No description available.

No description available.

Instantaneous total mortality rate Z can be partitioned into two components: fishing, F, and natural mortality, M. A number of data-poor methods have been developed to estimate Z, F, and M, and these methods tend to rely on fairly restrictive assumptions as well as on data types that are easy to obtain, e.g., length or other life history information. The overarching goal of this dissertation is to contribute to the advancement of methods for estimating these important and influential stock parameters. The relevant issues and the gaps in knowledge pertaining to these data-poor methods are outlined in the Introduction chapter (Chapter 1). The research papers presented in this dissertation fall into two main categories, namely, the evaluation of existing methods and the improvement of existing methods to estimate mortality rates. In Chapter 2, Monte Carlo simulation is utilized to compare the performance of two length-based methods developed by Beverton and Holt (1957) and Ehrhardt and Ault (1992), for estimating Z. I examine the impact of (1) variability in size at age and (2) the method of handling length truncation on the performance of the estimators. Results show that the Ehrhardt-Ault method exhibits complex patterns of bias and is not unequivocally better than the Beverton-Holt method. In Chapter 3, an existing non-equilibrium, mean length-based estimator of Z is modified to use additional information on fishing effort. The Z parameters are replaced with Z = qft+ M where q is the catchability coefficient and ft is the fishing effort in year t. Thus, only q, M, and the residual error need be estimated. This methodology appears promising for estimating F (= qf) and M, based on simulation studies. Furthermore, even if the estimates of F and M are imprecise and highly correlated, the resultant estimates of Z are year-specific and may be quite precise. The method may serve to bridge the gap between data-poor and data-rich methods to estimate Z. Chapter 4 addresses a long-standing gap in knowledge with respect to the ranking and predictive performance of existing empirical estimators of natural mortality of fish stocks. to address this question, a dataset of over 200 direct M estimates and corresponding life history parameters from unique fish species was compiled. Using this dataset, we were able to definitively quantify the predictive ability and update the equations of four widely used empirical estimators and their variants. Estimators based on maximum age perform substantially better than those based on growth parameters, either with or without consideration of water temperature. Results from this research will provide useful tools and guidelines for stock assessment scientists who need to estimate M and Z for both data-poor and data-rich stocks.

Incremental marks formed in the vertebral cartilage of most sharks, skates and rays are widely used as indicators of age in elasmobranch growth studies. Such information is essential for fisheries management, thus vertebral ageing has become an invaluable tool for investigating elasmobranch life history. Unfortunately, lack of information on the processes that regulate vertebral growth and mineralization limit efforts to correlate episodic stimuli with increment production. to address this research need, this dissertation investigated these processes through a detailed study on the vertebral cartilage of the clearnose skate, Raja eglanteria. Histologic observations indicated that changes in appositional cartilage growth are the catalyst for increment formation. Observations by scanning electron microscopy supported this conclusion by demonstrating that growth increments differ in the proportion of cells to mineralized matrix. In contrast, elemental analysis of vertebrae using energy-dispersive spectrophotometry demonstrated no change in mineral concentration between seasonal growth layers. This observation was advantageous, because uptake of the radiotracer &\sp{lcub}45{rcub}&Ca was ineffective in estimating the rates of vertebral calcification in captive R. eglanteria. Failure of this standard method appeared to reflect the free exchange of calcium between skeletal and serologic reservoirs. An in vitro method for measuring vertebral growth was developed using incorporation of &\sp{lcub}35{rcub}&S-sulfate as a marker for cartilage matrix synthesis. Certain conserved among elasmobranchs and higher vertebrates. The growth hormone-dependent serum factor insulin-like growth factor-I (IGF-I) increased vertebral matrix synthesis, suggesting an important role for this hormone in regulating elasmobranch skeletal growth. In contrast, corticosterone reduced &\sp{lcub}35{rcub}&S-sulfate uptake in vertebral cartilage, suggesting an inhibitory role for glucocorticosteroids in elasmobranch chondrogenesis. Calcitonin also inhibited vertebral matrix synthesis and, in vivo, may play some role in skeletal development or mineral homeostasis. Finally, nutritional status also appeared to influence vertebral growth in vitro, perhaps indirectly through effects on hormone production. In sum, this dissertation identified changes in cartilage growth as the impetus for vertebral growth zone production in R. eglanteria. Physiological mechanisms that likely regulate vertebral growth are described in this pioneer study on elasmobranch skeletal growth.

Atlantic bluefin tuna Thunnus thynnus (ABFT) support commercial and recreational fisheries throughout the North Atlantic Ocean. Due to heavy fishing pressure over the course of several decades, the eastern and western stocks of ABFT were overfished and the current biomass of the western stock is estimated to be approximately 19% of the biomass necessary for maximum sustainable yield. Despite a variety of management measures, including the implementation of minimum sizes and reductions of the total allowable catch (TAC) and country-specific quotas, little change was observed in the status of the western stock. The U.S. commercial and recreational ABFT fisheries are managed by the National Marine Fisheries Service (NMFS), which distributes the U.S. quota among domestic fisheries by gear type. The U.S. recreational fishery, which has historically targeted small or “school-size” (69-119cm) ABFT, is managed by open seasons, a minimum size, and bag limits (the number of ABFT allowed to be landed per vessel per day). Over the past 20 years, bag limits have been severely reduced due to decreased annual quotas, increasing the number of ABFT released each year, mostly within the school-size category. It is important, for the management of ABFT, to account for all sources of fishing mortality and the large number of releases in the recreational fishery each year could be a significant source of mortality. However, there is very little information available to assess post-release mortality of school-size ABFT in the U.S. recreational fishery. In this study, twenty pop-up satellite archival tags (PSATs) were deployed to estimate the post-release mortality of school-size ABFT captured under normal fishing conditions in the recreational fishery. PSATs recorded pressure (depth), temperature, and light data and were deployed on school-size ABFT caught using trolling methods. These tags were programmed to record data approximately every five minutes for a 31-day deployment. Nineteen tags (95%) reported to the satellites of the ARGOS system and approximately 85% (range: 34-100%) of all archived data were transmitted from each tag. Depth and temperature profiles were used to infer the survival of all 19 individuals whose tags reported (mortality=0% 95% CI=0%, 10%). Data from these tags were also used to investigate the short-term habitat utilization of school-size ABFT. During June to October, these fish spent the majority of their time in the upper 40m of the water column and at temperatures between 18 and 24oC.Individuals were more likely to make vertical excursions to depths exceeding 30m during the day than at night.

While the importance o f predation in controlling many natural bivalve populations is well established, it is often overlooked in the restoration strategies for depleted populations. Adult bay scallops {Argopecten irradians concentricus) along the U.S. mid-Atlantic coast spawn multiple times per year, typically once in the early summer and again in the early fall. Larvae generally settle on seagrass leaves to avoid benthic predators, but shift to the sediment surface around 20 mm in size when they become less vulnerable to predation. The objectives o f this study were to 1.) Determine proportional survival o f two distinct size classes o f A. irradians in different seasons related to the two naturally occurring cohorts found in this region, 2.) Determine the identity o f key predators o f A. irradians in the Virginia coastal bays. The goal is to incorporate this information into a restoration strategy for a Virginia seaside lagoon system where A. irradians have been absent since the disappearance o f eelgrass in the 1930s. Tethering experiments, conducted in re-established eelgrass during summer and fall o f 2013 and 2014, o f small (~10 mm SH) and large (~32 mm SH) juvenile A. irradians showed significant differences between the two years, likely due to the differences in the predator community. They also showed much higher predation rates in July than in August or October for both 2013 and 2014. Blue crabs are significant predators o f A. irradians and were present only in 2014, affecting survival, especially o f the large size class. Fish predators, such as pinfish, pigfish, striped burrfish, and sheepshead appear to have controlled the survival o f small juvenile A. irradians. Our results generally show greater survival in the fall, thus perhaps focusing on deploying small A. irradians in the fall would increase survival. However, the large differences in survival between 2013 and 2014 point to the importance o f employing an adaptive restoration approach which incorporates real-time abundances o f predators into restoration activities, allowing for the release o f A. irradians at smaller sizes dependent on the composition o f the predator assemblage.

No description available.

Located between upland and riverine systems, extensive tidal freshwater wetlands are influenced by a variety of recharging water sources and their respective nutrient contents. Conversely, tidal wetlands discharge interstitial waters and solutes to surface waters during periods of aerial exposure. Geohydrologic and model simulation methodology were utilized in order to aid in the understanding of wetland subsurface flow dynamics, its influence upon pore water nutrient chemistry, and its role in nutrient exchange with adjacent surface waters. Interstitial water nutrient chemistry was monitored along three transects extending from the uplands to the creekbank edge. Surface waters were also monitored throughout the 13 month study period. Measurements of soil dry bulk density, percent organic matter, fiber content, and horizontal hydraulic conductivity were conducted along a 118 meter transect from the creekbank edge to the high marsh/upland interface. Results indicate vertical and lateral heterogeneity of these physical and hydraulic soil properties within the upper one meter soil profile. Multivariate statistical techniques best described the transect as four separate soil types. General regions of soil types followed wetland elevational regions, these include: the creekbank, levee, low marsh flat, and high marsh regions. Fiber content was identified as the measured parameter which best explained variations in wetland soil permeability. Vertical and horizontal hydraulic head fluctuations were monitored utilizing piezometer/well arrays along the 118 meter transect. Direct measurement of interstitial water seepage flow from the subaquaeous portion of the creekbank to adjacent surface water was determined. Model simulation of subsurface hydrodynamics were made in order to provide water table fluctuations, estimates of horizontal seepage, and pore water budgets along the transect. Field measurements of marsh surface elevations and hydraulic soil properties were incorporated into the model to allow for comparison between simulated and observed results. Spatial variations in soil properties, and subsurface hydrodynamics indicate that an extensive tidal freshwater wetland cannot be considered as a homogeneous unit. It may be described more accurately as three distinct, yet interactive regions (creekbank, low marsh flat,and high marsh), with varying potentials for surface and interstitial water exchange. The creekbank, experiencing large water table oscillations and hydraulic gradients, was the most dynamic and tidally influenced region. These hydrodynamic characteristics resulted in substantial subsurface water transport and dilution of interstitial waters by recharging surface waters within the creekbank region. Due to extremely low hydraulic gradients and ponding of water, horizontal seepage was minimal within the low marsh flat. Moderate hydraulic gradients in conjunction with highly permeable soils were conducive for significant horizontal seepage within the high marsh. Hydrologic evidence indicates a potential for nutrient rich shallow groundwater recharge within the high marsh region. Sensitivity analysis within the creekbank region indicates that aquifer depth exhibits the largest influence on interstitial water discharge followed by soil permeability and specific yield properties of the aquifer respectively. Inverted results, as those found within the creekbank region, were obtained for the high marsh region. Interstitial water nitrogen and total phosphorus levels were variable and a function of depth, location, and time. However, several generalities and patterns appeared relatively consistant. Creekbank pore waters were relatively enriched with oxidized inorganic forms of nitrogen relative to low and high marsh regions. Creekbank ammonium, total nitrogen and phosphorus interstitial pools were intermediate, whereas, dissolved organic nitrogen levels was the lowest of the three regions sampled. The low marsh flat was inorganic nitrogen poor, and intermediate with respect to dissolved organic nitrogen, relative to creekbank and high marsh regions. Pore waters within the low marsh were significantly enriched with dissolved total phosphorus as compared to the creekbank and high marsh regions. High marsh interstitial waters displayed reduced levels of nitrate and nitrite, while levels of ammonium, dissolved organic and total nitrogen were elevated in relation to the creekbank and low marsh flat. Interstitial total phosphorus levels within the high marsh were significantly lower than the low marsh and approximately equal to the creekbank region. The role and influence of subsurface hydrodynamics upon pore water nutrient concentrations and spatial variations are discussed. Spatial and temporal potential patterns of nutrient exchange between surface water and pore waters of various wetland regions are identified. Dissolved oxidized inorganic forms of nitrogen were imported throughout the sampling period by the creekbank, low marsh flat, and high marsh regions. Ammonium flux, due to seepage , was predominantly from the wetland to surface waters; the high marsh exhibited a greatest potential for ammonium export. The high marsh was a source of dissolved organic nitrogen throughout the study, while the low marsh flat and creekbank regions may best be characterized as sources during winter, spring, and summer months, and potential sinks during the fall. The high marsh exhibited the potential to export dissolved total nitrogen throughout the year, whereas, the low marsh flat and creekbank exhibit export potential during spring and summer months. Patterns of total phosphorus exchange were from high marsh, and low marsh regions throughout the year, while exchange between creekbank and surface waters was minimal and temporally variable. Hydrodynamics within each wetland region must be considered in conjunction with pore water chemistry, in order to fully understand nutrient and solute transport potentials.

The blue crab fishery is the most valuable commercial fishery in Chesapeake Bay. The Chesapeake Bay stock recently experienced a period of overfishing, which has resulted in below average abundances, and the spawning stock has experienced an 84 % decline in biomass relative to levels in the late 1980s. The status of the stock is determined by comparing current estimates of mortality to biological reference points. Given the current focus on blue crab conservation, there is a need to obtain reliable, empirical estimates of survival to compare to the biological reference points. A tagging program was initiated on the terminally-molted, mature female component of the Bay-wide blue crab stock to estimate annual and semi-annual survival rates and to assess the effectiveness of the Virginia blue crab spawning sanctuary. Crabs were obtained from five fishery-independent research surveys throughout Chesapeake Bay and were measured, tagged, and released on-site. Tagging was conducted primarily during winter (late October to March) and summer (May to August) from November 2001 to March 2005. Recaptures of tagged crabs were reported by commercial and recreational fishers. Annual survival rates and tag recovery rates were estimated independently for the winter and summer tagging data using a Brownie model. The two independent estimates of annual survival based on winter tagging (0.08 ± 0.02 SE) and summer tagging (0.08 ± 0.02 SE) data were virtually identical and very low. The estimated tag recovery rate was 24 % based on the winter tagging data and 17 % based on the summer tagging data. The estimated monthly survival rate during winter, 0.87 ± 0.02 SE, was significantly higher than the monthly survival rate during summer, 0.74 ± 0.02 SE. The low estimates of annual survival are consistent with (i) historical estimates of the percentage of age 2+ females in the winter dredge fishery, and (ii) recent estimates of survival derived from estimates of exploitation rate obtained from the ratio of catch to pre-season abundance. To assess the effectiveness of the spawning sanctuary, mature females were tagged and released inside and outside the sanctuary in the summers of 2002, 2003 and 2004. A comparison of the probability of recapture for crabs tagged outside the sanctuary to crabs tagged inside the sanctuary using relative risk provided a means of assessing the sanctuary effectiveness quantitatively. Probability of recapture for crabs released outside of the sanctuary was 6.3, 5.2, and 2.8 times the probability of recapture for crabs tagged inside the sanctuary for 2002, 2003 and 2004, respectively. Consequently, a significant proportion of adult female blue crabs remained in the sanctuary to spawn and was not captured by the fishery. Hence, the blue crab spawning sanctuary in Chesapeake Bay is an effective means of protecting females migrating to or residing in the spawning grounds. These findings indicate that survival rates of mature female blue crabs in Chesapeake Bay have remained extremely low during a period of low abundance, which may be preventing stock recovery. Although the blue crab sanctuary is effective in protecting the females that have entered its borders, it only offers protection for 3.5 months of the year. A low annual survival rate suggests that very few adult females live long enough to spawn in more than one year. Current management must be altered for sustainable exploitation of the blue crab in Chesapeake Bay. This study represents one of the few to derive field estimates of semi-annual survival of an invertebrate species using Brownie models. This investigation also serves as one of the few empirical tests to date of the effectiveness of a marine reserve designed to protect spawning stock.

Aquaculture of the hard clam, Mercenaria mercenaria, has steadily increased in the United States over the past 25 years. In some areas, including along Virginia’s coast, this industry has grown to such a scale that aquaculture systems now exist as agroecosystems within coastal waters. Several features of these aquaculture systems, such as high clam densities and protective mesh netting, may modify the availability of habitat, food, and nutrients and may be expected to affect benthic and nektonic community structure. Few studies, however, have evaluated these effects. We conducted seasonal (May – September) sampling in a tidal tributary of Chesapeake Bay in three shallowwater habitat types – clam aquaculture, seagrass beds, and unvegetated bottom. Multiple trophic levels were sampled, including attached macroalgae, resident epibenthos, and mobile demersal and nektonic fishes and crustaceans. We compared biomass and abundance of trophic assemblages across the three habitat types. Trophic linkages were quantified through diet analyses of top predators. Results indicate that less-mobile communities showed more differentiation across the three site types. Assemblages supported by clam aquaculture and seagrass sites tend to be more similar to each other than they are to the assemblages at the unvegetated sites. Prey assemblages demonstrate a distinct shift, however, from May/July to September; during May/July the seagrass site supports the highest biomass and abundance of prey, but in September the clam aquaculture site supports the highest biomass and abundance of prey. This shift seems to correlate with the presence of structure (natural and man-made) within each site. Highly mobile top-predators show little preference among clam aquaculture, seagrass, and unvegetated sites. Diets of top predators do not exhibit a consistent pattern based on the site in which the individual was caught, but the diets of some species do correlate to the prey communities at a specific site. The added structure and organic matter deposition associated with clam aquaculture sites appear to create functional redundancy and temporal variability within Cherrystone Creek, potentially enhancing the stability of the creek system.