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VII.â Birds A.â Background The approximately 9.4 km2 of water surface area in Drakes Estero and Estero de Limantour, some fraction of which is exposed at low tide, are ecologically significant for wintering shorebirds and waterfowl, with at least 73 species being recorded in a series of mid-winter counts (White, 1999). Dominant species are small ducks and shorebirds, with winter counts of five species, ruddy ducks, bufflehead, dunlin, western sand- piper, and least sandpiper exceeding 1,000 (Page and White, 1999). Drakes Estero is an important staging area for migrating black brant geese (Branta bernicla nigrans) (Shuford et al., 1989). This estero is also named a wetland of critical importance in the United States Shorebird Conservation Plan (Hickey et al., 2003). B.â What is the Body of Scientific Studies on the Impact of the Oyster Farm on Drakes Estero? Despite the importance of Drakes Estero as habitat for shorebirds, not much is known about the influence of the oyster farm on these popula- tions. Press (2005) reported that green algal mats on the intertidal flats of Drakes Estero affect foraging behavior and success of black-bellied plo- vers, marbled godwits, and western sandpipers. The black-bellied plovers were attracted to the algal mats and exhibited improved foraging success, probably taking advantage of the increased abundances of gammarid amphipod prey (Press, 2005). Likewise, marbled godwits had higher for- 57
58 SHELLFISH MARICULTURE IN DRAKES ESTERO aging rates in the green algal mats and showed a slight trend of increased use of algal mat areas. Only the western sandpiper avoided algal mats, but those feeding within them exhibited feeding rates similar to those on unvegetated sand flats (Press, 2005). Oyster culture bags placed on inter- tidal flats in Drakes Estero clearly prevent access by probing shorebirds to the sediments beneath them, thereby removing typical foraging habitat for many species. Bags also, however, act as substrate for attachment and growth of green algae. A testable hypothesis is that shorebirds that can pick surface prey may benefit from enhanced abundances of phytal prey like some amphipods on seasonal green algae associated with oyster bags, whereas pure probers like western sandpipers will not use these phytal invertebrates and suffer displacement by intertidal oyster bags. No test of this hypothesis exists. C.â What Effects Can Be Directly Demonstrated by Research Conducted in Drakes Estero Itself? No studies address directly the effect of the oyster farm on bird behav- ior or abundance. Lengthy time series of wintering bird numbers do exist, courtesy of the Audubon Societyâs Christmas Bird Counts (Wimpfheimer, 2008). Summary numbers based on a 15-mile âcount circleâ include both Drakes Estero and Tomales Bay and thus represent a spatially broader per- spective. The duration of the time span of these standardized bird counts, 1970â2007, is impressive. Population trends for two species and their potential roles in the ecosystem of Drakes Estero are discussed below. D.â What Effects Can Reasonably Be Inferred from Research Conducted in Similar Ecosystems? Black brant breed in the high arctic and migrate along the Pacific coast. Estuaries provide staging (feeding) areas for flights of up to 5,000 km. Their primary source of energy is the eelgrass Zostera marina (Gan- ter, 2000), the dominant aquatic vegetation in many estuaries, including Drakes Estero. Small flocks of black brant are regular winter residents of Drakes Estero (White, 1999) and thousands may be seen during their migrations. Wimpfheimer (2008) reports an upward trend from 1970 to the present, with a low of 8 individuals and a maximum count of 2,550. The trophic relationship between brant and eelgrass deserves more atten- tion. Brant foraging does disturb Z. marina meadows seasonally, and eelgrass provides a critical habitat for salmon smolts, other fishes, and numerous invertebrates. Brant foraging generates eelgrass detritus, and their feces probably represent a nutritional subsidy. The relative bal- ance between these negative and positive influences remains generally
BIRDS 59 unknown, although eelgrass in Drakes Estero has increased in areal cover- age even as black brant abundances have increased. Peregrine falcons (Falco peregrinus) feed predominately on other birds. Some falcons are resident in Drakes Estero; others migrate along the Pacific flyway (Anderson et al., 1988). Winter numbers in the count circle have increased dramatically since 1970 (Wimpfheimer, 2008) from an average of 2.4 birds in the 1970â1974 interval to 18.7 from 2005â2007. Peregrine attacks generally involve flushing their prey. Falcons represent a natural source of disturbance, and along with kayakers and motorboat traffic, all cause roosting or feeding waterbirds and shorebirds to take flight. Studies on âflush distancesâ in San Francisco Bay for eight species also found in the Drakes Estero suggest an average response distance of 51.5 m (Evans, unpublished). Stillman et al. (2007) have shown for a set of European waterbirds exposed to disturbances that over-winter mortal- ity is increased, implying a connection between the energetic cost of risk avoidance and diminished demographic performance. Whether oyster culture is âa potentially very significant environmental impactâ to water- birds (Dixon, 2007) is undetermined and should be quantified and placed in the same context as other, more natural disturbances. Nevertheless, the activity of culturists, especially their boat traffic, is likely to cause many waterbirds and shorebirds to flush, but population consequences are not known. Kelly et al. (1996) studied how oyster mariculture in nearby Tomales Bay affected use of tidal flats by wintering shorebirds by comparing 2- hectare low-intertidal plots, some with rows of plastic mesh bags (in a mix of two positions: on the ground and elevated on racks) and others without culture bags. Two of the most abundant shorebirds, dunlin and western sandpipers, demonstrated significant avoidance of mariculture plots. One shorebird, the willet, exhibited significant attraction to mariculture plots, and four others (black-bellied plover, marbled godwit, sanderling, and least sandpiper) did not vary in abundance as a function of the presence of culture bags. The dunlin and western sandpiper, which probe into the sediments for prey, foraged between rows of culture bags when on mariculture plots, while the least sandpiper, which uses a visual search for surface prey, was often found foraging on tops of culture bags. Willets foraged between rows of bags and on tops of bags: this species is known for its diverse feeding methods and diet. The presence and activity of mariculture workers on plots did not affect the distribution of shorebirds analyzed for many species and no movements in or out of culture plots were associated with culturist activity. These results from such a similar system, involving the same species of shorebirds that use Drakes Estero and the same plastic mesh culture bags, albeit not only placed on the ground but also on elevated racks, are probably directly transferrable to
60 SHELLFISH MARICULTURE IN DRAKES ESTERO Drakes Estero. Consequently, only the obligate probers are likely to be negatively affected by mariculture on intertidal flats in Drakes Estero, while most species remain unaffected and some that forage visually on surface prey may benefit from invertebrates associated with culture bags and epibiotic growth on the bags and oysters. Feeding shorebirds do not seem prone to being flushed by normal activities of culturists, but insuf- ficient information exists to know how closely culturists can approach the birds without causing retreat by walking or flying.