Thomas M. Soniat

Stable isotope sclerochronology of Pleistocene and Recent oyster shells (Crassostrea virginica)

Sclerochronology, the study of periodic increments in skeletal organisms, can decipher the life history and environmental records preserved in fossil shells. Two powerful tools used in sclerochronologic studies are oxygen and carbon isotope profiles. Although many studies have applied isotopic analyses to marine shells, few have examined isotope profiles from estuaries. One common inhabitant of estuaries is the crassostreine oyster, which preserves a sclerochronologic record on the ligamental area surface in the form of external convex and concave bands. This study examines delta 18 O and delta 13 C profiles across ligamental increments found in Crassostrea virginica shells from the recent of Terrebonne Bay, Mississippi Delta, and the Pleistocene of Chesapeake Bay, Virginia. Results show that Mississippi Delta oysters calcified their shells close to isotopic equilibrium with respect to their environment. Three skeletal breaks found in one oyster formed from cessation of shell growth when summer temperature exceeded 27 degrees C. delta 18 O profiles show that ligamental increments formed annually from varying growth rates through the year, with convex bands forming from rapid growth during spring and summer, and concave bands forming from slower growth during winter. Chesapeake oysters have well-defined ligamental increments with deep concave bands that formed from slow growth and cessation during winter. Skeletal growth breaks suggest that temperature decreased below 10 degrees C during winter. Estimated Pleistocene delta 18 O water values are similar to published recent delta 18 O water values measured at the mouth of the Chesapeake Bay. Two important factors affecting shell delta 13 C values are metabolic changes and occurrence of phytoplankton blooms. Ranges of shell delta 13 C values are similar to delta 13 C dic values measured from water samples, which indicate that oyster delta 13 C profiles are tracking changes in environmental DIC, such as changes related to phytoplankton blooms and benthic respiration. Summer delta 13 C maxima may be related to recovery from gametogenesis. These results demonstrate that stable isotope sclerochronology of oyster shells facilitates the interpretation of past estuarine environments and oyster life histories.

Field validation of a habitat suitability index model for the American oyster.

A habitat suitability index (HSI) model, developed for the American oyster,Crassostrea virginica, along the Gulf of Mexico, was field tested on 38 0.1-ha reef and nonreef sites in Galveston Bay, Texas. The HSI depends upon six (HSI1) or, optionally, eight (HSI2) variables. The six variables are percent of bottom covered with suitable cultch (V1), mean summer water salinity (V2), mean abundance of living oysters (V3) (a gregarious settling factor), historic mean water salinity (V4), frequency of killing floods (V5), and substrate firmness (V6). The optional variables are the abundance of the southern oyster drillThais haemostoma (V7), and the intensity of the oyster pathogenPerkinsus marinus (V8). The HSI values were lowest at high and low salinity sites and highest at intermediate-salinity sites. To validate the model, the hypothesis that the output of the HSI model was correlated with oyster density was therefore tested. A significant correlation was found between HSI1 and oyster density (Kendall Tau Beta correlation coefficient, τ=0.674, p<0.001, n=38); however, a statistical independence problem exists with the above test, that is, oyster density is both the independent standard for the test and a variable in the model. A regression model was constructed to test the relationship between log-transformed oyster density values (dependent variable) and the other variables of the model (independent variables). Most variation (r2=0.72, r=0.85) in the log-transformed density values were explained by a regression model that contained V2, V4, V5, V6, V7, and V8 as independent variables. The regression model was useful in constructing a modified HSI model (MHSI). A significant correlation (τ=0.674, p<0.05, n=10) was found between MHSI1 values and oyster densities from reefs closed to harvesting. The MHSI improves upon the original model by (i) simplifying the model structure, (ii) removing the requirement to measure V3, (iii) accounting better for the negative effects of high salinity, disease, and parasitism upon oysters, and (iv) eliminating the statistical independence problem by dropping V3 from the model. The MHSI should be tested against a new, independently-collected data set.

UNDERSTANDING THE SUCCESS AND FAILURE OF OYSTER POPULATIONS: CLIMATIC CYCLES AND PERKINSUS MARINUS

Abstract Perkinsus (= Dermocystidium) marinus is a major cause of mortality in eastern oysters, Crassostrea virginica. Because initiation of infection and progression of disease are favored by high temperature and high salinity, we hypothesized that climatic cycles influence cycles of disease. Analyses of a 10-y time series of disease prevalence and intensity, chlorophyll a, suspended sediments, water temperature and salinity from a Louisiana site, using a wavelet technique, show a teleconnection between the El Niño-Southern Oscillation (ENSO) and oyster disease in the northern Gulf of Mexico. Salinity increases precede increased disease prevalence by several months. The changes in salinity that trigger changes in disease prevalence and intensity are strongly driven by ENSO events. Interannual variation is important in the initiation and intensification of disease, and salinity is the primary driving factor. The patterns in the environmental and disease time series suggest that epizootics can be initiated within 6 mo of a La Niña event, which produces increased water temperature and salinity. This relationship suggests an approach for predicting epizootics of P. marinus from climate models, which in turn can inform the management of oyster populations.

Predicting the effects of proposed Mississippi River diversions on oyster habitat quality; application of an oyster habitat suitability index model

ABSTRACT In an attempt to decelerate the rate of coastal erosion and wetland loss, and protect human communities, the state of Louisiana developed its Comprehensive Master Plan for a Sustainable Coast. The master plan proposes a combination of restoration efforts including shoreline protection, marsh creation, sediment diversions, and ridge, barrier island, and hydrological restoration. Coastal restoration projects, particularly the large-scale diversions of fresh water from the Mississippi River, needed to supply sediment to an eroding coast potentially impact oyster populations and oyster habitat. An oyster habitat suitability index model is presented that evaluates the effects of a proposed sediment and freshwater diversion into Lower Breton Sound. Voluminous freshwater, needed to suspend and broadly distribute river sediment, will push optimal salinities for oysters seaward and beyond many of the existing reefs. Implementation and operation of the Lower Breton Sound diversion structure as proposed would render about 6,173 ha of hard bottom immediately east of the Mississippi River unsuitable for the sustained cultivation of oysters. If historical harvests are to be maintained in this region, a massive and unprecedented effort to relocate private leases and restore oyster bottoms would be required. Habitat suitability index model results indicate that the appropriate location for such efforts are to the east and north of the Mississippi River Gulf Outlet.

UNDERSTANDING THE SUCCESS AND FAILURE OF OYSTER POPULATIONS: PERIODICITIES OF PERKINSUS MARINUS, AND OYSTER RECRUITMENT, MORTALITY, AND SIZE

ABSTRACT Ten-year time series (1992 to 2002) of salinity, Dermo disease, and size-class structure and mortality measured for an eastern oyster (Crassostrea virginica) population at a reef in Bay Tambour, Terrebonne Parish, LA, were analyzed using wavelet techniques to determine dominant frequencies and correlations. Along the Gulf Coast of the United States, Dermo disease (caused by Perkinsus marinus) responds to the El Niño-Southern Oscillation (ENSO) climate signal through its response to salinity. During the La Niña portion of ENSO, decreased rainfall leads to an increase in salinity, which triggers a rise in Dermo disease prevalence and intensity, producing increased oyster mortality. Although disease responds to the 4-y periodicity of ENSO and salinity, the oyster population dynamics do not appear to be controlled by disease at this site. A significant 4-y coherency exists between recruitment and salinity, with recruitment being higher during periods of high salinity. Recruit numbers and submarket numbers also exhibit a strong 4-y periodicity. However, a relationship between the recruit time series and the subsequent change in market-size abundance did not exist. The complexity of postsettlement processes and the extended time over which these processes interact decrease the predictability of the recruit-to-market transition. Even the strong pulse of recruits associated with La Niña and its locally elevated salinities did not result in an exceptional abundance of market oysters. Understanding the environmental and biotic factors that favor the production of large oysters is critical because large oysters not only supply the fishery, but, upon their death, contribute the bulk of the shell required for reef sustainability.

Chemical and Physiological Measures on Oysters (Crassostrea virginica) from Oil-Exposed Sites in Louisiana

ABSTRACT Potential lethal and sublethal effects of oil from the Deepwater Horizon spill to oysters (Crassostrea virginica) in Louisiana east of the Mississippi River were examined along a biophysical gradient of oil pollution, salinity, and disease. Approximately 6 mo after the capping of the Deepwater Horizon wellhead, no polycyclic aromatic hydrocarbons were detected in oysters from oil-exposed sites. Variations in oyster condition and reproductive state, and infection with the oyster parasite Perkinsus marinus are consistent with natural differences along the salinity gradient and not with impacts of polycyclic aromatic hydrocarbon contamination.

Estimating Sustainable Harvests of Eastern Oysters, Crassostrea Virginica

ABSTRACT Sustainability of a fishery is traditionally and typically considered achieved if the exploited population does not decline in numbers or biomass over time as a result of fishing relative to biological reference point goals. Oysters, however, exhibit atypical population dynamics compared with many other commercial species. The population dynamics often display extreme natural interannual variation in numbers and biomass, and oysters create their own habitat—the reef itself. With the worldwide decline of oyster reef habitat and the oyster fisheries dependent thereon, the maintenance of shell has received renewed attention as essential to population sustainability. We apply a shell budget model to estimate the sustainable catch of oysters on public oyster grounds in Louisiana using no net shell loss as a sustainability reference point. Oyster density and size are obtained from an annual stock assessment. The model simulates oyster growth and mortality, and natural shell loss. Shell mass is increased when oysters die in place, and is diminished when oysters are removed by fishing. The shell budget model has practical applications, such as identifying areas for closure, determining total allowable catch, managing shell planting and reef restoration, and achieving product certification for sustainability. The determination of sustainable yield by shell budget modeling should be broadly applicable to the eastern oyster across its entire range.

A Robust, Spatially Explicit Model for Identifying Oyster Restoration Sites: Case Studies on the Atlantic and Gulf Coasts

ABSTRACT The eastern oyster (Crassostrea virginica) is a reef-forming organism commonly found in estuaries throughout the Atlantic and Gulf coasts of North America. Eastern oyster reefs provide several ecosystem services, including water filtration, habitat diversity, and storm surge protection, among others. Oyster abundance has declined precipitously during the past century along the Atlantic and Gulf coasts as a result of overfishing, disease and predation, and large-scale human-mediated events. Given the importance of oysters, both ecologically and economically, there have been significant efforts during the past 20 y to reestablish and/or restore oysters to historical levels. Successful reef restoration depends on choosing sites that optimize survival, which requires an understanding of the environmental factors that influence the life stage of an oyster. For most restoration projects, time and budget constraints prevent long-term field studies; therefore, modeling is often used to determine the best locations for restoration. In this study, we developed a spatially explicit, flexible, 4-parameter habitat suitability index model that can be used to determine locations suitable for restoration of eastern oyster reefs throughout the western Atlantic and Gulf coasts. The model captures the minimum environmental parameters required for successful restoration suitability and was applied in 2 studies: (1) Chesapeake Bay, a data rich environment, and (2) northern Gulf of Mexico (western Mississippi Sound), a data poor environment. It illustrates the implications of using data of varying quality when applying the model for identifying restoration potential. In both locations, the model was most sensitive to the presence of appropriate substrate, but not as sensitive to salinity values. This model provides a scientifically based support tool for natural resource managers and project planners, and local conditions may require further consideration.

Interactive effects of water temperature and salinity on growth and mortality of eastern oysters, Crassostrea virginica: A meta-analysis using 40 years of monitoring data

ABSTRACT Despite nearly a century of exploitation and scientific study, predicting growth and mortality rates of the eastern oyster (Crassostrea virginica) as a means to inform local harvest and management activities remains difficult. Ensuring that models reflect local population responses to varying salinity and temperature combinations requires locally appropriate models. Using long-term (1988 to 2015) monitoring data from Louisianas public oyster reefs, we develop regionally specific models of temperature- and salinity-driven mortality (sack oysters only) and growth for spat (<25 mm), seed (25–75 mm), and sack (>75 mm) oyster size classes. The results demonstrate that the optimal combination of temperature and salinity where Louisiana oysters experience reduced mortality and fast growth rates is skewed toward lower salinities and higher water temperatures than previous models have suggested. Outside of that optimal range, oysters are commonly exposed to combinations of temperature and salinity that are correlated with high mortality and reduced growth. How these combinations affect growth, and to a lesser degree mortality, appears to be size class dependent. Given current climate predictions for the region and ongoing large-scale restoration activities in coastal Louisiana, the growth and mortality models are a critical step toward ensuring sustainable oyster reefs for long-term harvest and continued delivery of the ecological services in a changing environment.

The Exposed Surface Area to Volume Ratio: Is Shell More Efficient than Limestone in Promoting Oyster Recruitment?

ABSTRACT Planting oyster cultch is a common management approach used to enhance recruitment. The two most popular cultch materials are shell and limestone. Both are sold by volume or weight; however, once deposited on oyster grounds, only a small portion of the total surface area of each particle is available for recruitment. Shell and limestone have different surface area to volume properties, and thus provide differential settlement opportunities. Exposed surface area to volume (expSA/V) ratios of oyster shell and limestone fragments were compared, as an indicator of their recruitment potential and cost-effectiveness for cultch planting. Samples were collected from the Primary Public Oyster Seed Grounds in Louisiana by vibracore, and from the Pass Christian TongingGrounds in Mississippi by dredge. Shell (including whole shell and fragments) and limestone particles greater than or equal to 8 mm by geometric shape were classified and their expSA/V was calculated. Mean expSA/V ratios of shell were approximately three to nine times higher than limestone. For limestone of similar particle size to provide an equivalent recruitment benefit for the same cost would require that the cost of purchase, transport, and planting be three to nine times lower than shell. Thus, shell is likely to be a more efficient material than limestone for recruitment enhancement. Nevertheless, the higher variability in expSA/V of shell and other factors such as the expected lifetime and the relative performance of small and large particles of materials should also be considered. Analysis of a Louisiana limestone plant and associated oyster cultch showed that the proportion of small and large limestone particles and the relative proportion of whole shells and fragments can greatly alter expSA/V. In this case, the a priori expectation that oyster shell would outperform limestone did not materialize because of the quantity of small limestone particles of favorable shapes in the deployed material. Even so, as yet unknown is the possible reduction in performance in situ of smaller particles that might occur if they increase the one-dimensionality of the plant.