Consensus Report

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Drakes Estero, 25 miles northwest of San Francisco, is a marine estuary home to harbor seals, waterfowl, fish, and other marine organisms. Congress designated the estuary a Potential Wilderness in 1976, signifying the intention to incorporate the area into an existing Wilderness area in Point Reyes National Seashore. Drakes Estero is also the site of commercial oyster farming since the 1930s, and Drakes Bay Oyster Company continues to operate today under a permit. The permit is set to expire in 2012, which would effectively close the oyster farm unless it is extended by congressional action. As the expiration date approaches, the Park Service has issued a series of reports presenting scientific information that describes negative effects of the oyster farm on the Drakes Estero ecosystem.

Key Messages

  • 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.
  • Effects of the mariculture on birds. 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.
  • 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).
  • 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.
  • Effects of the mariculture on fishes. 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.
  • Effects of the mariculture on harbor seals. 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 Ni�o and oyster production level.
  • 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.