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Shellfish Mariculture in Drakes Estero, Point Reyes National Seashore, California III. Fish A. BACKGROUND Fishes have been widely shown to associate with natural three-dimensional biogenic structures that emerge off the bottom like eelgrass and bivalve reefs (Heck et al., 2003; Peterson et al., 2003; Coen and Grizzle, 2007; Horinouchi, 2007), but less work has been conducted on the effects of shellfish mariculture on fish populations and communities. Studies of mostly off-bottom mariculture operations have shown higher abundances of some fishes and invertebrates in areas with mariculture structures than in nearby areas with eelgrass, unstructured open mudflat, and even nearby oyster reefs and rocky substrates, although eelgrass generally also harbors a few unique species (DeAlteris et al., 2004; Clynick et al., 2008; Erbland and Ozbay, 2008). Powers et al. (2007) demonstrated that densities of fish and nektonic invertebrates were as high over plastic bottom netting used to cover infaunal cultured clams and colonized by macroalgae and epifauna as in eelgrass beds in North Carolina, with much lower densities over unvegetated bottom. However, abundance estimates are not a definitive indication of how structured habitat benefits fishes because structures often attract fishes without necessarily enhancing their productivity (reproduction, growth, or survival). Nevertheless, a substantial body of experimental research has shown that structure provides nektonic organisms with protection against predation, thereby offering a survival advantage, especially to more vulnerable juvenile life stages.
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Shellfish Mariculture in Drakes Estero, Point Reyes National Seashore, California B. WHAT IS THE BODY OF SCIENTIFIC STUDIES ON THE IMPACT OF THE OYSTER FARM ON DRAKES ESTERO? Only one study (Wechsler, 2004) has been conducted on the potential effects of oyster mariculture on fish communities in Drakes Estero, which was described as a preliminary study in the project report submitted to NPS (Elliott-Fisk et al., 2005). The lack of any additional fish research or population monitoring in this estero is notable. Wechsler sampled the eelgrass fish community using multiple techniques (trawls, traps, and experimental gill nets) in three settings: next to oyster racks in Drakes Estero, 75 m away from those racks, and in neighboring Estero de Limantour, which lacks mariculture operations. Because of difficulties collecting acceptable samples, only seven of the nine approximately monthly sampling dates were used in the analyses—from December 2002 to January 2004. No significant difference in fish abundance or species richness was detected among the three sampling sites; however, there is an indication that the composition of fish assemblages was modified near oyster racks by enhanced numbers of the guild characterized as structure-associated fishes. This pattern was driven by one species (kelp surfperch, Brachyistius frenatus) typically associated with hard substrate (Wechsler, 2004; Elliot-Fisk et al., 2005). C. WHAT EFFECTS CAN BE DIRECTLY DEMONSTRATED BY RESEARCH CONDUCTED IN DRAKES ESTERO ITSELF? The only study of fish in Drakes Estero (Wechsler, 2004) failed to detect impacts of oyster mariculture on fish abundances or community composition. This study appeared to have low statistical detection power. D. WHAT EFFECTS CAN REASONABLY BE INFERRED FROM RESEARCH CONDUCTED IN SIMILAR ECOSYSTEMS? Few reports address the effects of oyster mariculture on fish communities in circumstances that allow extrapolation to Drakes Estero, but there are numerous studies documenting enhanced densities of fish in structured habitats that include natural bivalve reefs (summarized in Peterson et al., 2003; Coen and Grizzle, 2007). Mariculture studies include one demonstrating that juvenile sole utilized oyster trestle culture areas for protection during the day and foraged over adjacent sand flats at night (Laffargue et al., 2006). A study in Narragansett Bay, Rhode Island, found that scup (Stenotomus chrysops) grew slightly faster on adjacent rocky habitat than on oyster grow-out cages, although tagging suggested that they had greater fidelity to the cages (Tallman and Forrester, 2007). Other studies of fish around mariculture operations from U.S. West
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Shellfish Mariculture in Drakes Estero, Point Reyes National Seashore, California Coast estuaries provide useful insights into processes that may occur in Drakes Estero. In Humboldt Bay, California, oyster long-lines were found to harbor more fish than either eelgrass or open mudflats (Pinnix et al., 2005). In addition, Rumrill and Poulton (2004) observed substantial numbers of staghorn sculpin (Leptocottus armatus) and juvenile Dungeness crab (Cancer magister) within baited minnow-traps deployed beneath oyster long-lines in Humboldt Bay. In Willapa Bay, Washington, few statistically significant differences in density were found among the more than 20 species of fish and crabs collected at intertidal locations when oyster bottom culture, eelgrass, and open mudflat were comparatively sampled (Hosack et al., 2006). In both studies, some individual species like tubesnouts (Aulorhynchus flavidus) were more abundant in structured habitats. Larger mobile invertebrates have also been shown to display species-specific and even life stage-specific behavior around structure in response to the availability of prey and/or protection from larger predators. Juvenile Dungeness crabs (C. magister) rely on structured habitat for protection while older individuals utilize open mudflat to forage; however, red rock crabs (Cancer productus) prefer bottom oyster culture habitat (Holsman et al., 2006). These functional associations with habitat and links to population processes are little explored, especially on a larger spatial scale where it is known that patch size, connectivity, and proximity to other habitats are also important such that patchy habitats with more edges may actually enhance diversity and abundance (Bostrom et al., 2006; Selgrath et al., 2007). Based upon the (non-significant) trend of enhanced abundances of structure-associated fishes associated with oyster racks reported in Wechsler (2004) and Elliott-Fisk et al. (2005) and the often demonstrated affinity of many fishes to structural habitat, including oyster culture structures, it is reasonable to expect that some species of fishes are attracted to oyster racks in Drakes Estero. Additional research would be necessary to test this expectation and evaluate the significance of any such responses to the overall fish community of the estero.