Mark W. Luckenbach

Oyster Reefs at Risk and Recommendations for Conservation, Restoration, and Management

Native oyster reefs once dominated many estuaries, ecologically and economically. Centuries of resource extraction exacerbated by coastal degradation have pushed oyster reefs to the brink of functional extinction worldwide. We examined the condition of oyster reefs across 144 bays and 44 ecoregions; our comparisons of past with present abundances indicate that more than 90% of them have been lost in bays (70%) and ecoregions (63%). In many bays, more than 99% of oyster reefs have been lost and are functionally extinct. Overall, we estimate that 85% of oyster reefs have been lost globally. Most of the worlds remaining wild capture of native oysters (> 75%) comes from just five ecoregions in North America, yet the condition of reefs in these ecoregions is poor at best, except in the Gulf of Mexico. We identify many cost-effective solutions for conservation, restoration, and the management of fisheries and nonnative species that could reverse these oyster losses and restore reef ecosystem services.

Developing success criteria and goals for evaluating oyster reef restoration: Ecological function or resource exploitation?

Abstract Habitat restoration encompasses a broad range of activities, emphasizing very different issues, goals, and approaches depending on the operational definition of ‘restoration’. This is particularly true for many shellfish (molluscan) dominated systems (e.g. oyster reefs, mussel beds, vermetid gastropod reefs). In contrast to other well-studied biogenic habitats, such as seagrasses, mangroves, or salt marshes, bivalves are directly consumed as a resource. Hence resource extraction has direct consequences for habitat health. Restoration objectives have typically included reduction of public health risks through improved water quality to increase harvest. Restoration or enhancement of populations of commercially exploited shellfish depressed by overharvesting and/or reduced environmental quality remains the principal motivation behind most shellfish ‘restoration’ efforts. Direct and indirect ecosystem services (e.g. filtering capacity, benthic–pelagic coupling, nutrient dynamics, sediment stabilization, provision of habitat, etc.) derived from oyster habitat have been largely ignored or underestimated. Only recently, the restoration of lost ecological function associated with shellfish communities has been included in our discussions and related research examining habitat development and function through a scientific approach. The former area has been reviewed extensively and will not be our focus here. In this review, we examine some of the restoration efforts made in the name of fisheries enhancement, address their effectiveness, and discuss some of the issues associated with realizing the broader goal of ecological restoration. We note the importance of linking success criteria to specific goals and make the case for a greater need in clarifying the ecological functions of shellfish and shellfish habitats. We recognize the limitations of existing datasets and summarize ongoing attempts to address oyster habitat restoration throughout the broad geographic distribution of the American oyster, Crassostrea virginica (Gmelin). In many ways this topic parallels the ongoing debate over ‘attraction versus production’ associated with artificial reef management. We consider how local conditions (e.g. tidal range, bottom topography, turbidity, salinity) and resulting habitat traits affect restoration strategies. We also discuss the underappreciated value of shellfish populations from those areas designated as closed to harvesting due to their intrinsic worth as habitat/larval reserves. The necessity of ecosystem (adaptive) management strategies emerges from this discussion. Finally, this overview supports our contention that shellfish habitat should be included in discussions of ‘essential fish habitats’ (or EFH).

Historical ecology with real numbers: past and present extent and biomass of an imperilled estuarine habitat

Historic baselines are important in developing our understanding of ecosystems in the face of rapid global change. While a number of studies have sought to determine changes in extent of exploited habitats over historic timescales, few have quantified such changes prior to late twentieth century baselines. Here, we present, to our knowledge, the first ever large-scale quantitative assessment of the extent and biomass of marine habitat-forming species over a 100-year time frame. We examined records of wild native oyster abundance in the United States from a historic, yet already exploited, baseline between 1878 and 1935 (predominantly 1885–1915), and a current baseline between 1968 and 2010 (predominantly 2000–2010). We quantified the extent of oyster grounds in 39 estuaries historically and 51 estuaries from recent times. Data from 24 estuaries allowed comparison of historic to present extent and biomass. We found evidence for a 64 per cent decline in the spatial extent of oyster habitat and an 88 per cent decline in oyster biomass over time. The difference between these two numbers illustrates that current areal extent measures may be masking significant loss of habitat through degradation.

Growth and mortality of oysters (Crassostrea virginica) on constructed intertidal reefs: effects of tidal height and substrate level

Abstract Intertidal oyster reefs, 3-dimensional structures created by years of successive settlement of larval oysters on adult oyster shells, provide levels of surface and interstitial heterogeneity that are rare in marine ecosystems. Surprisingly, little is known about the ecological benefits for oysters (Crassostrea virginica) in these aerially exposed, structurally complex systems. In this study a 210 m×30 m intertidal reef was constructed. During three, 28-day sampling periods in the summer and early fall, growth and mortality of two size classes of oysters placed in cages at three tidal heights (25 cm above mean low water (MLW), MLW, and 90 cm below MLW) and at two substrate levels (reef surface and 10 cm below the reef surface) within the reef setting were examined. Mid-intertidal oysters residing within the reef interstices grew faster and enjoyed greater survival than mid-intertidal oysters at the reef surface during certain times of the year. Conversely, subtidal oysters inhabiting subsurface environments did not grow faster than surface dwelling subtidal oysters, but some oysters within the reef fabric (larger size class) experienced significantly higher survival. Along a tidal continuum, oysters at the reef surface grew best in subtidal locations, yet experienced the lowest mortalities in the low intertidal zone (MLW). Reef heterogeneity, which allows for residence along both interstitial and tidal gradients, may provide physical and biological refugia for oysters and furnish ideal spatial platforms for growth. Therefore, location within the reef habitat has a significant impact on the biology and ecology of C. virginica.

SETTLEMENT OF CRASSOSTREA ARIAKENSIS LARVAE: EFFECTS OF SUBSTRATE, BIOFILMS, SEDIMENT AND ADULT CHEMICAL CUES

Abstract The Suminoe oyster (Crassostrea ariakensis) is being considered for introduction into the Chesapeake Bay. However, our current understanding of the biology and ecology of C. ariakensis is insufficient to predict whether an introduction will be successful, provide desired benefits, or have adverse impacts. Behavior of native Eastern oyster (C. virginica) pediveligers has been studied for many years and it is well established that they use a variety of habitat characteristics when selecting a site for colonization. Perhaps the most important of these are chemical cues emitted by adult conspecifics, which can lead to gregarious larval settlement and dense, persistent reef communities. Conversely, almost nothing is known about the mechanisms that regulate larval settlement and metamorphosis for C. ariakensis or how pediveligers might respond to conditions found in Chesapeake Bay. In a comparative study with C. virginica, we examined how environmental factors such as substrate type, natural biofilms, sediment and waterborne chemical cues influence larval settlement for two C. ariakensis strains (“south China” and “west coast”). Our results demonstrate many similarities but also potentially important differences. Both species and strains of larvae greatly prefer natural substrates (e.g., shell) covered with biofilms for colonization but the west coast strain of C. ariakensis exhibited greater attachment onto manmade substrates (e.g., fiberglass) than C. virginica. Waterborne chemical cues emitted by adult oysters were also found to enhance substrate attachment for all larval forms but cues do not appear to be species specific. These results provide critical insight to the ability of C. ariakensis larvae to identify and colonize suitable substrates in the Chesapeake Bay, which will have a large impact on recruitment success and their ability to establish self-sustaining populations.

Swimming velocities and behavior of blue crab (Callinectes sapidus Rathbun) megalopae in still and flowing water

Habitat selection capabilities of the recruiting larval stages of marine invertebrates are limited, in part, by their ability to maneuver in flowing water. Distributional and experimental evidence suggest that blue crab (Callinectes sapidus) megalopae may preferentially settle into vegetated habitats. However, the behavior and swimming capabilities of megalopae in flowing water have not previously been investigated. Laboratory experiments were conducted in a small, recirculating seawater flume to determine the swimming response of megalopae to varying flow velocities. Nighttime trials were conducted at six flow velocities: 0, 1.9, 3.6, 4.8, 6.3, and 9.3 cm s−1. Behavior and swimming velocities of field-collected C. sapidus megalopae were video recorded. Megalopae exhibited negative phototaxis and were found in the water column at all flows in the dark. The maximum sustained swimming speed observed was 12.6 cm s−1 and the mean swimming speed in still water was 5.0 cm s−1, with short bursts in excess of 20 cm s−1. Megalopae frequently oriented into the current and were capable of swimming upstream against the current at flow speeds <4.8 cm s−1; at greater velocities they were not able to do so. The results suggest that at low to moderate current velocities C. sapidus megalopae have the ability to actively move in search of settlement sites and to maintain their positions in desirable sites rather than relying strictly on passive movements by currents.

LESSONS LEARNED FROM EFFORTS TO RESTORE OYSTER POPULATIONS IN MARYLAND AND VIRGINIA, 1990 TO 2007

ABSTRACT A century-long decline of the fishery for the Eastern oyster Crassostrea virginica (Gmelin, 1791) in Maryland and Virginia stimulated numerous efforts by federal, state, and nongovernmental agencies to restore oyster populations, with limited success. To learn from recent efforts, we analyzed records of restoration and monitoring activities undertaken between 1990 and 2007 by 12 such agencies. Of the 1,037 oyster bars (reefs, beds, or grounds) for which we obtained data, 43% experienced both restoration and monitoring, with the remaining experiencing either restoration or monitoring only. Restoration activities involved adding substrate (shell), transplanting hatchery or wild seed (juvenile oysters), bar cleaning, and bagless dredging. Of these, substrate addition and transplanting seed were common actions, with bar cleaning and bagless dredging relatively uncommon. Limited monitoring efforts, a lack of replicated postrestoration sampling, and the effects of harvest on some restored bars hinders evaluations of the effectiveness of restoration activities. Future restoration activities should have clearly articulated objectives and be coordinated among agencies and across bars, which should also be off limits to fishing. To evaluate restoration efforts, experimental designs should include replication, quantitative sampling, and robust sample sizes, supplemented by pre- and postrestoration monitoring.

Effects of adult infauna on new recruits: implications for the role of biogenic refuges☆

Abstract Abundances of many macrofaunal species are enhanced around biogenic structures, yet some species show a reduced survival around such structures. High densities of macrofauna occur around the tubes of Diopatra cuprea (Bosc), but successful recruitment of the bivalve Mulinia lateralis (Say) is inhibited by high post-settlement mortality. Laboratory experiments were conducted to test the effects of two polychaetes, Nereis succinea (Frey and Leuckart) and Streblospio benedicti (Webster), on the survival of newly-recruited Mulinia lateralis. In the field these polychaetes are abundant around Diopatra cuprea tubes and rare in the same habitat when tubes are absent. Recently metamorphosed Mulinia lateralis were added to azoic sediment cores in the laboratory which contained 0, 1×, 2× or 4× field densities of either Nereissuccinea or Streblospio benedicti. Survival of Mulinia lateralis in treatment cores ranged from 29 to 70% in control cores and both Nereis succinea and Streblospio benedicti were found to reduce survival of the bivalve. These results suggest that, in the field, Diopatra cuprea indirectly modify the environment by altering local fauna which then inhibit the recruitment of some taxa.

A NEW IN SITU METHOD FOR MEASURING SESTON UPTAKE BY SUSPENSION-FEEDING BIVALVE MOLLUSCS

Abstract The most commonly used methods for measuring the amount of seston removed from the water column (uptake) by populations of suspension-feeding bivalve molluscs involve taking discrete water samples followed by laboratory analyses. Here we describe a new method based on in situ fluorometry that provides rapid measurement of seston removal rates. The new system is comprised of two identical units, each consisting of an in situ fluorometer, data logger and peristaltic pump with plastic tube attached to a deployment device. The deployment device allows precise placement of the fluorometer probe and intake end of the plastic tube so that in situ fluorescence (chlorophyll a) can be measured and water can be sampled for seston analyses in the laboratory from the same height. The typical setup involves placing one unit upstream and the other downstream of the study area and sampling the water at periodic intervals. Changes in seston concentration are revealed in the field by the fluorometers, and the sampled water can be analyzed in the laboratory for various seston parameters. Comparisons of the in situ data with data from laboratory analyses of pumped water samples were made for three species at four study sites: the eastern oyster (Crassostrea virginica), hard clam (Mercenaria mercenaria), and blue mussel (Mytilus edulis). Comparisons of measured upstream versus downstream seston concentrations indicated significant (t-tests, P < 0.05) differences (uptake) for six of eight trials based on in situ fluorometry, but only marginally significant (P < 0.10) differences at two of the four trials using laboratory chlorophyll a measurements. These data demonstrate that compared with sampling methods requiring laboratory analyses, the new in situ method provides much more rapid quantitative assessments and may provide more accurate estimates.

Sediment transport, biotic modifications and selection of grain size in a surface deposit-feeder

Deposit-feeders often select for particles on the basis of grain size. The available pool of particles at the sediment surface may be modified both by deposit-feeder activity and by sediment transport, but the effects of these alterations on deposit-feeder diet composition have received little attention. In laboratory experiments the spionid polychaeteParaprionospio pinnata altered the grain-size composition in its foraging area, and these alterations were reflected in grain-size changes in the diet. After simulated transport of fine-grain sediments,P. pinnata diets also changed in grain-size composition. Field data were collected from 9 m depth in the lower Chesapeake Bay. A video camera, deployed near the bottom, identified times of sediment transport over a 6-h Period;P. pinnata were collected concurrently for gut analysis. Consistent with predictions from the laboratory experiments,P. pinnata ingested primarily small-grain sizes. During periods of no sediment transport this feeding pattern reduced the relative availability of small particles; larger sediments were incorporated into the diet. Sediment transport may resupply the foraging area with fine-grain particles which are then incorporated into the diet. On these small spatial and time scales, deposit-feeder activity may affect the availability of food resources.