by volunteers with NPS oversight, were not designed to test the influence of shellfish mariculture on the seal population, but have been used to track trends in the size of seal colonies at the main locations, or sites, in the Point Reyes region where seals come ashore. Within a site, there may be several distinct features, such as a sandbar or rock outcrop, which are monitored as haul-out subsites. Since the restriction of kayakers from the estero during the breeding season (March 1 to June 30), mariculture has become the main anthropogenic activity in the upper estero at that time of year. Statistical analyses of Drakes Estero harbor seal count data during the breeding season suggest a possible relationship between mean counts at two of three subsites where seals haul out on sand bars in the upper estero and the combined signals from the 1998 El Niño and oyster production level. Although these results cannot be used to infer cause and effect for many reasons, as explained in the body of this report, they highlight the need for a more detailed assessment of the extent to which different disturbance sources may impact harbor seals both on land and in the water. In Europe buffers of up to 500–1,500 m have been established around seal haul-outs to protect them from disruption by human activities. No studies have determined whether short-term responses to disturbance have long-term population consequences for harbor seals, but if the disturbance affects behavior during the breeding season, a precautionary approach to management would seek to reduce these types of disturbance to avoid potentially significant population effects. Past, present, and future effects of mariculture on nonnative species. The oysters and clams cultured in Drakes Estero are nonnative species that have some risk of establishing self-sustaining populations. In the past, importations of nonnative oysters were associated with the introduction of a salt marsh snail, Batillaria attramentaria, and the oyster pathogenic parasite, Haplosporidium nelsoni. Currently, the oyster farm imports oyster larvae and spat that meet certification requirements as specific pathogen free, which greatly reduces the potential for new introductions but does not eliminate the possibility of transmission of all oyster pathogens such as oyster herpes viruses. The invasive clonal tunicate, Didemnum vexillum, is considered a pest because it fouls submerged surfaces, including eelgrass to a small degree, and has rapidly overgrown valuable shellfish beds in some other areas. This nonnative tunicate has become established in Drakes Estero, where it covers much of the subtidal hard surfaces provided by oyster shells, racks, and other structures. The cultured oysters together with wooden culture racks and plastic mesh bags increase the availability of hard surface for colonization by tunicates in Drakes Estero, which has few natural hard substrates such as rocky bottom, although shells of native Olympia oysters would have provided
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Shellfish Mariculture in Drakes Estero, Point Reyes National Seashore, California substrate had they not been over-harvested to virtual extinction. The high coverage of tunicates increases the potential for spread of this invasive species within Drakes Estero and Estero de Limantour and possibly beyond through transport of the short-lived larvae and body fragments capable of regeneration. Effects of the mariculture on birds. Drakes Estero is recognized as an ecologically significant overwintering site for shorebirds and waterfowl, especially for overwintering and migrating black brant. The oyster farm likely has some impacts on birds caused by culture bags lying on intertidal sand flats, which limit access to and availability of soft-sediment invertebrate prey. Other shorebirds may benefit from enhanced foraging on small crustaceans and other invertebrates growing on and around intertidal bags and other mariculture structures. Birds can be flushed by the activities of the culturists, particularly while driving boats to and from culture sites, with unknown population consequences. Effects of the mariculture on economics, recreation, and aesthetics. The effect of oyster farming in Drakes Estero on the aggregate economic values generated by PRNS is likely to be small relative to recreational value (on the order of $100 million per year) and value of ecological services (on the order of $20 million to $30 million per year). Recreation is probably not influenced by the mariculture operation except to the degree that the construction and improvement of the road into the land-based oyster farm facilities provides access for launching kayaks and canoes. Visits to DBOC could be considered a form of recreation and the oyster farm represents part of the cultural history of the estero. Conversely, the low-tide appearance of culture racks holding oysters partially covered by the invasive, yellow tunicate and the sight of plastic mesh culture bags lying on some intertidal flats conflict with the aesthetics of the vistas expected in a National Park Wilderness Area. After evaluating the limited scientific literature on Drakes Estero and the relevant research from other areas, the committee concludes that there is a lack of strong scientific evidence that shellfish farming has major adverse ecological effects on Drakes Estero at the current (2008–2009) levels of production and under current (2008–2009) operational practices, including compliance with restrictions to protect eelgrass, seals, waterbirds, and other natural resources. Adaptive management could help address effects, if any, that emerge with additional scientific research and monitoring to more fully understand the Drakes Estero ecosystem and the effects of shellfish farming. Based on their own conclusions on how shellfish mariculture affects the Drakes Estero ecosystem, NPS made some documented decisions. NPS and DBOC reached agreement in April 2008 on the Special Use Per-
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Shellfish Mariculture in Drakes Estero, Point Reyes National Seashore, California mit giving the company the right to operate in Drakes Estero until 2012, when the current RUO expires, and specifying conditions of operations. According to the Department of the Interior (DOI), the Superintendent of Point Reyes National Seashore does not have the authority to extend the RUO because of the congressional mandate designating Drakes Estero as a Potential Wilderness. Under this interpretation of the Wilderness Act, NPS has the mandate to convert a Potential Wilderness to Wilderness status as soon as the non-conforming activity can be removed. Because the likely beneficial functions of oysters in biogeochemical processes were not acknowledged, they did not appear to play a role in NPS decision making. Similarly, NPS did not mention that Olympia oysters were part of the historic ecological baseline condition of Drakes Estero and that, in the past, Olympia oysters could have played a significant role in the biogeochemical processes of the estero. The ultimate decision to permit or prohibit shellfish farming in Drakes Estero necessarily requires value judgments and tradeoffs that can be informed, but not resolved, by science. This report provides information that may be used by policymakers to reach a decision on the DBOC request to extend the mariculture lease beyond 2012, but statements in the report should not be interpreted as recommendations in support of or in opposition to an extension of the lease. If the legal opinion of the DOI Solicitor’s Office stands, then NPS has no authority to offer a new RUO because the mariculture operation is judged as a nonconforming use in a Wilderness area, preventing conversion to full Wilderness status. If a decision were made to extend the lease of DBOC, science would be required to help establish and adjust permit conditions to enhance the benefits (derived largely from the presence and biological activities of oysters) and minimize the risks (derived largely from the activities of the culturists) of the mariculture operation. The challenges faced by the Point Reyes National Seashore in managing their natural and cultural assets in Drakes Estero represent an example of the NPS dilemma nationwide. NPS receives inadequate financial support to conduct the research necessary to follow its dual mandate of promoting access and enjoyment by the public yet preserving natural processes and values of its public trust assets. Research conducted to meet NPS management needs in Drakes Estero would have broader applicability to local, state, and federal agencies and would contribute to basic scientific understanding. Science could also contribute to educational exhibits if an educational demonstration project were developed to illustrate the beneficial ecological roles of oysters, the challenges of nonnative species, best practices for mariculture, the history of shellfish harvesting reflected in Coast Miwok middens beside the estero, conservation issues, and the biology of oyster propagation. Another possible application for
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Shellfish Mariculture in Drakes Estero, Point Reyes National Seashore, California science in management could emerge from a partnership between NPS and DBOC or other entity to restore the native Olympia oyster to Drakes Estero. Research needs for effective management of Drakes Estero include most importantly evaluation of how to manage and control potential human disturbances to harbor seals from mariculturists, kayaks, hikers, and other sources. Assessing impacts of disturbance on seals, shorebirds, and waterbirds at the population level would help determine the long-term population impacts. Further research is needed to understand why eelgrass is expanding in Drakes Estero and not in some other systems. Additional research could be directed at understanding the relative habitat value of eelgrass, mud flat, and mariculture structures for fish at a landscape scale in the estero and similar systems. Additional observational and experimental studies are needed for use in carrying capacity models to determine how many oysters can be cultured in Drakes Estero without depleting phytoplankton and organic matter also used by native species and to avoid sediment anoxia from excess production of feces and pseudofeces by oysters. Research on control of abundance and risk of spread of the invasive tunicate, Didemnum vexillum, is urgently needed, not just in Drakes Estero, but worldwide. And research into the most effective way to control diseases that may be spread with transport of shellfish is important to the public trust and the mariculture industry broadly.