See Hicks and Coull 1983 and Fleeger
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Epibenthic Invertebrates at Two Beaches After Addition of Olympia Oysters, with Particular Reference to Prey of Juvenile Pacific Salmon Jeffery Cordell, Brenda Bachman, and Lucinda Tear University of Washington, School of Aquatic and Fishery Sciences 2 Summary 1. Epibenthic pump samples designed to quantify juvenile Pacific salmon prey taxa were taken at two beaches in Puget Sound, Washington, before and after addition of Olympia oysters. Before-oyster sampling occurred at two plots at each beach in 2006 once in April and once in May. In 2007, after oysters had been added to one of the plots at each beach, sampling occurred again. 2. Seventy-one taxa of epibenthic organisms were collected at the two beaches, twelve of which were juvenile prey salmon taxa. 3. Taxa richness before and after addition of oyster was similar at both beaches. 4. Samples were dominated by harpacticoid copepods, particularly the family Ectinosomatidae, and the juvenile salmon prey taxon Tisbe sp. 5. Densities of most taxa were higher in 2007 than in 2006. 6. Four-way Analysis of Variance, using treatment (oyster, no oyster), month, year, and location indicated that the treatment*year interaction was significant for many of the taxon densities, including total epibenthic organisms, total harpacticoids, and Tisbe sp. This resulted because the change in mean density from 2006 to 2007 was much greater in the oyster plots than in the no oyster plots. 7. We conclude that the addition of Olympia oysters to the two beaches studied increased overall epibenthic organism densities and those of the most abundant salmon prey harpacticoid copepods. This enhancement is probably due to increased habitat complexity which has been extensively demonstrated to increase abundance and diversity of intertidal organisms. 3 Introduction Extensive population growth and development in the Puget Sound region has led to a drastic decline in intact natural shorelines (Levings and Thom 1994). Overall, approximately one third of the Puget Sound shoreline is modified, with much larger proportions modified in counties in and around the metropolitan centers of Seattle and Tacoma (e.g., 68% modified in King County) (WDNR 1999). The construction of piers, docks, and shoreline armoring has resulted in extensive disruption of shoreline and intertidal habitats (Levings and Thom 1994, Nightingale and Simenstad 2001, Sobocinski 2003). The cumulative impact of this shoreline development is unknown, but is of increasing concern, particularly because of its potential to affect important species such as juvenile Chinook and chum salmon (Oncorhynchus tshawytscha, O. keta) that have been listed as “threatened” under the Endangered Species Act. It is known that “ocean- type salmon” (i.e., those that out-migrate to estuarine and nearshore marine waters at a relatively small size, soon after hatching), such as Chinook and chum extensively use nearshore habitats such as wetlands and seagrasses in estuaries, shallow embayments, and outmigration corridors near river mouths (Kaczynski et al. 1973, Congleton 1978, Healey 1980, Healey 1982, Simenstad et al. 1982, D’Amours 1987, Shreffler et al. 1992, Webb 1989). One of the important functions of these wetlands is providing juvenile salmon with foraging opportunities, and patterns of invertebrate prey that the salmon use in these habitats are relatively well understood. For example, emergent and riparian vegetation produces insect prey such as larval and emergent midges, and shallow mudflat and eelgrass habitats produce crustacean prey such as amphipods and harpacticoid copepods. Although considerably less studied than estuarine wetlands, recent work has suggested that nearshore marine habitats in Puget Sound play a similar role for juvenile ocean-type salmon. A detailed examination of the stomachs of 819 juvenile Chinook salmon caught along central Puget Sound beaches in 2001 and 2002 revealed a link between intertidal habitats and the salmon diets (Brennan et al. 2004). In particular, this study found that intertidal polychaete worms, gammarid amphipods, and barnacles were common and sometimes dominant in the juvenile Chinook salmon diets Epibenthic and epibiotic organisms provide food and shelter for many types of organisms and are important components of the biogenic structure of coastal habitats (Underwood et al. 1991). There is considerable scientific evidence that increasing habitat complexity also increases availability of suitable shelter, benefiting the survival and/or fitness of these intertidal organisms in several ways. First, shelter may reduce physiological stresses such as temperature extremes, desiccation, osmotic fluctuations, etc. (review by Newell 1970; Gibbons 1988, Lohrer et al. 2000). Second, animals without appropriate shelter are probably more vulnerable to terrestrial and avian predators during low tide and to fish and invertebrate marine predators during high tide (Moksnes et al. 1998). Third, highly complex habitats may enable some mobile species to forage with reduced threat of predation, whereas organisms with limited shelter may face risks of predation that force them to curtail their foraging activities (Sih 1987, Holbrook 1988). In addition to differences in availability of shelter, complex habitats may provide a greater quantity and/or variety of substrate types relative to structurally simple habitats; crevices and depressions associated with complex habitats accumulate sediment and organic material, 4 increasing opportunities for both infaunal and epifaunal species (Kelaher et al. 2001, Olabarría et al. 2002, Kelaher and Castilla 2005) and the relatively great surface area of complex habitats potentially increases available space for prey items. Relatively little work has been done on the effects of oyster placement on invertebrate assemblages. However, oyster reefs are known to contain diverse and abundant communities of benthic meiofauna that provide potential prey for fish and decapods (Castel et al. 1989), and in Willapa Bay, Washington, Hosack et al. (2006) found that the densities of epibenthic invertebrates, including known prey of fish and decapods, were significantly higher in oyster and eelgrass habitats compared to adjacent unstructured mudflats. In this project we evaluate paired oyster-no-oyster intertidal plots at two sites in Puget Sound, Washington, both before and after placement of Olympia oysters, and our working hypothesis is that increased habitat complexity associated with oyster placement increases diversity and abundance of intertidal epibenthic invertebrates. Thus, the main goals of the study were to determine (1) if oyster habitat increases numbers and diversity of epibenthic invertebrates; and (2) if observed increases include juvenile salmon prey species. The information generated will help scientists, aquaculturists, and habitat managers in improving and maintaining biological function associated with aquaculture and other human impacts in intertidal habitats. Methods Mobile macro- and meiofaunal invertebrates such as amphipods and harpacticoid copepods that live on the surface of the substrate were sampled using an epibenthic pump (14.8 cm diameter, 150-µm mesh size; Fig. 1). This type of pump has been used extensively in intertidal habitats in the Pacific Northwest to sample juvenile salmon prey invertebrates (Simenstad et al. 1991). The pump works by vacuuming invertebrates inside a cylinder of known volume from the surface layer of substrate. Seven replicate samples were taken on each plot in April and May 2006 (pre-oyster placement) and 2007. Samples were fixed in 10% buffered formalin in the field. Invertebrate taxa from known salmon prey groups such as large harpacticoid copepods, gammarid amphipods, other peracarid crustaceans, and mobile polychaete worms were identified to species; other taxa will be identified to lower taxonomic levels. For purposes of this study, calanoid copepods, which are water-column dwelling plankton that were probably obtained during pump-clearing between samples, and nematode worms, which are primarily benthic infauna, were omitted from analyses. Mean densities of selected taxa from each plot were graphed by year and coded by month, location, and treatment to show patterns of change from year one to year two. Four way Analysis of Variance was conducted using treatment (oyster, no oyster), month (April, May), year (2006, 2007), location (Dogfish, Lemolo), and all interaction terms except those including month. ANOVA results were interpreted in the context of the patterns observed in corresponding graphs. Since both oyster and no oyster plots began with no oysters and oysters were added after the 2006 sampling, the ANOVA must be interpreted carefully to in order to isolate the terms that express the way that densities changed between 2006 and 2007 in the two treatments. The ANOVA factor that most directly describes differences in how the treatment plots changed is the treatment*year 5 interaction term. If this term is significant, then changes over time (average for both sites and both sampling months) in the two treatments were not equal. If this term is not significant, then changes in the two treatments were similar. The treatment factor, which describes the differences between the mean densities in the two treatments (average of both sites, months, and years), is not the best term for summarizing the change after oysters were added, since it includes densities in the oyster plots before oysters were added. Consequently, in this ANOVA, the "treatment"