Oyster Density Research Articles

The importance of population density and inter-individual distance in conserving the European oysterOstrea edulis

Fertilization success will determine the rate at which a population can expand and is especially important when considering small, establishing or enduring communities. Introduced species frequently fail to establish reproductively functional populations due to strong Allee effects associated with low densities. The native European oyster,Ostrea edulisbroods its fertilized eggs in the pallial cavity for a period of 8–10 days before releasing the larvae. It is considered a partial broadcast spawner and was used as a model species to assess the importance of Allee effects such as inter-individual distance on reproductive success. Distances between individual oysters within test plots in areas of known oyster density were used in conjunction with standardized brood size (n larvae g−1total wet weight) to assess fertilization success. A significant, positive relationship was observed between brood size and oyster density. Oysters with a nearest neighbour ≤1.5 m were found to brood significantly more larvae than individuals with nearest neighbours ≥1.5 m. Therefore, high density sites need to be maintained to ensure the recovery and enhancement of this OSPAR Convention recognized species in decline.

Gear and survey efficiency of patent tongs for oyster populations on restoration reefs.

Surveys of restored oyster reefs need to produce accurate population estimates to assess the efficacy of restoration. Due to the complex structure of subtidal oyster reefs, one effective and efficient means to sample is by patent tongs, rather than SCUBA, dredges, or bottom cores. Restored reefs vary in relief and oyster density, either of which could affect survey efficiency. This study is the first to evaluate gear (the first full grab) and survey (which includes selecting a specific half portion of the first grab for further processing) efficiencies of hand-operated patent tongs as a function of reef height and oyster density on subtidal restoration reefs. In the Great Wicomico River, a tributary of lower Chesapeake Bay, restored reefs of high- and low-relief (25–45 cm, and 8–12 cm, respectively) were constructed throughout the river as the first large-scale oyster sanctuary reef restoration effort (sanctuary acreage > 20 ha at one site) in Chesapeake Bay. We designed a metal frame to guide a non-hydraulic mechanical patent tong repeatedly into the same plot on a restored reef until all oysters within the grab area were captured. Full capture was verified by an underwater remotely-operated vehicle. Samples (n = 19) were taken on nine different reefs, including five low- (n = 8) and four high-relief reefs (n = 11), over a two-year period. The gear efficiency of the patent tong was estimated to be 76% (± 5% standard error), whereas survey efficiency increased to 81% (± 10%) due to processing. Neither efficiency differed significantly between young-of-the-year oysters (spat) and adults, high- and low-relief reefs, or years. As this type of patent tong is a common and cost-effective tool to evaluate oyster restoration projects as well as population density on fished habitat, knowing the gear and survey efficiencies allows for accurate and precise population estimates.

Density-dependent capture efficiency of a survey dredge and its influence on the stock assessment of eastern oysters (Crassostrea virginica) in Delaware Bay

A reliable measure of gear capture efficiency is required to calculate unbiased estimates of population size and fishing mortality from survey data in a stock assessment. However, capture efficiency can vary spatially and temporally due to changes in abundance, stock area, the environment, and the sampling gear itself. Therefore, periodic reassessment of this parameter is necessary to ensure that the catchability coefficients being applied accurately reflect the capture efficiency of the survey sampling gear, especially when catchability is being estimated outside of the stock assessment model. Using data from field experiments conducted in 1999, 2000, 2003, and 2013, we evaluated spatial and temporal variability in capture efficiency for a commercial dredge used to conduct a fishery-independent survey of the eastern oyster (Crassostrea virginica) population in Delaware Bay, USA. A spatial gradient in capture efficiency was detected, but no temporal trend. Capture efficiency was a function of the density of oysters in the sampled area. To our knowledge this is the first time density-dependent capture efficiency has been identified for a sessile invertebrate stock survey. Since density dependence in capture efficiency leads to hyperstable catch-per-unit-effort, caution is advised when deriving oyster abundance from dredge survey catch-per-effort data, especially at low oyster density and when high spatial resolution estimates of survey dredge capture efficiency are not available.

Limited impact of an invasive oyster on intertidal assemblage structure and biodiversity: the importance of environmental context and functional equivalency with native species

Impacts of invasive species are context dependent and linked to the ecosystem they occur within. To broaden the understanding of the impact of a globally widespread invasive oyster, Crassostrea (Magallana) gigas, intertidal surveys were carried out at 15 different sites in Europe. The impact of C. gigas on macro- (taxa surrounding oyster > 1 cm) and epifaunal (taxa on oyster < 1 cm) benthic communities and α and β-diversity was assessed and compared to those associated with native ecosystem engineers, including the flat oyster Ostrea edulis. Whilst the effect of C. gigas on benthic community structures was dependent on habitat type, epifaunal communities associated with low densities of O. edulis and C. gigas did not differ and changes in benthic assemblage structure owing to the abundance of C. gigas were therefore attributed to the presence of oyster shells. Macrofaunal α-diversity increased with C. gigas cover in muddy habitats, while epifaunal α-diversity decreased at greater oyster densities. Macrofaunal β-diversity was greatest at low densities of C. gigas; however, it did not differ between samples without and increased densities of oysters. In contrast, epifaunal β-diversity decreased with increasing oyster cover. Different environmental contexts enabled more independent predictions of the effect of C. gigas on native communities. These were found to be low and more importantly not differing from O. edulis. This indicates that, at low densities, C. gigas may be functionally equivalent to the declining native oyster in terms of biodiversity facilitation and aid in re-establishing benthic communities on shores where O. edulis has become extinct.

Integrating Larval Dispersal, Permitting, and Logistical Factors Within a Validated Habitat Suitability Index for Oyster Restoration

Habitat suitability index (HSI) models are increasingly used to guide ecological restoration. Successful restoration is a byproduct of several factors, including physical and biological processes, as well as permitting and logistical considerations. Rarely are factors from all of these categories included in HSI models, despite their combined relevance to common restoration goals such as population persistence. We developed a Geographic Information System (GIS)-based HSI for restoring persistent high-relief subtidal oyster (Crassostrea virginica) reefs protected from harvest (i.e., sanctuaries) in Pamlico Sound, North Carolina, USA. Expert stakeholder input identified 17 factors to include in the HSI. Factors primarily represented physical (e.g., salinity) and biological (e.g., larval dispersal) processes relevant to oyster restoration, but also included several relevant permitting (e.g., presence of seagrasses) and logistical (e.g., distance to restoration material stockpile sites) considerations. We validated the model with multiple years of oyster density data from existing sanctuaries, and compared HSI output with distributions of oyster reefs from the late 1800’s. Of the 17 factors included in the model, stakeholders identified four factors—salinity, larval export from existing oyster sanctuaries, larval import to existing sanctuaries, and dissolved oxygen—most critical to oyster sanctuary site selection. The HSI model provided a quantitative scale over which a vast water body (~6,000 km2) was narrowed down by 95% to a much smaller suite of optimal (top 1% HSI) and suitable (top 5% HSI) locations for oyster restoration. Optimal and suitable restoration locations were clustered in northeast and southwest Pamlico Sound. Oyster density in existing sanctuaries, normalized for time since reef restoration, was a positive exponential function of HSI, providing validation for the model. Only a small portion (10-20%) of historical reef locations overlapped with current, model-predicted optimal and suitable restoration habitat. We contend that stronger linkages between larval connectivity, landscape ecology, stakeholder engagement and spatial planning within HSI models can provide a more holistic, unified approach to restoration.

Effects of landscape setting on oyster reef structure and function largely persist more than a decade post‐restoration

Long‐term monitoring is vital to understanding the efficacy of restoration approaches and how restoration may enhance ecosystem functions. We revisited restored oyster reefs 13 years post‐restoration and quantified the resident and transient fauna that utilize restored reefs in three differing landscape contexts: on mudflats isolated from vegetated habitat, along the edge of salt marsh, and in between seagrass and salt marsh habitat. Differences observed 1–2 years post‐restoration in reef development and associated fauna within reefs restored on mudflats versus adjacent to seagrass/salt marsh and salt marsh‐only habitats persisted more than 10 years post‐restoration. Reefs constructed on open mudflat habitats had the highest densities of oysters and resident invertebrates compared to those in other landscape contexts, although all restored reefs continued to enhance local densities of invertebrate taxa (e.g. bivalves, gastropods, decapods, polychaetes, etc.). Catch rates of juvenile fishes were enhanced on restored reefs relative to controls, but to a lesser extent than directly post‐restoration, potentially because the reefs have grown vertically within the intertidal and out of the preferred inundation regime of small juvenile fishes. Reef presence and landscape setting did not augment the catch rates of piscivorous fishes in passive gill nets, similar to initial findings; however, hook‐and‐line catch rates were greater on restored reefs than non‐reef controls. We conclude that ecosystem functions and associated services provided by restored habitats can vary both spatially and temporally; therefore, a better understanding of how service delivery varies among landscape setting and over time should enhance efforts to model these processes and restoration decision‐making.

Oyster Demographics in Harvested Reefs vs. No-Take Reserves: Implications for Larval Spillover and Restoration Success

Fishery species that reside in no-take, marine reserves often show striking increases in size and abundance relative to harvested areas, with the potential for larval spillover to harvested populations. The benefits of spillover, however, may not be realized if the populations or habitats outside of reserves are too degraded. We quantified oyster population density and demographics such as recruitment, growth, mortality, and potential larval output as a function of two types of oyster management strategies in Pamlico Sound, North Carolina, USA: (1) natural reefs + harvested and (2) restored reefs + harvested. We compared these data to demographic data collected as a function of a third type of management strategy, (3) restored reefs + protected from harvest. Mean oyster recruitment was ~12 times higher in restored + harvested reefs than in natural + harvested reefs. Mean total oyster density was ~8- to 72-times higher in restored + protected reefs than in restored + harvested or natural + harvested reefs, respectively. Moreover, harvested reefs exhibited truncated size structure, and few or no individuals greater than legal size (75 mm), whereas protected reefs typically had a polymodal size structure, including many large individuals. We estimate that restored + protected reefs have ~4 to 700 times greater potential larval output m-2 than restored + harvested or natural + harvested reefs, respectively. After accounting for total sound-wide areal coverage of each reef type, total potential larval output from restored + protected reefs was ~6 times greater than that from natural + harvested and restored + harvested reefs. Marine reserves can potentially subsidize harvested populations via larval spillover, however, in the case of oyster reefs in Pamlico Sound, the relatively degraded conditions of natural reefs (e.g., low vertical relief, low shell volume per square meter) may not provide much in the way of suitable settlement substrate to realize the benefits of larval spillover from reserves. Restoration of oyster reefs, even with a thin veneer of substrate, may improve settlement substrate to increase the benefits of larval spillover from reserves.

Range expansion of a fouling species indirectly impacts local species interactions.

We investigated how recent changes in the distribution and abundance of a fouling organism affected the strength of interactions between a commercially important foundation species and a common predator. Increases in the abundance of boring sponges that bioerode the calcified shells of oysters and other shelled organisms have been attributed to increased salinization of estuarine ecosystems. We tested the hypothesis that fouling by boring sponges will change the interaction strength between oysters and a common predator (stone crabs). We generated five oyster density treatments crossed with two sponge treatments (sponge and no sponge). We contrasted the interaction strength between stone crabs and fouled and non-fouled oysters by comparing the parameters of fitted functional response curves based on Rogers random predation model. We found that fouled oysters suffered higher predation from stone crabs, and that the increased predation risk stemmed from a reduction in the handling time needed to consume the fouled oysters. These findings highlight the importance of understanding the effects of abiotic changes on both the composition of ecological communities, and on the strengths of direct and indirect interactions among species. Global climate change is altering local ecosystems in complex ways, and the success of restoration, management, and mitigation strategies for important species requires a better appreciation for how these effects cascade through ecosystems.

Oyster (Crassostrea virginica) Aquaculture Shifts Sediment Nitrogen Processes toward Mineralization over Denitrification

Filter-feeding bivalves, like oysters, couple pelagic primary production with benthic microbial processes by consuming plankton from the water column and depositing unassimilated material on sediment. Conceptual models suggest that at low to moderate oyster densities, this deposition can stimulate benthic denitrification by providing denitrifying bacteria with organic carbon and nitrogen (N). While enhanced denitrification has been found at oyster reefs, data from oyster aquaculture are limited and equivocal. This study measured seasonal rates of denitrification, as well as dissimilatory nitrate reduction to ammonium (DNRA), and dissolved inorganic N fluxes at a rack and bag eastern oyster (Crassostrea virginica) aquaculture farm. Consistent with models, denitrification was enhanced within the farm, with an average annual increase of 350% compared to a reference site. However, absolute denitrification rates were low relative to other coastal systems, reaching a maximum of 19.2 μmol m−2 h−1. Denitrification appeared to be nitrate (NO3 −) limited, likely due to inhibited nitrification caused by sediment anoxia. Denitrification may also have been limited by competition for NO3 − with DNRA, which accounted for an average of 76% of NO3 − reduction. Consequently, direct release of ammonium (NH4 +) from mineralization to the water column was the most significant benthic N pathway, with seasonal rates exceeding 900 μmol m−2 h−1 within the farm. The enhanced N processes were spatially limited however, with significantly higher rates directly under oysters, compared to in between oyster racks. For commercial aquaculture farms like this, with moderate oyster densities (100–200 oysters m−2), denitrification may be enhanced, but nonetheless limited by biodeposition-induced sediment anoxia. The resulting shift in the sediment N balance toward processes that regenerate reactive N to the water column rather than remove N is an important consideration for water quality.

Oyster toadfish (Opsanus tau) boatwhistle call detection and patterns within a large-scale oyster restoration site.

During May 2015, passive acoustic recorders were deployed at eight subtidal oyster reefs within Harris Creek Oyster Sanctuary in Chesapeake Bay, Maryland USA. These sites were selected to represent both restored and unrestored habitats having a range of oyster densities. Throughout the survey, the soundscape within Harris Creek was dominated by the boatwhistle calls of the oyster toadfish, Opsanus tau. A novel, multi-kernel spectral correlation approach was developed to automatically detect these boatwhistle calls using their two lowest harmonic bands. The results provided quantitative information on how call rate and call frequency varied in space and time. Toadfish boatwhistle fundamental frequency ranged from 140 Hz to 260 Hz and was well correlated (r = 0.94) with changes in water temperature, with the fundamental frequency increasing by ~11 Hz for every 1°C increase in temperature. The boatwhistle call rate increased from just a few calls per minute at the start of monitoring on May 7th to ~100 calls/min on May 10th and remained elevated throughout the survey. As male toadfish are known to generate boatwhistles to attract mates, this rapid increase in call rate was interpreted to mark the onset of spring spawning behavior. Call rate was not modulated by water temperature, but showed a consistent diurnal pattern, with a sharp decrease in rate just before sunrise and a peak just after sunset. There was a significant difference in call rate between restored and unrestored reefs, with restored sites having nearly twice the call rate as unrestored sites. This work highlights the benefits of using automated detection techniques that provide quantitative information on species-specific call characteristics and patterns. This type of non-invasive acoustic monitoring provides long-term, semi-continuous information on animal behavior and abundance, and operates effectively in settings that are otherwise difficult to sample.

Oyster Density and Demographic Rates on Natural Intertidal Reefs and Hardened Shoreline Structures

The ubiquitous loss of natural intertidal oyster reefs and associated ecosystem services has fueled restoration efforts throughout the world. Effective restoration requires an understanding of the distribution, density, and demographic rates (growth and survival) of oysters inhabiting existing natural reefs and how these may vary as a function of landscape-scale factors, such as tidal range and fetch distances. Furthermore, natural intertidal habitats are increasingly being replaced with hardened shoreline structures that may be colonized by oysters, yet little is known about habitat quality (as indexed by oyster density and demographic rates) of these hardened structures relative to natural habitats. The present study sought to compare oyster density, demographic rates, and population estimates (1) across estuarine landscape settings to inform natural intertidal oyster reef restoration (i.e., comparing natural intertidal reefs within adjacent water bodies that vary in tidal regimes and fetch distances) and (2) across natural habitats and human-made structures to assess variation in habitat quality between natural reefs and hardened shorelines. Oyster density, growth rates, and population estimates on natural intertidal reefs were greatest within the smaller, more tidally influenced Core Sound versus the larger, wind-driven Pamlico Sound, with no significant difference in survivorship identified between the two water bodies. Natural intertidal reefs and hardened shoreline structures were compared within Pamlico Sound only, with natural intertidal reefs hosting three to eight times higher oyster densities than hardened shoreline structures. When mean oyster density/m2 was multiplied by reef area to estimate population size, natural intertidal reefs within Pamlico Sound hosted considerably greater populations of oysters relative to hardened shorelines. The present study fills an existing need to understand oyster density and demographic rates on natural intertidal reefs and hardened shorelines to better inform future restoration and shoreline management scenarios.

Estimating the impact of oyster restoration scenarios on transient fish production

AbstractOyster reef restoration projects are increasing in number both to enhance oyster density and to retain valuable ecosystem services provided by oyster reefs. Although some oyster restoration projects have demonstrated success by increasing density and biomass of transient fish, it still remains a challenge to quantify the effects of oyster restoration on transient fish communities. We developed a bioenergetics model to assess the impact of selected oyster reef restoration scenarios on associated transient fish species. We used the model to analyze the impact of changes in (1) oyster population carrying capacity; (2) oyster population growth rate; and (3) diet preference of transient fish on oyster reef development and associated transient fish species. Our model results indicate that resident fish biomass is directly affected by oyster restoration and oyster biomass, and oyster restoration can have cascading impacts on transient fish biomass. Furthermore, the results highlight the importance of a favorable oyster population growth rate during early restoration years, as it can lead to rapid increases in mean oyster biomass and biomass of transient fish species. The model also revealed that a transient fish's diet solely dependent on oyster reef‐derived prey could limit the biomass of transient fish species, emphasizing the importance of habitat connectivity in estuarine areas to enhance transient fish species biomass. Simple bioenergetics models can be developed to understand the dynamics of a system and make qualitative predictions of management and restoration scenarios.

Niche dimension differs among life-history stages of Pacific oysters in intertidal environments

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 562:113-122 (2016) - DOI: https://doi.org/10.3354/meps11961 Niche dimension differs among life-history stages of Pacific oysters in intertidal environments Brenda Walles1,2,3,*, Aad C. Smaal1,2, Peter M. J. Herman3, Tom Ysebaert1,3 1Wageningen Marine Research, Wageningen University & Research, PO Box 77, 4400 AB Yerseke, The Netherlands 2Aquaculture and Fisheries Group, Wageningen University and Research, PO Box 338, 6700 AH Wageningen, The Netherlands 3NIOZ Yerseke, Royal Netherlands Institute for Sea Research and Utrecht University, PO Box 140, 4400 AC Yerseke, The Netherlands *Corresponding author: brenda.walles@nioz.nl ABSTRACT: Structure-building, autogenic ecosystem engineers are recognized worldwide as potential tools for coastal protection, which depends on long-term sustainability and persistence of their structures. For reef-building oysters, reefs are maintained through accumulation of shell material, which depends on recruitment and growth and which provides substrate for new generations. Knowledge of the fundamental niche of oysters in relation to biotic and abiotic conditions helps define the area where ecosystem engineers grow and survive well and where their structures are likely to persist and be effective for coastal protection. Response curves of different life-history stages (larvae, juveniles and adults) of the Pacific oyster Crassostrea gigas were investigated along a tidal emersion gradient in a manipulative field study. Density of juvenile oysters was maximal around 36% tidal emersion. Shell growth of juveniles and adults, condition index (CI) of adults and mortality of adults were negatively related to tidal emersion, whereas mortality of juveniles was positively related to tidal emersion. Tidal emersion had a strong effect on oyster survival and growth, with the most favourable being around 20 to 40% tidal emersion, as indicated by recruit density and the CI of adults. These findings suggest that the area of 20 to 40% tidal emersion is optimal for reef development, which is in line with other investigations of C. gigas and the eastern oyster C. virginica across different systems, supporting a broader geographical application of these findings. KEY WORDS: Oyster reef · Ecosystem engineering · Recruitment · Growth · Niche · Ecosystem-based management · Tidal flats Full text in pdf format Supplementary material PreviousNextCite this article as: Walles B, Smaal AC, Herman PMJ, Ysebaert T (2016) Niche dimension differs among life-history stages of Pacific oysters in intertidal environments. Mar Ecol Prog Ser 562:113-122. https://doi.org/10.3354/meps11961 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 562. Online publication date: December 29, 2016 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2016 Inter-Research.

Experimental test of oyster restoration within eelgrass

Abstract Both seagrasses and oysters are foundation species valued for their wide range of ecosystem services, but their space competition sets a constraint on joint benefits. A reserve for native Olympia oysters (Ostrea lurida) was established in lower Hood Canal (Washington State, USA) more than a century ago but is now devoid of that species and dominated by native eelgrass (Zostera marina). This situation sets up a conservation conflict because management activities for one species are at odds with the protection of another. In experimental enhancement plots, Olympia oysters were outplanted at low density, which successfully maintained eelgrass density and production. One method was used in 2013 (seeded cultch, 8% cover) and two additional methods in 2015 (anchored cultch and single oysters, the latter at 4% cover). For all outplant methods, oysters experienced 99% annual mortality, associated with the attraction of non‐native and native predators. Shell cover remained steady for a year and then declined rapidly, as shell accumulation did not exceed sedimentation rates. Eelgrass per se does not preclude Olympia oysters, given that the two species were observed to co‐occur at a coastal estuarine site (Willapa Bay, Washington). However, even when socio‐political constraints on restoration activities were overcome, ecological constraints remained from predation. Competition between these two protected species was avoided, but it may be the case that top‐down control on oysters was particularly acute owing to low oyster density and/or the environmental conditions of eelgrass beds.

Oyster reef restoration: substrate suitability may depend on specific restoration goals

AbstractA limited supply of oyster shell for restoration practices has prompted investigations of alternative substrates used in construction of artificial oyster reefs. The success of oyster reef restoration projects is increasingly focused not only on oyster densities, but also on habitat provisioning for associated fauna. A subtidal oyster reef complex (0.24 km2) was restored in the Mission‐Aransas Estuary, Texas, U.S.A., in July 2013 using replicated mounds of concrete, limestone, river rock, and oyster shell substrates. Oyster and reef‐associated fauna characteristics were quantified quarterly for 15 months, using sampling trays that were deployed 3 months after construction. The highest densities of oyster spat occurred 9 months after tray deployment (July 2014, 1,264/m2), whereas juvenile oyster densities increased throughout the study period to 283/m2. Concrete (1,022/m2) and limestone (939/m2) supported the highest number of oysters over all dates. Oyster shell (1,533/m2) and concrete (1,047/m2) substrates supported the highest densities of associated motile fauna. Faunal diversity (Hill's N1) did not vary by substrate material, but did show seasonal variation. A simple benefit–cost ratio was used to indicate the localized monetary value for each of the substrates. Oyster shell and concrete substrates returned the highest benefit–cost ratio for motile fauna, while concrete yielded the highest benefit–cost ratio for oyster abundance. Incorporating benefit–cost ratios in restoration planning will allow practitioners to better integrate substrate‐specific ecological values with economic considerations and project goals to maximize return on restoration investments.

Ecological impacts of non-native Pacific oysters (Crassostrea gigas) and management measures for protected areas in Europe

Pacific oysters are now one of the most ‘globalised’ marine invertebrates. They dominate bivalve aquaculture production in many regions and wild populations are increasingly becoming established, with potential to displace native species and modify habitats and ecosystems. While some fishing communities may benefit from wild populations, there is now a tension between the continued production of Pacific oysters and risk to biodiversity, which is of particular concern within protected sites. The issue of the Pacific oyster therefore locates at the intersection between two policy areas: one concerning the conservation of protected habitats, the other relating to livelihoods and the socio-economics of coastal aquaculture and fishing communities. To help provide an informed basis for management decisions, we first summarise evidence for ecological impacts of wild Pacific oysters in representative coastal habitats. At local scales, it is clear that establishment of Pacific oysters can significantly alter diversity, community structure and ecosystem processes, with effects varying among habitats and locations and with the density of oysters. Less evidence is available to evaluate regional-scale impacts. A range of management measures have been applied to mitigate negative impacts of wild Pacific oysters and we develop recommendations which are consistent with the scientific evidence and believe compatible with multiple interests. We conclude that all stakeholders must engage in regional decision making to help minimise negative environmental impacts, and promote sustainable industry development.

Suspended sediment projections in Apalachicola Bay in response to altered river flow and sediment loads under climate change and sea level rise

AbstractSuspended sediments, or total suspended solids (TSS), are an important factor for oyster habitat. While high concentrations of suspended sediments can cause a reduction of oyster density, some level of suspended sediment is required to supply oysters with necessary nutrients. In this study, characteristics of TSS variations in response to sea level rise (SLR) at two oyster reefs in Apalachicola Bay are investigated by coupled estuarine hydrodynamic and sediment transport modeling. A storm event in 1993 and a year‐long period in 2010 under recent sea level conditions are selected as the baseline conditions. Scenarios of river flow and sediment loads under SLR and climate change are obtained by downscaled global climate modeling. Compared to the baseline conditions, simulations of TSS indicate that predicted SLR yields a substantial decrease in TSS near the two oyster reefs. However, TSS levels differed at the two study locations. TSS changes by SLR revealed minimal impact on oyster habitat at the Dry Bar site (to the west of the mouth of the Apalachicola River) but are projected to have a significant impact at the Cat Point site (to the east of the Apalachicola River). At Cat Point, because SLR causes the increase of salt water intrusion from the Gulf through a large tidal inlet (East Pass), maximum sediment concentration is near zero for 0.2‐m SLR and equal to zero for 0.5‐ and 1.2‐m SLR. Therefore, SLR may result in a substantial loss of nutrients from suspended sediment in the oyster reef at Cat Point.

Wave Exposure Structures Oyster Distribution on Natural Intertidal Reefs, But Not on Hardened Shorelines

Intertidal oyster reefs can protect estuarine shorelines from wave erosion and sea-level rise, and recognition of these ecosystem services has fueled global efforts to conserve and restore these reefs. Although intertidal oyster reefs are valued for attenuating wave erosion, little attention has been paid to the effects of wave exposure on their distribution. The present study characterized the role of wave exposure in determining the distribution of natural intertidal oyster reefs and of oysters on hardened shorelines (bulkhead and riprap revetments). Wave exposure was determined using the National Oceanic and Atmospheric Administration (NOAA)-developed Wave Exposure Model (WEMo), which integrates adjacent water depth, fetch, and processed wind information, among other variables. Field mapping of oyster reefs, defined as ≥10 oysters m−2, in Pamlico and Core sounds, North Carolina, USA, was conducted in summer 2014. Hardened shorelines and associated oyster densities were mapped for Pamlico Sound only. A narrow wave exposure threshold (~500 J m−1) was identified above which natural intertidal reefs did not occur and below which reef presence was apparently dependent on other structuring variables, such as salinity at the time of sampling and the grain size of surrounding sediments. Wave exposure was not correlated with the presence of oysters on hardened shorelines. The application of WEMo in the present study should be useful for selecting locations and materials for intertidal oyster reef restoration.

Modeling the transmission of Perkinsus marinus in the Eastern oyster Crassostrea virginica

Dermo disease caused by the protistan Perkinsus marinus in Eastern oysters Crassotrea virginica is an important source of mortality impacting oyster population dynamics resulting in substantial losses in fisheries and aquaculture. The rapid transmission and spread of the disease minimized the importance of transmission models and past models (proliferation-based models) assumed simple density-dependent transmission or rapid infection post-settlement. This approach is a good approximation only for low population densities. A transmission model was developed for P. marinus in Eastern oysters that accounts for the seasonal change in disease dynamics and density-dependent foraging (of suspended particles) interference among hosts. The model, verified and evaluated against field observations, incorporates parasite release to the water column from live and dead individuals, parasite consumption by living oysters, the diffusion of parasites in the water, body burden-based dose-dependent transmission, recruitment, and disease-caused mortality. The model returns a basic reproduction number R0 for Dermo much greater than unity (R0=90) in accordance with the current persistence and pandemic nature of this disease in oysters. No population density is obtained that is low enough to suppress R0 below 1 (i.e. disease extinction). R0 is also estimated for high oyster densities (>300 individuals m−2) and particularly for relatively large oysters (∼90mm), today rare but once common before generalized overfishing occurred on healthy oyster reefs. In this scenario, R0 drops below 1, indicating that high oyster density can limit disease invasion through foraging interference and depletion of parasites in the water column. High intensity recruitment events allow the oyster population to attain such densities and limit the development of epizootics. These results provide insight into the transition from past populations, where Dermo is inferred to have been limited in its impact, to the current persistent and pandemic nature of this disease. Further coupling of this model into metapopulation and hydrodynamic models could be a promising tool to support management decision-making for bivalve populations impacted by Dermo disease.

Quantitative Validation of a Habitat Suitability Index for Oyster Restoration

Habitat suitability index (HSI) models provide spatially explicit information on the capacity of a given habitat to support a species of interest, and their prevalence has increased dramatically in recent years. Despite caution that the reliability of HSIs must be validated using independent, quantitative data, most HSIs intended to inform terrestrial and marine species management remain unvalidated. Furthermore, of the eight HSI models developed for eastern oyster (Crassostrea virginica) restoration and fishery production, none has been validated. Consequently, we developed, calibrated, and validated an HSI for the eastern oyster to identify optimal habitat for restoration in a tributary of Chesapeake Bay, the Great Wicomico River (GWR). The GWR harbors a high density, restored oyster population, and therefore serves as an excellent model system for assessing the validity of the HSI. The HSI was derived from GIS layers of bottom type, salinity, and water depth (surrogate for dissolved oxygen), and was tested using live adult oyster density data from a survey of high vertical relief reefs (HRR) and low vertical relief reefs (LRR) in the sanctuary network. Live adult oyster density was a statistically-significant sigmoid function of the HSI, which validates the HSI as a robust predictor of suitable oyster reef habitat for rehabilitation or restoration. In addition, HRR had on average 103-116 more adults m^−2 than LRR at a given level of the HSI. For HRR, HSI values ≥0.3 exceeded the accepted restoration target of 50 live adult oysters m^−2. For LRR, the HSI was generally able to predict live adult oyster densities that meet or exceed the target at HSI values ≥0.3. The HSI indicated that there remain large areas of suitable habitat for restoration in the GWR. This study provides a robust framework for HSI model development and validation, which can be refined and applied to other systems and previously developed HSIs to improve the efficacy of native oyster restoration.