In lower Chesapeake Bay, the spawning season of the bay anchovy Anchoa mitchilli in 1988 was from early May to mid-September. Spawning was temporally synchronized and lasted for about 1.5 h each night. Spawning frequency per individual was every 4 d in early June and 1.3-1.9 d in other months. Batch fecundity was a linear function of fork length and body weight; regression slopes on 6 July and 4 August were significantly higher than those on 6 June and 31 August. Estimated mean total spawnings per female in 1988 was 54. Total egg production for a fish of average size was 45,110, which is equivalent to 346% of body biomass energy. Age determination based on lagenar otoliths showed that some fish spawned when as young as 2.5-3 months. Transport of the adult bay anchovy in darkness was studied in laboratory and field experiments. In a hydraulic flume, 99% of all fish were transported to the end of the flume in darkness at a current speed of 30 cm s&\sp{lcub}-1{rcub}&. In field experiments, fish marked with neutral red dye and released in a creek at flood tide were recaptured 5.1 km upstream 4 h after release at night, and were recaptured within 200 m of the release site 3 h after release in daylight. This nocturnal transport phenomenon may help in understanding behavior and distribution of pelagic estuarine fishes. The standardized CPUE data show long-term population fluctuations on the order of ten fold. The bay anchovy population also has extensive seasonal variations. A Fourier analysis removed the seasonal (short-term) variation from the long-term data series. An autoregressive analysis of the residual series indicated that it contained a significant first-order autoregressive process component (r&\sp2& = 0.26, P &\le& 0.0066), which was interpreted as a spawner-recruit relationship. Cross-correlation analysis indicated that bay anchovy population abundance was positively correlated with winter water temperature (r = 0.663, P &\le& 0.0001) and river flow (r = 0.376, P &\le& 0.027), but negatively correlated with the abundances of white perch (r = &-&0.437, P &\le& 0.011), and the squared function of residual wind speed (r = &-&0.377, P &\le& 0.026). A multiple regression model indicated that temperature, white perch abundance and wind made significant contributions (accounting for 78% of the variation) to the model.

Numbers of sandbar sharks, Carcharhinus plumbeus, in the Northwest Atlantic have experienced drastic declines since the early 1980's reaching their minima during the early 1990's. Catch rates in the early 1990's were a mere 25% of those during the 1980's. Such drastic reductions in other fish stocks have often caused compensatory responses, most notably the cod stocks in the Northwest Atlantic. Compensatory responses in depressed populations may include decreased natural mortality, increased fecundity, or increased growth rates. Compensation for population fluctuations below carrying capacities have been recognized for many terrestrial and oceanic r-selected organisms, but few instances have been noted for K-selected species. Due to slow-growth and late maturity, compensatory responses in K-selected species such as the sandbar shark probably require generation-scale time periods to become evident. A previous age and growth study discovered slight increases in juvenile sandbar shark growth rates when vertebral centra samples obtained in 1980-81 and 1990-1992 were compared. The Virginia Institute of Marine Science shark long-line survey reported the lowest abundance of sandbar sharks in 1992. Animals pupped during this time may display greater differences in growth rates due to drastically reduced population size. Samples obtained over the 2001-2004 time period were compared to the aforementioned time periods to investigate potential compensatory responses in the sandbar shark population in the Northwest Atlantic. Growth estimates for the sandbar shark, Carcharhinus plumbeus, in the Northwestern Atlantic were estimated using a reparameterized von Bertalanffy growth model. Sharks were tagged in Virginia waters with roto-tags and double return nylon dart tags from 1992 to 2006 by the shark longline survey of the Virginia Institute of Marine Science. Captured sharks were measured, tagged, and released by VIMS scientists. Age and growth estimates were determined for the sandbar shark, Carcharhinus plumbeus, from Oahu, Hawaii in the central Pacific Ocean. Age estimates were obtained through vertebral centra analysis of 187 sharks. We verified our age estimates through marginal increment analysis of centra and oxytetracycline marking methods of at-liberty sandbar sharks. Sizes of sampled sharks ranged from 46 cm to 147 cm pre-caudal length. Four growth models were fitted to length-at-age data; two forms of the von Bertalanffy growth model, the Gompertz growth model, and a logistic growth model. Males and females exhibited statistically significant differences in growth, indicating that females grow slower and attain larger sizes than males. Growth parameter estimates revealed slower growth rates than previously estimated (based on captive specimens) for Hawaiian sandbar sharks. The von Bertalanffy growth model using empirical length-at-birth provided the best biological and statistical fit to the data. This model gave parameter estimates of L infinity .=138.5 cm PCL and k=0.12 year-1 for males and Linfinity=152.8 cm PCL, k=0.10 year-1 for females. Male and female sandbar sharks mature at approximately 8 and 10 years of age respectively. The population of sandbar sharks in the Hawaiian Islands is an unfished population. The presents a unique opportunity to conduct demographic analyses on a virgin population of sandbar sharks. Most populations of sandbar sharks have been heavily exploited due to near coastal and estuarine habitat preferences and high demand for fins. Conversely the population of sandbar sharks in the Northwest Atlantic (NWA) has suffered severe declines since the early 1980's. Previous studies have suggested compensatory growth is occurring within this population, but the true effect at the population level has not been estimated. Life history parameters estimated for the Hawaii population, the NWA population in 1980-1981 and 2000-2004 time periods were used in stochastic age-based life tables and Leslie matrices to estimate demographic parameters. Yield recruit-1 relationships were estimated for the Hawaii population to determine optimal harvest strategies that would maintain a population at equilibrium. Population growth for the Hawaii population was estimated to be 1.014 year-1. Yield recruit-1 analyses suggested harvest of sharks 15 years of age and older would provide the greatest yield while not causing population decline. Population growth for sandbar sharks in the NWA was 1.009-1 year for the 1980-1981 time period and 1.030-1 year for the 2000-2004 time period. (Abstract shortened by UMI.).

Intraspecific genetic relationships within yellowfin tuna, Thunnus albacares, and three mackerels of the genus Scomber were studied by restriction fragment length polymorphism (RFLP) analysis of mitochondrial DNA (mtDNA). The comparison of these scombrids, with different larval distributions, adult distributions, and vagilities, served to investigate the hypothesis that population structure in marine fishes results from geographic and physical oceanographic processes that limit dispersal of early life history stages. Samples of 20 yellowfin tuna were examined from each of five Pacific locations and one Atlantic location. MtDNA analysis with 12 informative restriction endonucleases demonstrated considerable genetic variation, as evidenced by overall nucleon diversity of 0.84 and mean nucleotide sequence diversity of 0.31%. Estimates of within-sample variation were consistent across all six locations. Common genotypes occurred with similar frequencies in all samples, and with one exception, all genotypes that were represented by more than one individual occurred at more than one location. The null hypothesis that the sampled populations of yellowfin tuna share a common gene pool was not rejected. In contrast, analysis of species of Scomber revealed considerable intraspecific differentiation. A total of 15 samples averaging 19 individuals each of Scomber japonicus, S. australasicus, and S. scombrus were collected from geographically isolated populations throughout the ranges of each species. Genotypic diversities ranged from 0.28 to 0.95, and nucleotide sequence diversities from 0.13 to 0.76%. East and west Atlantic populations of S. scombrus exhibited significant differentiation, but no fixed restriction site differences. This species differed by 11.9% nucleotide sequence divergence from the other two species. In S. japonicus, fixed restriction site differences were revealed among Pacific samples, but not among Atlantic samples; although significant heterogeneity occurred within the Atlantic. In S. australasicus, North and South Pacific samples were highly differentiated. One of two divergent mtDNA matrilines observed in this species was restricted to southern samples. The study demonstrated that population structure is greater in the species of Scomber than in yellowfin tuna. It further revealed that adult dispersal, in addition to geographic and physical oceanographic processes that control dispersal of early life history stages, are of significant importance in shaping population structure in scombrids.

Coastal seagrass ecosystems are complex habitats that are increasingly influenced by human perturbations. Disturbances that affect the strength of bottom-up (i.e. resource availability) and top-down (i.e. consumer) controls may also influence biomass distribution between trophic levels, sediment biogeochemistry, and seagrass ecosystem metabolism. Here, I experimentally tested how top-down and bottom-up perturbations interact with community structure (diversity, food chain length of epibenthic consumers) to alter sediment biogeochemistry and ecosystem metabolism in an experimental eelgrass (Zostera marina ) system. My data indicated that resource availability influenced SOM composition and ecosystem metabolism. Light availability tended to be a stronger determinant of SOM composition while nutrient enrichment affected secondary production of invertebrate grazers more strongly than primary producers or SOM. Top-down predator effects on SOM composition and ecosystem flux rates tended to be weak. However, the strength of the trophic cascade may partly be a function of grazer community composition and grazer susceptibility to predation. Finally, my results indicated that grazer species identity and community composition strongly influenced SOM composition. In addition to the main effects of light, nutrients, predators, and grazers there were a variety of interactive effects between resources and food web composition. Consequently, the effects of resource availability and food web composition on seagrass ecosystem functioning should not be considered in isolation.

The northern diamondback terrapin (Malaclemys terrapin ) is the only estuarine turtle residing along western Atlantic and Gulf of Mexico coasts of North America. Through predation, terrapins may be a key species in coastal marine habitats by exerting top-down control on marsh invertebrates, and thereby help to maintain healthy marshes. Despite the terrapin's ecological importance, there has not been a thorough study of terrapin foraging ecology and movements in lower Chesapeake Bay. In this dissertation, the research focused on the foraging habits of diamondback terrapins and their effects within salt marsh and seagrass habitats of lower Chesapeake Bay. I provided detailed analysis of the diet of diamondback terrapins captured from both salt marsh and seagrass habitats that showed preferred foraging in seagrass beds when present, as well as prey differentiation by terrapin size, i.e., small vs. large terrapins. Three general patterns in dietary overlap based on terrapin size were common for M. terrapin. By habitat, there was substantial overlap in prey choice, though further analyses determined selectivity for barnacles Balanus spp. by small terrapins in seagrass beds. Small terrapins from marsh-mudflat habitats consumed more periwinkles than large terrapins from marshes or all terrapins from marsh-SAV habitats. The size of ingested periwinkles was related to terrapin size with snails ingested by mature females presenting a bimodal distribution. Two mesocosm prey choice experiments showed that terrapins had a preference for juvenile blue crabs (Callinectes sapidus) over periwinkles and mottled dog whelks (Nassarius vibex). A third revealed terrapins were less successful in finding and consuming blue crabs with increasing percent cover of vegetation. An acoustic telemetry study confirmed that small terrapins, which included all males and juvenile females, had better-defined home ranges than adult females and stayed mostly in shallow near-shore water in seagrass beds of lower York River, whereas large females entered these areas on flood and high tides. A logistic regression model predicted that small terrapins were less likely to move out of an area than large females. Males made infrequent, long distances trips within the study area not associated with mating. Both genders and size classes also frequently moved between non-connected Zostera marina beds in the study area. This research provided the first field evidence that terrapins ingested viable eelgrass seeds incidental to consuming Z. marina epifauna. A second logistic regression model revealed that small terrapins were more likely to consume seeds than large terrapins. The diamondback terrapin is considered a "species of concern" in Virginia, but it has minimal protection with little enforcement. This dissertation provides empirical evidence supporting the diamondback terrapin's ecological importance within Chesapeake Bay that can be incorporated into conservation strategies to promote recovery and protection of the species within Virginia.

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