No description available.

Allee effects occur when fitness suffers at low population size or density. This can lead to reduced population growth rate and a critical density below which extinction occurs. Allee effects can occur in reproduction or survival. Critical densities are higher in reproductive Allee effects than in juvenile survival Allee effects, making reproductive Allee effects more severe (higher extinction probability). In terms of the critical Allee effect threshold, reproductive Allee effects are more severe in r-selected populations, while juvenile survival Allee effects are more severe in K-selected populations. For populations with negative density dependence, there is a maximum critical Allee threshold, above which a population of any density goes extinct. Critical thresholds are altered by changes in mortality, such that Allee effects can interact with exploitation to cause non-linear population collapse. Adding two Allee effects together has unpredictable consequences depending on the life history. Adding negative density dependence makes Allee effects more severe. I tested Allee effects in the field in the queen conch Strombus gigas, a marine gastropod. Mating and egg production are reduced in low-density areas. This could be due to low encounter rates, habitat quality or ontogenetic differences. to distinguish between these hypotheses, mature conch were translocated between high and low density sites. Conch were at equal high density in all enclosures, but high source-site density produced higher reproductive activity. Conch from high-density source sites also had thicker shell lips, indicating that they were older. These data reject the hypothesis of an Allee effect; possibly reproduction is delayed after morphological maturity. In model simulations, a reproductive Allee effect and delayed reproduction could both produce non-linear population collapse. I show that predators can generate an Allee effect in prey if they have a Type II functional response and the aggregative or numerical response is not Type III (or vice versa). A literature review reveals several unrecognised examples of predation-driven Allee effects from the literature. This mechanism for Allee effects does not arise from the specifics of prey life history, which makes it difficult to predict and prepare for.

No description available.

The Asian clam, Corbicula fluminea, is one of the most invasive bivalves in the world but there is limited research on its presence in tidal freshwater systems. Despite its introduction into Chesapeake Bay tributaries in the 1970s, the initial colonization and subsequent development of populations of C. fluminea in the Mattaponi and Pamunkey sub-tributaries of the York River, Virginia, is mostly undocumented. This study assessed the spatial distribution and population structure of C. fluminea in tidal freshwater sections of these rivers (~45km) with benthic surveys during summer 2011 – 2012. Benthic grabs (2.4L, 0.023m2) taken at 40 sites in each river were analyzed for clam abundance and size. In addition, relationships between abiotic factors and clam distribution within each river were evaluated using Akaike’s Information Criterion (AIC) to compare a set of generalized linear models. C. fluminea was present at the majority of sites in both rivers during both years, with mean densities (m-2) during 2011 and 2012 of 660 and 410 for Mattaponi River; 1,451 and 834 for Pamunkey River. Populations were dominated by > 90% juvenile clams (< 6mm shell length), which is common for C. fluminea populations during recruitment periods. Both rivers had lower abundance during 2012, suggesting that C. fluminea is actively reproducing but not necessarily accumulating in the system. High juvenile and adult mortality are characteristic of C. fluminea populations. Compared to other invaded systems, C. fluminea in Mattaponi and Pamunkey Rivers is a low-to-moderate level invasion based on clam density. Using AIC analysis, the bestsupported models included factors of distance upriver (km), % sand, depth (m), and year. Distance and % sand showed positive relationships with C. fluminea abundance and had significant parameter estimates in all models (" = 0.05). Spatial analysis in GIS showed 3 that C. fluminea was widely distributed throughout the rivers but achieved higher densities further upriver and in sandier habitats. Despite these trends, Corbicula densities were highly variable, highlighting eurytopic habitat preferences of this species that have led to its successful invasion of tidal freshwater habitats. Populations may also be controlled by the high degree of physical disturbance in tidal freshwater systems and predation by fish and waterfowl.

A panel of 48 single nucleotide polymorphism markers (SNPs) was developed for use in a population genetic analysis of the Eastern Oyster Crassostrea virginica sampled from the lower Chesapeake Bay. The SNPs were developed from published and unpublished sequencing data and developed to be used on a Fluidigm Biomark. A selection of 95 SNPs were chosen initially for development and the best 48 were selected for downstream applications. This project was a collaboration with the non-profit Chesapeake Bay Foundation (CBF) to examine their oyster reef restoration project in the Lafayette River, Virginia. The CBF wanted to test a hydrodynamic connectivity model designed to predict where oyster larvae produced in the Lafayette River would settle within the river. To test the model, oysters from Tangier Island, VA and the Haskin NEH hatchery strain were planted in the Lafayette River at locations corresponding to locations within the model with the expectation that the oysters would spawn the following summer. Baseline geographic oyster samples were taken from the nearby rivers; the Lafayette, Elizabeth and James Rivers before deployment of the planted test oysters. Newly recruited oyster spat were sampled from the Lafayette River in the summer following deployment of the planted test oysters. The baseline samples and spat were genotyped and compared to each other with the panel of 48 SNPs. Assignment tests were performed to identify the source population(s) for the spat. There was no population structure defined by FST values among oysters sampled from the lower Chesapeake Bay. The Haskin NEH oysters were genetically different from the other oysters in the study; however, the Tangier Island oysters were not different from the oysters in the lower Chesapeake Bay. The low FST values among the oysters from the lower Chesapeake Bay suggest that the connectivity of the reefs is high. The hydrodynamics of the region mix drifting larvae produced by oysters across the region as seen in the genotypic profile of the spat recovered in the Lafayette River. Heterozygote deficiencies suggestive of a Wahlund effect were observed; however, high rates of migration likely work to counterbalance stable population substructure. Alternatively, the heterozygote deficiencies could represent hidden variation not accessible by the methods used in this thesis. Some population structure exists with increasing geographic distance consistent with a pattern of isolation by distance among the populations sampled for this project. Assignment tests did not identify any spat as a product of the NEH oysters and assignment of spat to Tangier Island origin is inconclusive. The genetic data obtained were not able to provide unequivocal support for the predictions of oyster spat distributions by the connectivity model, although, the data do support the overall circulation patterns in the region predicted by the model.

No description available.

In 1999 a mass mortality of the American lobster (Homarus americanus) occurred in western Long Island Sound (WLIS). Although the etiology remains unknown, warm bottom water temperature, hypoxia, heavy metal poisoning, and pesticides have been suggested as casual factors. Subsequently, lobsters from WLIS have continued to display symptoms of morbidity that include lethargy and cloudy grey eyes, caused by idiopathic lesions. The effects of these lesions on lobster vision are unknown. We therefore used electoretinography (ERG) to document changes in visual function in lobsters obtained from WLIS, while simultaneously using histology to quantify the extent of damage. Of the lobsters collected from WLIS, seventy three percent showed damage to photoreceptors and optic nerve fibers including necrosis of the optic nerve, breakdown of the rhabdom, and hemocyte infiltration through the basement membrane into the ommatidia. Animals with more than 15% of photoreceptors exhibiting histological damage also exhibited markedly reduced responses to 10 ms flashes of a broad-spectrum white light. Specifically, the maximum voltage (Vmax) response was significantly lower and occurred at a lower light intensity as compared to responses from lobsters without idiopathic lesions. Lobsters from outside WLIS did not show such reduced changes to their vision. Lobsters from WLIS still appear to be subjected to an unknown stressor with an idiopathic etiology that is causing significant functional damage to their visual system.

Establishing the relationships between recruitment indices and estimates of adult abundance using fishery-independent data continues to remain one of the principal challenges faced by fisheries scientists due to the lack of concurrent monitoring programs designed to target different life stages of the same species. In Chesapeake Bay, however, multiple, fishery-independent surveys currently monitor the relative abundance of YOY and adult fishes. Using the available data from these surveys, the relationships between estimates of relative abundance for young-of-year and adults of striped bass (Morone saxatilis), weakfish (Cynoscion regalis), and Atlantic croaker (Micropogonias undulatus) were examined. Year-class strength was reflected in subsequent estimates of age-specific adult abundance; however, the strength of the relationships varied greatly with age. For all three species, the initial lack of significant correlations across all age classes indicated the need for improving the recruitment indices to more appropriately reflect YOY abundance. To ensure that the recruitment indices reflect patterns in abundance of YOY fishes, the following information was examined: assignment of the index period and strata and the distributional assumptions of the YOY catch data. For striped bass, a Bay-wide recruitment index appears to more accurately reflect year-class strength than the individual VA and MD recruitment indices. The recruitment indices for weakfish and Atlantic croaker improved when changes were made to the index period; however, further investigation is necessary to determine how depth influences the distribution and, ultimately, abundance of these two species. Identifying the distribution of the YOY catch data from the VIMS juvenile finfish surveys is also critical for obtaining unbiased recruitment indices. Here, the striped bass and weakfish catch data were gamma distributed; whereas, the Atlantic croaker catch data were lognormally distributed. The application of the delta-lognormal distribution did not improve the recruitment indices for any of the species in this study. An ageing study was conducted to determine if historically-defined length threshold values accurately distinguish YOY fish from older individuals in present day samples of striped bass, weakfish, and Atlantic croaker collected from the juvenile finfish surveys. The length threshold value for striped bass was determined to be approximately 30 mm too high. Although the current recruitment index for striped bass is not likely influenced by the small number of 1-year olds measured as YOY fish, reducing the length threshold value would ensure that only YOY fish are included in the calculation of the recruitment index. Further research is needed to determine if the length threshold values are appropriate for weakfish. For Atlantic croaker, length threshold values for the early portion of the index period (May, June) were appropriate; whereas, values used for the latter half of the index period (July, August) were too high, allowing for older individuals to be considered YOY based on their length. Consequently, the use of an earlier index period for Atlantic croaker would ensure that older fish are not being considered as YOY fish based upon their length.

Polar zooplankton and fish safeguard against the seasonality of food availability by using the summer months to build large reserves of lipids, which in turn are utilized to meet the metabolic demands of apex predators such as penguins, seals, and whales. A warming trend in the northern part of the western Antarctic Peninsula (WAP) has led to a decrease in perennial and summer sea ice, an increase in heat content over the shelf, and lower phytoplankton biomass, which could affect prey quality. We compared prey quality, including elemental (C, N) content and ratios, total, neutral, and polar lipid content, and energy densities, of known top-predator prey items (krill Euphaush1 superba, 17Jysanoessa macrura, and Euphausia crystallorophias; and fish Pleuragramma antarcticum, and Electrona antarctica) along the W AP latitudinal gradient in January of 2009-20 II as part of the Palmer Antarctica Long-Term Ecological Research study. E. antarctica had the highest prey quality in terms of total lipid content and energy density, followed by T. macrura and P. antarcticum, then E. c1ystallorophias and E. superba. For all species, variations in carbon and nitrogen content were best correlated with by the animals' neutral lipid content, in that animals with larger neutral lipid stores had significantly higher carbon and lower nitrogen content. Across all sexes and maturity stages, E. superba in the South had ca. 20% higher total lipid content than E. superba in the North. Total lipid content was also significantly higher in the South for E. crystallorophias, though this was largely due to the presence of larger individuals in the south combined with a significant positive relationship between length vs. weight-specific total lipid content for this species. For all prey species except T. macrura, there was a positive relationship between latitude or 0-120 m integrated Chi a vs. lipid content (neutral, polar, or total lipids), and a negative relationship between 0-120 m mean water temperature vs. lipid content. Trends opposite to those above found for T. macrura, suggest an optimal habitat for this species in the northern W AP which is characterized by warmer temperatures and lower Chi a. Patterns in Chi a were more important than upper water column temperature in explaining the observed latitudinal trends. If regional warming persists, the prey quality trends described for E. superba, combined with their regional abundance decline in the northern, coastal W AP could affect the ability of apex predators that rely on E. superba to meet their energetics demands.

An energetics model was constructed as an alternative method to length frequency analysis for the estimation of growth for juvenile silver perch, Bairdiella chrysoura. The model approach was adopted since estimation of juvenile estuarine fish growth by only collecting length or weight frequency data is difficult due to the large bodies of water and variety of microhabitats that must be regularly sampled over short time intervals. Field and laboratory data was included in analysis of model compartments for growth, metabolism, food intake and energy loss in waste products. Food habit studies indicated that silver perch is a planktivore that feeds on calanoid copepods during the day and mysids at night. Estimated energy utilization for silver perch at maintenance and maximum rations were similar to literature values for yellow perch, Perca flavescens, and brown trout, Salmo trutta. Model simulation of growth for silver perch in the York River, Virginia from July through October 1981, contained less variation in the size prediction than growth rates determined by length frequency analysis. The model did not contain periods of negative growth as did the field data. By the end of the season, model predictions of silver perch length matched the average length of silver perch captured in the lower York River. The construction of energy utilization models through complementary laboratory and field research has been demonstrated to be a viable method for estimating growth for juvenile fishes. Length frequency analysis is limited to only expressing growth over time. The advantages of an energetics model are that it also defines trophic and ecological interactions as well as environmental factors that impact growth.