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Scientific Conclusions That Can Be Drawn The previous sections present the available information from the on- site investigations of potential impacts of DBOC operations on the Drakes Estero ecosystem. Those studies could be fairly characterized as pre- liminary results that would require additional focused research to allow definitive conclusions to be reached about the presence, absence, or mag- nitude of any ecological impacts. Although studies conducted in other systems may help support results from initial studies in Drakes Estero, the comparability is not always sufficient to reach confident conclusions on the most important issues of relevance to management decisions. Nevertheless, some limited results have sufficient scientific support to reach the following conclusions: 1.â The presence and biological activities of cultured oysters in Drakes Estero results in locally enhanced filtration of the water column and deposition of feces and pseudofeces onto the bottom. One study was conducted in which sediment cores were taken in eelgrass as a function of distance away from oyster culture racks in Drakes Estero and also in nearby Estero de Limantour, which lacks oyster culture. No enhancement of sediment organics and no reduction in oxygen content of sediments were detectable near the culture racks or in Drakes Estero as compared to Estero de Limantour, indicating a lack of detectable negative impacts of oyster biodeposition, probably because of relatively low oyster stocking densities and high tidal flushing, which disperses the deposits. Limited water quality sampling did not demonstrate elevated levels of nutrients, 67
68 SHELLFISH MARICULTURE IN DRAKES ESTERO pathogens (fecal coliform bacteria), or hypoxia. Ecologically, oyster cul- ture in Drakes Estero replaces, to an unknown extent, the filtering capac- ity and biogeochemical processing that was lost in the mid-19th century and subsequent decades with the overharvest and functional elimination of the native Olympia oyster. Although mariculture of the Pacific oyster is not a direct replacement of the native populations of the Olympia oyster, it may be viewed as providing similar biogeochemical functions and ecological resilience. The structural habitat provided by cultured oysters and the racks holding them is elevated off the bottom such that habitat services associated with provision of hard substrates may differ from those provided by native oysters residing on the bottom. Intertidal oyster bags with oysters inside may come closer to matching the habitat provided by native oysters, although periodic flipping of the oyster bags likely disturbs (and is intended to inhibit) the epifaunal fouling commu- nity growing on the bags. 2.â The absence of eelgrass directly under oyster culture racks repre- sents a small-scale and localized impact on the biogenic habitat. Numer- ous boat propeller scars in the eelgrass beds, partially affecting a total area of about 50 acres, are also evident and attributable to oyster cultur- ists because they are the only ones allowed to use motorized vessels in the estero. Nevertheless, the total percentage of eelgrass area lost (1%) or partially degraded by propeller scars (7%) and thus attributable to oyster mariculture represents about 8% of all eelgrass habitat in Drakes Estero as of 2007. Eelgrass has approximately doubled in areal cover in Drakes Estero from 1991 to 2007, implying little systemic threat from the exist- ing intensity of oyster culturing activities. Oysters have the potential to benefit eelgrass because their filtering activity improves local water clarity (and hence light penetration) and because they release biodeposits and ammonium (plant nutrients). 3.â Definitive conclusions about potential impacts of DBOC activities on fish cannot be reached. The only study done of fishes of Drakes Estero was unable to detect significant differences in fish abundance, species diversity, or community composition between eelgrass fishes of Drakes Estero at varying distances from culture racks or between Drakes Estero and the Estero de Limantour, which has no oyster culture operation. There is some indication that the guild of fishes typically associated with hard substrates may be locally enhanced around culture racks, driven largely by response of one species, the kelp surfperch. 4.â The only study of how the benthic invertebrates of Drakes Estero may be affected by the mariculture operation found no significant differ- ences in invertebrate abundance or composition of eelgrass infauna as a function of distance from mariculture racks or between Drakes Estero and Estero de Limantour. Nevertheless, amphipods tended to be enhanced in
SCIENTIFIC CONCLUSIONS THAT CAN BE DRAWN 69 the sediments under racks, while a tanaid was depressed. A statistically significant increase in sandiness of sediments was detected under racks, but the magnitude of the change was small. 5.â Drakes Estero represents an important site for harbor seals, sup- porting about 20% of the mainland California population. The design of the program to monitor harbor seal abundances and disturbance events at sub-sites within the estero does not permit explicit tests of the impacts of mariculture. Seal disturbance data have been collected during surveys designed primarily to monitor seal abundance trends, and observations of disturbance are not sufficiently representative to infer the proportionate contribution of mariculture-related disturbance relative to other sources of disturbance to hauled-out seals. In particular, critical information is lacking on potential disturbances from mariculture activities at higher tidal levels, which could inhibit hauling out and other important activi- ties. There is a lack of information on how various sources of disturbance affect seals on an individual basis. Nonetheless, precautionary measures to reduce the likelihood of disturbance of seals are consistent with current management practices in the United States and in other countries. 6.â Oyster culturing increases the existing amount of hard substrate in Drakes Estero, thereby sustaining high cover (up to about half of subtidal hard surface) of an introduced tunicate, Didemnum vexillum, and smaller populations of at least three other nonnative epifaunal invertebrates. The oyster that is cultured, Crassostrea gigas, is itself a nonnative species. Some culturing of the nonnative Manila clam, V. philippinarum, also occurs in the estero. There is risk that ongoing culture of C. gigas and V. philippinarum at Drakes Estero could lead to establishment of self-sustaining naturalized populations. Past importation of nonnative oysters at Drakes Estero is associated with the unintentional introduction of a nonnative haplospo- ridian oyster disease and a salt marsh snail, Batillaria attramentaria, whose introduction has had locally limited detrimental effects on the native marsh snail. However, the current mariculture practice of importing only larvae or seed from hatcheries practicing High Health Programs of inspec- tions carries low risk of future new introductions of exotic species into Drakes Estero. Sustaining high cover of Didemnum vexillum elevates risk of its spread to other habitats, especially to eelgrass, and locations within Drakes Estero and Estero de Limantour. 7.â No study has been conducted to test the impacts of mariculture on birds of Drakes Estero. Drakes Estero represents an important site for overwintering and seasonally migrating shorebirds- and waterfowl, with special significance as a feeding and staging site for migrating black brant geese. Boat travel by the mariculturists is likely to disturb and flush seaducks, shorebirds, and other waterbirds. Furthermore, the presence of lines of oyster bags on the intertidal flats is likely to diminish the feeding
70 SHELLFISH MARICULTURE IN DRAKES ESTERO area for some probing shorebirds, while enhancing food supplies for other shorebirds willing to consume epibiotic amphipods and other inverte- brates associated with algal growth on mariculture bags. 8.â The effect of oyster farming in Drakes Estero on the aggregate economic values generated by Point Reyes National Seashore 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).