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Shellfish Mariculture in Drakes Estero, Point Reyes National Seashore, California
nated by smaller deposit feeders (usually polychaetes). Such responses are associated with low current flow, very dense shellfish culture, or both (Castel et al., 1989; Nugues et al., 1996; Mirto et al., 2000; Christensen et al., 2003; Forrest and Creese, 2006; Lu and Grant, 2008). Such modifications of the benthos are generally absent where stocking density is low or moderate or where currents are strong enough to disperse the biodeposits (Crawford et al., 2003; Mallet et al., 2006).
Tidal currents have previously been shown to enhance erosion around the base of mariculture structures (Pregnall, 1993; Everett et al., 1995), and this process probably explains the slightly coarser substrate found under oyster culture racks in Drakes Estero. Such an explanation is consistent with the conclusion that tidal flows are sufficient to disperse the biodeposits far enough to prevent detectable organic loading at the relatively low oyster stocking densities used in Drakes Estero. The absence of eelgrass underneath the racks also implies faster near-bottom flows than underneath the eelgrass canopy, which baffles flow velocity by friction. Slower flows underneath seagrass canopies induce deposition of fine particles and thus create finer sediment sizes (Madsen et al., 2001).
The observed enhancement of amphipods and reduction in tanaids underneath racks might represent a response to (1) sedimentary changes induced by local loss of eelgrass, allowing faster flows under racks; (2) some aspect of oysters and epibiota on racks that influences the soft-sediment benthos below; or (3) secondary effects of rack structure acting on predators of benthic macro-invertebrates. For example, racks might attract predatory fishes that feed on tanaids. This suggestion is supported by the recognition that small benthic crustaceans are often preferred prey by demersal fishes and by the trend of more structure-oriented predators like kelp surfperch near the racks (Wechsler, 2004). On the other hand, amphipods are also preferred prey for many demersal fishes. Some amphipods have been shown to associate with oysters and structures, or else with the macroalgae and fouling organisms found on them, both on the west coast (Eogammarus and Amphithoe: Dumbauld et al., 2000; Dumbauld et al., 2001) and elsewhere (Gammarus: Rodney and Paynter, 2006). Scientific studies of both on-bottom culture (Trianni, 1995; Hosack et al., 2006; Ferraro and Cole, 2007) and off-bottom culture in other west coast estuaries (Pregnall, 1993; Rumrill and Poulton, 2004) generally indicate that the benthic community associated with oyster culture is more diverse than that of unstructured bottom, and either equal to or slightly less diverse than that of eelgrass habitat. Enhanced diversity in structured habitat has also been documented for epibenthic meiofauna, which represent important food items in fish diets (Castel et al., 1989; Simenstad and Fresh, 1995; Hosack et al., 2006) and may respond to oyster racks, but this has not been studied in the estero to date.