Oyster Habitat Research Articles

Invasive species cause large-scale loss of native California oyster habitat by disrupting trophic cascades

Although invasive species often resemble their native counterparts, differences in their foraging and anti-predator strategies may disrupt native food webs. In a California estuary, we showed that regions dominated by native crabs and native whelks have low mortality of native oysters (the basal prey), while regions dominated by invasive crabs and invasive whelks have high oyster mortality and are consequently losing a biologically diverse habitat. Using field experiments, we demonstrated that the invasive whelk's distribution is causally related to a large-scale pattern of oyster mortality. To determine whether predator-prey interactions between crabs (top predators) and whelks (intermediate consumers) indirectly control the pattern of oyster mortality, we manipulated the presence and invasion status of the intermediate and top trophic levels in laboratory mesocosms. Our results show that native crabs indirectly maintain a portion of the estuary's oyster habitat by both consuming native whelks (density-mediated trophic cascade) and altering their foraging behavior (trait-mediated trophic cascade). In contrast, invasive whelks are naive to crab predators and fail to avoid them, thereby inhibiting trait-mediated cascades and their invasion into areas with native crabs. Similarly, when native crabs are replaced with invasive crabs, the naive foraging strategy and smaller size of invasive crabs prevents them from efficiently consuming adult whelks, thereby inhibiting strong density-mediated cascades. Thus, while trophic cascades allow native crabs, whelks, and oysters to locally co-exist, the replacement of native crabs and whelks by functionally similar invasive species results in severe depletion of native oysters. As coastal systems become increasingly invaded, the mismatch of evolutionarily based strategies among predators and prey may lead to further losses of critical habitat that support marine biodiversity and ecosystem function.

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A Comparison of the Macrofaunal Communities Inhabiting a Crassostrea virginica Oyster Reef and Oyster Aquaculture Gear in Indian River Bay, Delaware

The eastern oyster (Crassostrea virginica) is a vital species in the estuaries of the mid-Atlantic United States. Whereas their filtering activity and biodeposition play an important role in the ecology of these systems, the reefs they form are one of the few sources of hard bottom habitat for fouling organisms and are the foundation of a rich biological community. This species has experienced drastic declines throughout the mid-Atlantic region in the last two centuries due to overfishing, habitat loss, and disease. Much interest and effort is now focused on restoring this important commercial and ecological resource. Whereas oyster reef restoration is central to the recovery of this species and the habitat it creates, oyster aquaculture can provide many of these same services albeit on a smaller scale. This is a two-part study, assessing the macrofaunal communities associated with subtidal modified rack and bag aquaculture, conducted over the summer and fall of 2006. The first part of this study compares the macro-epifaunal communities associated with two oyster habitats: a created oyster reef and oyster aquaculture cages. Both habitats were sampled with lift nets and compared using two-way ANOVAs between habitat and time. Secondly, we compared the sediment composition and macro-infaunal communities below the oyster cages with a nearby reference area of open sand/mud bottom. A significantly greater (P < 0.05) total abundance and species richness was found in the oyster cages, but significantly greater (P < 0.05) species evenness was found on the reef with species diversity similar between habitats. The sediments below the oyster cages showed a slight but significant (P < 0.05) reduction in it's silt/clay fraction. The results of this study show that, this method of oyster aquaculture supports additional populations of ecologically and economically important macrofauna compared with a created oyster reef. Furthermore this study demonstrates that off-bottom oyster aquaculture operations in the mid-Atlantic United States are a beneficial addition to host estuaries and associated natural communities.

Seasonal and Spatial Variations in Fish and Macroinvertebrate Communities of Oyster and Adjacent Habitats in a Mississippi Estuary

In Grand Bay National Estuarine Research Reserve (Grand Bay NERR), Mississippi, we used quantitative drop sampling in three common shallow estuarine habitats—low profile oyster reef (oyster), vegetated marsh edge (VME), and nonvegetated bottom (NVB)—to address the dearth in research comparing nekton utilization of oyster relative to adjacent habitats. The three habitats were sampled at two distinct marsh complexes within Grand Bay NERR. We collected a total of 633 individual fishes representing 41 taxa in 22 families. The most diverse fish family was Gobiidae (seven species) followed by Blennidae and Poeciliidae (three species each). We collected a total of 2,734 invertebrates representing 24 taxa in 11 families. The most diverse invertebrate family was Xanthidae (six species) followed by Palaemonidae (five species). We used ordination techniques to examine variation in species relative abundance among habitats, seasons, and sampling areas, and to identify environmental gradients correlated with species relative abundances. Our resulted indicated that oyster provided a similarly complex and important function as the adjacent VME. We documented three basic trends related to the importance of oyster and VME habitats: 1) Oyster and VME provide habitat for significantly more species relative to NVB, 2) Oyster and VME provide habitat for rare species, and 3) Several species collected across multiple habitats occurred at higher abundances in oyster or VME habitat. We also found that salinity, temperature, and depth were associated with seasonal and spatial shifts in nekton communities. Lastly, we found that the relative location of the two marsh complexes we studied within the context of the whole estuary may also explain some of the temporal and spatial differences in communities. We conclude that oyster habitat supported a temporally diverse and spatially distinct nekton community and deserves further attention in research and estuarine conservation efforts.

Vertical swimming behavior influences the dispersal of simulated oyster larvae in a coupled particle-tracking and hydrodynamic model of Chesapeake Bay

Because planktonic organisms have swimming speeds that are orders of magnitude lower than horizontal current velocities, it is unclear whether behavior of weak-swimming bivalve larvae could influence dispersal distance, encounters with suitable habitat, or subpopulation connec- tivity. We used a numerical approach to investigate whether these processes could be affected by species-specific differences in larval vertical swimming behavior of 2 oyster species (Crassostrea vir- ginica and C. ariakensis) in Chesapeake Bay, a partially mixed estuary. A coupled particle-tracking and hydrodynamic model was forced with observed winds and freshwater flow and included the best available estimate of present-day oyster habitat. Model scenarios were conducted with hydrody- namic predictions from June to September, 1995 to 1999, to simulate a range of environmental con- ditions. Simple larval swimming behaviors were parameterized for the 2 oyster species with results from preliminary laboratory experiments and literature. To isolate the effect of circulation, settlement habitat, and larval behavior on the spatial trajectories of particles, vertical swimming velocity was the only biological process represented in the model; egg production and larval growth were not included. Differences in larval swimming behavior had significant consequences for particle trans- port in Chesapeake Bay by influencing dispersal distances, transport success, and the degree of con- nectivity between 'subpopulations' in different tributaries. Most particles (>96%) did not return to the same reef on which they were released, and there were behavior-dependent differences in spa- tial patterns of the 'source' and 'sink' characteristics of oyster reefs. Simulated larval behavior had greater influence on spatial patterns of transport success than did interannual differences in circula- tion patterns. These model results have implications for fisheries management and oyster restoration activities.

Impact of seston characteristics on qualitative particle selection sites and efficiencies in the pseudolamellibranch bivalve Crassostrea gigas

To date, knowledge of the qualitative particle selection sites and conditions in the widely-distributed bivalve Crassostrea gigas is incomplete, having focussed either on heterogeneous particles, or on particles intentionally too large to enter the gill principal filament tracts. We used endoscope-directed sampling and the intact diatom-empty, cleaned frustule approach to unambiguously establish qualitative selection sites and the influence of seston quality (varying proportions of intact diatoms and empty, cleaned frustules) and quantity (particle loads) on the degree of qualitative selection. Normally-feeding oysters were presented test mixtures of the naturally-occurring Actinoptychus senarius (small enough to enter the gill principal filaments), and the potential selection sites (gill: dorsal and ventral collecting tracts; labial palps: anteriorly-deposited pseudofaeces), were sampled for comparison with the proportions and concentrations of the ambient medium. Qualitative selection was demonstrated at both the gills and labial palps. Gill selection efficiency was shown to be directly proportional to seston quality and quantity, using a technique independant of pseudofeces mucus content. The oyster gill is thus able to increase ingested food quality when environmental food quality is low and / or when seston concentrations are high, which is typical of oyster habitats. Palp selection efficiency was directly proportional to seston quality, but at the highest concentration tested, no qualitative selection was observed on the labial palps, probably due to overload on these smaller organs. The partial functional redundancy of these key processing organs in heterorhabdic species such as oysters and scallops may enhance their success in high-turbidity habitats.

Restoration of the Chesapeake Bay Using a Non-Native Oyster: Ecological and Fishery Considerations

In 2004, the State of Maryland and Commonwealth of Virginia proposed to introduce a non-native oyster species, Crassostrea ariakensis, into the tidal waters of Maryland and Virginia. The states planned to take action as soon as a rigorous, scientifically based Environmental Impact Statement and Record of Decision could be prepared. The benefits advanced by the supporters of this introduction included: improving water quality; providing important habitat for oysters, finfish, crabs and a diversity of other species; rehabilitating an oyster population capable of supporting an economically viable oyster industry; and preserving Chesapeake Bay communities and culture. As part of the broader Environmental Impact Statement (EIS), the authors were contracted to evaluate the sociocultural risks and benefits of introducing C. ariakensis, as compared to a number of alternative actions. In this article, we describe some emergent findings from the cultural and socioeconomic risk analysis portion of the EIS analysis. ...

Development of ecosystem indicators for the Suwannee River estuary: Oyster reef habitat quality along a salinity gradient

The Suwannee River watershed is one of the least developed in the eastern United States, but with increasing urbanization it is facing potential long-term alterations in freshwater flow to its estuary in the Gulf of Mexico. The purpose of this study was to develop biological indicators of oyster reef state along a natural salinity gradient in the Suwannee River estuary in order to allow the rapid assessment of the effect of changing freshwater input to this system. Percent cover and density of three size classes of living oysters, as well as the abundance of several predominant reef-associated invertebrates, were measured along a broad salinity gradient in the estuary and were correlated with salinity estimates from a long-term database for the preceding 12–24 mo. All eastern oyster,Crassostrea virginica, parameters (percent cover and density of three size classes) were significantly and negatively related to salinity. Data from samples collected near the lower intertidal were more closely dependent upon salinity than were samples from the higher intertidal at the same sites. Salinity differences were most closely reflected in differences in total oyster cover. This relationship corresponded with a general decline in oyster habitat with increasing distance from the mouth of the Suwannee River. Species richness was significantly and positively correlated with allC. virginica parameters (percent cover and density of three size classes), but the relationship explained only about half the variability. Density data of the hooked mussel,Ischadium recurvum, and a mud crab,Eurypanopeus depressus, were positively and strongly correlated withC. virginica parameters, likely reflecting the abundance of habitat provided byC. virginica shells. All of the biological indicators measured responded similarly along the salinity gradient, indicating they provide reliable indices of the effect of changing salinities in the Suwannee River estuary over the previous 1 or 2 yr. Some areas of positive relief defined as reefs 30 years ago are no longer oyster habitat, suggesting an ongoing decline, but nearshoreC. virginica were abundant. *** DIRECT SUPPORT *** A02BY003 00002

Relative importance of habitat selection and predation for the distribution of blue crab megalopae and young juveniles

We assessed the relative role of 3 potentially important processes affecting the distribu- tion of young juvenile blue crabs Callinectes sapidus Rathbun: (1) habitat selection at settlement, (2) selection of habitats by dispersing juveniles, and (3) habitat-specific predation rates, using cage experiments. The results suggest that active habitat selection by postlarvae and young juvenile crabs determines the habitat-specific distribution of juvenile blue crabs. Densities of blue crab settlers (megalopae and first instar crabs) in caged habitat patches were high and similar in artificial sea- grass, live shoal grass Halodule wrightii, and live oyster habitats (on average 69 to 82 settlers m -2 ), but significantly lower in mud (26 settlers m -2 ), indicating active selection for any structurally com- plex habitat at settlement. Second and third instar juvenile blue crabs (J2 and J3 crabs) also colonized the structurally complex habitats in higher numbers (34 to 115 crabs m -2 ), compared to mud (14 crabs m -2 ), demonstrating that young juvenile blue crabs are highly mobile and redistribute soon after metamorphosis. Densities of J2 and J3 blue crabs in uncaged treatments were significantly lower in all habitats than in caged treatments (on average 44% loss), suggesting high predation mortality. However, the loss of settlers and juvenile crabs was similar in all habitats, and had no significant effect on the juvenile crab distribution. Densities of potential predators were on average 5 times higher in the structurally complex habitats than in mud. An aggregation of predators in the refuge habitats, coupled with a refuge at low prey densities in unstructured habitats, appears to decrease the proximate effect of predation on the distribution of juvenile crabs. Concurrent high settlement of a small xanthid crab (cf. Eurypanopeus sp.) occurred almost exclusively in the oyster habitat, sug- gesting that species with a limited ability to redistribute after metamorphosis may display a more specific selection of the adult habitat at settlement.

APOPTOSIS OF THE PROTOZOAN OYSTER PATHOGEN PERKINSUS MARINUS IN VIVO AND IN VITRO IN THE CHESAPEAKE BAY AND THE LONG ISLAND SOUND

Abstract Perkinsus marinus, a protozoan pathogen of the eastern oyster, Crassostrea virginica, infects oysters at high prevalences along the east coast of the United States. P. marinus was previously reported to be frequently apoptotic among the intestine epithelial cells in oysters collected from Long Island Sound. In this work, we study whether apoptotic activity of P. marinus cells is consistent with the distribution patterns of the parasite in the field in Long Island Sound and Chesapeake Bay. Prevalences and intensities of P. marinus infections were compared between Chesapeake Bay and Long Island Sound oysters during a 5-year period, from 1997 to 2001. In situ hybridization for apoptosis was performed on archived oyster histological tissues to detect differences in apoptotic indices (% of apoptotic P. marinus cells) between Chesapeake Bay and Long Island Sound oysters. Parasite apoptotic indices in Chesapeake Bay oysters were compared between different oyster habitat salinities. Two different P. mari...

Assessment of Recent Habitat Conditions of Eastern Oyster Crassostrea virginica Bars in Mesohaline Chesapeake Bay

AbstractEastern oysters Crassostrea virginica in Chesapeake Bay, USA, have been subjected to intense harvest pressure since about 1850 and to disease epizootics since the 1950s. Combined, these processes have degraded the once extensive eastern oyster bottom; the remaining oyster shells, which are required as substrate for successful larval oyster settlement, are thus rendered increasingly susceptible to siltation. Between 1999 and 2001, we used an acoustic seabed classification system and underwater videography to assess oyster habitat conditions throughout Maryland's portion of Chesapeake Bay relative to eastern oyster recruitment and habitat restoration activities. We performed 16 surveys that covered a total of 39 km2 of bottom that were classified in 1911 as supporting productive oyster populations. Over 90% of this area has degraded from productive oyster bottom to mud, sand, or heavily sedimented oyster shell. Seventy percent of the locations we identified as containing unsedimented shell came from within the approximately 90 km2 of oyster bottom that had been subject to the management activity of spreading large volumes of shell at some time between 1960 and 1999. However, video observations of 49 of these locations that had been replenished with shell at various times indicated that any improvements in oyster habitat were short lived, as interstitial spaces in spread shell became highly sedimented after an average period of 5.5 years (n = 49; SE = 1.03). We conclude that the majority of oyster bottom in Maryland is extremely degraded and that no reasonable increase in the scale of present management practices will reverse this habitat decline.

High resolution mapping and classification of oyster habitats in nearshore Louisiana using sidescan sonar

Sidescan sonar holds great promise as a tool to quantitatively depict the distribution and extent of benthic habitats in Louisiana’s turbid estuaries. In this study, we describe an effective protocol for acoustic sampling in this environment. We also compared three methods of classification in detail: mean-based thresholding, supervised, and unsupervised techniques to classify sidescan imagery into categories of mud and shell. Classification results were compared to ground truth results using quadrat and dredge sampling. Supervised classification gave the best overall result (kappa=75%) when compared to quadrat results. Classification accuracy was less robust when compared to all dredge samples (kappa=21–56%), but increased greatly (90–100%) when only dredge samples taken from acoustically homogeneous areas were considered. Sidescan sonar when combined with ground truth sampling at an appropriate scale can be effectively used to establish an accurate substrate base map for both research applications and shellfish management. The sidescan imagery presented here also provides, for the first time, a detailed presentation of oyster habitat patchiness and scale in a productive oyster growing area.

Conserving oyster reef habitat by switching from dredging and tonging to diver-harvesting

A major cause of the steep declines of American oyster (Crassos- trea virginica) fisheries is the loss of oyster habitat through the use of dredges that have mined the reef substrata during a century of intense harvest. Experiments comparing the efficiency and habitat impacts of three alternative gears for harvesting oys- ters revealed differences among gear types that might be used to help im- prove the sustainability of commercial oyster fisheries. Hand harvesting by divers produced 25−32% more oysters per unit of time of fishing than tradi- tional dredging and tonging, although the dive operation required two fish- ermen, rather than one. Per capita returns for dive operations may none- theless be competitive with returns for other gears even in the short term if one person culling on deck can serve two or three divers. Dredging reduced the height of reef habitat by 34%, sig- nificantly more than the 23% reduction caused by tonging, both of which were greater than the 6% reduction induced by diver hand-harvesting. Thus, con- servation of the essential habitat and sustainability of the subtidal oyster fishery can be enhanced by switch- ing to diver hand-harvesting. Man- agement schemes must intervene to drive the change in harvest methods because fishermen will face relatively high costs in making the switch and will not necessarily realize the long- term ecological benefits.

Comparative patterns of occupancy by decapod crustaceans in seagrass, oyster, and marsh-edge habitats in a Northeast Gulf of Mexico estuary

Decapod crustaceans occupying seagrass, salt marsh edge, and oyster habitats within the St. Martins Aquatic Preserve along the central Gulf coast of Florida were quantitatively sampled using a 1-m2 throw trap during July–August 1999 and March–April 2000. Relative abundance and biomass were used as the primary measures to compare patterns of occupancy among the three habitat types. Representative assemblages of abundant and common species from each habitat were compared using Schoener's Percent Similarity Index (PSI). In all, 17,985 decapods were sampled, representing 14 families and 28 species. In the summer sampling period, mean decapod density did not differ between oyster and seagrass habitats, which both held greater densities of decapods than marsh-edge. In the spring sampling period oyster reef habitat supported greater mean decapod density than both seagrass and marsh-edge, which had similar densities of decapods. Habitat-specific comparisons of decapod density between the two sampling periods indicated no clear seasonal effect. In summer 1999, when seagrasses were well established, decapod biomass among the three habitats was not significantly different. During spring 2000, decapod biomass in oyster (41.40 gm−2) was greater than in marshedge (4.20 gm−2), but did not differ from that of seagrass (9.73 g m−2). There was no significant difference in decapod biomas between seagrass and marsh-edge habitats during the spring 2000 sampling period. The assemblage analysis using Schoener's PSI indicated that decapod assemblages associated with oyster were distinct from seagrass and marshedge habitats (which were similar). The results of this study suggest that in comparison to seagrass and marsh-edge habitats, oyster reef habitats and the distinct assemblage of decapod crustaceans that they support represent an ecologically important component of this estuarine system.

The location, composition, and origin of oyster bars in mesohaline Chesapeake Bay

Sub-bottom profiling sonar surveys in the mesohaline Chesapeake Bay located pre-Holocene, hard terrace structures surrounded by or buried by softer sediments. The descending scarps of these hard terraces frequently correspond to the boundaries of charted oyster bars. The terraces are composed of a base of coarse Tertiary or Pleistocene sediments and may be covered by a layer of oyster shell, shell grit or mud. The presence of soft, methane gas-charged sediments adjacent to descending terrace scarps indicates that terrace/oyster bar configuration is due to the segmentation of the larger regional terrace structure by paleochannel drainage. These sediment filled channels, relicts of the last glacial period, extend into the estuary far beyond the current terrestrial shores and demarcate the oyster bar boundaries. Continual filling of the paleochannels segregating oyster bars, both by fluvial transport as well as in situ production of sediment, has lead to a decrease in relief in the terrace scarp profile. At some locations sediments are burying or have buried oyster bar terraces. Although widespread reduction of oyster bar relief in the Chesapeake Bay has been attributed to harvest activity, statistical analysis of bathymetry does not clearly support this premise. Our findings suggest that oyster habitat restoration should take place only on the clearly identified, hard terraces away from encroaching bottom sediments.

Influence of simulated bivalve biodeposition and microphytobenthos on sediment nitrogen dynamics: A laboratory study

Suspension‐feeding eastern oysters, Crassostrea virginica, were once abundant in Chesapeake Bay and may then have exerted top‐down control on phytoplankton and also reduced turbidities, thereby increasing light available to benthic plants. Alternatively, oysters may have simply recycled inorganic nutrients rapidly back to the water column, with no long‐lasting reduction in phytoplankton biomass resulting from oyster feeding activity. To help distinguish between these scenarios, we explored changes in nitrogen fluxes and denitrification in laboratory incubations of sediment cores held under oxic and anoxic conditions in response to loading by pelletized phytoplankton cells, an experimental analog for oyster feces and pseudofeces. When organics were regenerated under aerobic conditions, typical of those associated with oyster habitat, coupled nitrification‐denitrification was promoted, resulting in denitrification of ~20% of the total added nitrogen. In contrast, under anoxic conditions, typical of current summertime conditions in main‐stem Chesapeake Bay where phytoplankton is microbially degraded beneath the pycnocline, nitrogen was released solely as ammonium from the added organics. We postulate that denitrification of particulate nitrogen remaining in oyster feces and pseudofeces may enhance nitrogen removal from estuaries. In aerobic incubations with sufficient light (70 µmol m-2 s-1), a benthic microalgal/cyanobacterial community grew that not only absorbed the inorganic nitrogen released from the added organics but also fixed N2. This result suggests that an ecosystem dominated by benthic primary production may develop in shallow waters when reduced turbidity associated with bivalve feeding increases light penetration to a level that can sustain benthic microalgal production.

Remote Acoustic Habitat Assessment Techniques Used to Characterize the Quality and Extent of Oyster Bottom in the Chesapeake Bay

Underwater acoustic technologies may be used for remote sensing of estuarine and coastal benthic habitat in addition to more traditional methods of assessing the seabed. These systems use reflected sound energy to identify surface objects, texture, and subbottom density discontinuities, and to classify benthic habitat. We evaluated the ability of subbottom profiling systems, side scan sonar, and acoustic seabed classification systems (ASCS) to assess oyster habitat in the Chesapeake Bay. Our criteria were the systems' abilities to assess the quality and quantity of oyster shell resources and to integrate with Geographical Information Systems (GIS) technology for the charting of the seabed. Although all systems examined have previously been used for benthic habitat assessment, we found that ASCS offered the most promise as a stand alone system for mapping complex and heterogeneous habitat typical of oyster bottoms. The results from this study suggest that ASCS technology is highly suited for the identification and charting of oyster shell as well as distinguishing among different combinations of shell and fine sediments. Such systems also offer excellent linkage with GIS display and analysis capability.

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

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).

Subaqueous Soils Pedogenesis in a Submersed Environment

Proposals for the inclusion of permanently submersed materials in soil taxonomic systems have periodically been put forth since some time in the mid 1800s. The proposals were largely conceptual in nature, relying more on subjective reasoning rather than analytical and field data. Advances in computer and global positioning technology, and the continuing development of the discipline of pedology provided the opportunity to examine shallow water estuarine sediments within a pedological framework. Morphological and analytical data from 85 1.5‐ to 2.0‐m profiles from Sinepuxent Bay, Maryland, indicate that the four pedogenic processes of additions, losses, transformations, and transfers outlined in the generalized theory of soil genesis are active in a subaqueous environment. The evidence of pedogenic processes includes the addition of biogenic calcium carbonate (shells), organic fragments, and organic matter; the loss of organic matter and surficial material; the transfer of oxygen through diffusion and bioturbation processes; and the transformation of humic substances and sulfur (sulfidization). The change in the concept of estuarine substrates from sediment to soil has significant ramifications for pedologists, ecologists, estuarine researchers, and government agencies involved in soil resource inventory and estuarine restoration programs. Application of these concepts could help further our understanding of the relationships between subaqueous soil distribution and submersed aquatic vegetation (SAV); clam, scallop, and oyster habitat; and dredge sites with potential acid‐sulfate weathering. This work resulted in a change to the definition of soil in Soil Taxonomy to include subaqueous soils that are capable of or presently support rooted SAV.

Changes in the distribution of epifaunal reefs and oysters during 130 years of dredging for oysters in Foveaux Strait, southern New Zealand

1. Foveaux Strait, a narrow seaway that is exposed to heavy wave action and strong tidal currents, has been the subject of an oyster fishery for over 130 years. Before the oyster fishery commenced the seafloor was extensively covered by epifaunal reefs that were tidally-oriented, linear aggregations of patch reefs. 2. Patch reefs are formed by the bryozoan Cinctipora elegans cemented by encrusting bryozoa, ascidians, sponges, and polychaetes. The molluscan epifauna is dominated by the oyster, Tiostrea chilensis and bysally attached bivalves. Mortality of oysters is probably lower and recruitment and growth may be higher within the reef habitat. 3. Fishers found commercial densities of oysters occurred only on epifaunal reefs. Fishers exploited local groups of reefs. These groups form the patchily distributed oyster beds characteristic of this fishery. 4. Dredging for oysters progressively modified reefs until oysters were the only epifauna remaining. Dredges caught oysters more efficiently after the catch bag no longer became saturated with other epifauna. This heightened efficiency allowed fishers to rapidly reduce oyster density to commercial extinction. Oyster density has not rebuilt on oyster beds abandoned by fishers. 5. The rate of modification of epifaunal reefs was slower during the early years of the fishery but has accelerated, especially over the last 37 years. Frequency of disturbance increased as the numbers of vessels fishing grew and fishers developed speedier dredging methods. Intensity of disturbance also increased as heavier dredges were introduced and allowed focused fishing of reefs. 6. Oysters became reduced to low densities in the eastern and central areas that fishers then abandoned. The commercially exploited area subsequently expanded to the limits of Foveaux Strait. 7. With accelerated modification of oyster habitat, disease mortality has become more important. 8. Attempting to rebuild the fishery by oyster enhancement may be more successful conjoined with habitat restoration. Copyright © 1999 John Wiley & Sons, Ltd.