This dissertation examines the shrimp industry in the Gulf of California from a political ecology perspective. The interaction between fishermen and their marine environment is explored, as well as the historical factors that led to vastly different types of fishermen in the communities of Guaymas and Empalme. Some have specialized in the harvesting of shrimp; others are diversified, multiple species fishermen. Some are highly industrialized offshore shrimpers; others are small-scale fishermen, more modest in their technology but more resilient when facing the current crisis in the shrimp industry. The underlaying causes of this crisis are explored by looking at state development policies, the assumptions behind fisheries management, and the configuration of markets. These have all emphasized specialization in the production of shrimp while ignoring the high interannual variability characteristic of shrimp populations. The end result: an overcapitalized, overexpanded industry and a possible overexploitation of shrimp stocks. Rather than addressing the root causes of the crisis, recent policies have instead transferred rights to the offshore fishery from cooperatives to private investors. At the same time there has been a concerted attack against small-scale producers. It is believed that by getting rid of this sector, catch per boat in the offshore sector will increase and overall "efficiency" will be improved. I compare industrialized trawlers and the small-scale sector and argue that the latter is currently producing high quality shrimp at lower monetary and ecological costs. But small-scale fishing is not equated with sustainability. Instead, differences among small-scale fishermen are analyzed. I contend that those who belong to traditional fishing families and have access to collective knowledge about the marine environment that has accumulated through generations, are better able to deal with a highly unpredictable environment and minimize risk. Those who do not have access to this knowledge have specialized in the harvesting of shrimp. I argue that a strategy of diversification is both more profitable in the short-term and sustainable in the long-run. Avoidance strategies among small-scale fishermen in response to externally imposed regulations are also examined. Fishermen are analyzed as individual profit maximizers and as community members who break the rules to serve collective interests. Just as individuals act collectively to deal with an unpredictable environment, they also act collectively to effectively challenge the institutions of rule-making.

Geophysical surveys in the Gulf of California provided the data to construct contour maps of bathymetry, free-air anomalies and total field magnetic anomalies for the area north of 27° N. Major faults such as the Ballenas- Salsipuedes, Tiburón, Guaymas, and the South Cerro Prieto are clearly observable on these maps. Spectral analysis, using 2-D Fast Fourier Transform methods, of the magnetic anomalies north of 29° N, allowed the identification of at least three distinct magnetic source horizons. The shallowest depth magnetic horizon, with an average depth to the top of 3.1 km below sea level, is interpreted as the top of the magnetic basement. The intermediate depth magnetic horizon, with an average depth to the top of 5.3 km below sea level, may represent either a lithological discontinuity in continental crust, or a transition zone characterized by the intrusion of igneous rocks, faulting, and fracturing associated with rifling processes. Some lineaments observed in the contour map on this horizon are oriented about 15° counterclockwise from the expected orientation of faults. Other lineaments are almost perpendicular to those faults. The deepest magnetic horizon is not apparent at all locations. Computed depths to the bottom of the magnetized crust average 11.5 km below sea level. The depth to the bottom of the magnetic crust is interpreted as the depth of the Curie-point isotherm. Assuming a Curie-point temperature of 580°C and a thermal conductivity of 2.2 W/m °C, the calculated heat flow averages 114 mW/m². Using a two-dimensional Maximum Entropy Method (2DMEM) to obtain the power spectrum of the magnetic anomalies increased the horizontal spatial resolution of the depth determinations by a factor of 4. This method when used to compute the depth to the top of the intermediate horizon, shows an improvement in the delineation of structures. However, the other magnetic horizons and the depth to the bottom of the magnetic crust were not clearly observable using this technique. / Graduation date: 1988

Natural vents in the Gulf of Mexico are actively expelling water and hydrocarbons. They are ubiquitous along continental margins, and I characterize a single vent in the Ursa Basin at leaseblocks MC852/853. Seismic data reveal that the vent is elevated ~75 meters above the seafloor and is roughly circular with a ~1.2 km diameter. A transparent zone centered underneath the vent extends to ~1500 meters below seafloor; this zone is commonly interpreted to record the presence of gas. There is a strong negative polarity seismic reflection that rises rapidly at the vent’s boundaries and is horizontal within a few meters of the seafloor beneath the vent edifice. I interpret that this reflection records a negative impedance contrast, marking the boundary between hydrate and water above and free gas and water below: it is the bottom-simulating reflector. Salinities beneath the vent increase from seawater concentrations to >4x seawater salinity one meter below seafloor. Temperature gradients within the vent are ~15x the background geothermal gradient. I model the coexistence of high salinity fluids, elevated temperature gradients, and an uplifted bottom-simulating reflector with two approaches. First, I assume that high salinity fluids are generated by dissolution of salt bodies at depth and that these hot, saline fluids are expelled vertically. Second, I model the solidification of gas hydrate during upward flow of gas and water. In this model, free gas combines with water to form hydrate: salt is excluded and heat is released, resulting in the generation of a warm, saline brine. The two models result in predictable differences of salinity and temperature. A better understanding of the hydrogeological processes at vent zones is important for quantifying the fluxes of heat and mass from submarine vents and is important for understanding the conditions under which deep-sea biological vent communities exist. / text

Uniaxial consolidation tests of resedimented mudrocks from the offshore Gulf of Mexico reveal compression and permeability behavior that is in many ways similar to those of intact core specimens and field measurements. Porosity (n) of the resedimented mudrock also falls between field porosity estimates obtained from sonic and bulk density well logs at comparable effective stresses. Laboratory-prepared mudrocks are used as testing analogs because accurate in-situ measurements and intact cores are difficult to obtain. However, few direct comparisons between laboratory-prepared mudrocks, field behavior, and intact core behavior have been made. In this thesis, I compare permeability and compressibility of laboratory-prepared specimens from Gulf of Mexico material to intact core and field analysis of this material. I resediment high plasticity silty claystone obtained from Plio-Pleistocene-aged mudrocks in the Eugene Island Block 330 oilfield, offshore Louisiana, and characterize its compression and permeability behavior through constant rate of strain consolidation tests. The resedimented mudrocks decrease in void ratio (e) from 1.4 (61% porosity) at 100 kPa of effective stress to 0.34 (26% porosity) at 20.4 MPa. I model the compression behavior using a power function between specific volume (v=1+e) and effective stress ([sigma]'v): v=1.85[sigma]'v-⁰̇¹⁰⁸. Vertical permeability (k) decreases from 2.5·10-¹⁶ m² to 4.5·10-²⁰ m² over this range, and I model the permeability as a log-linear function of porosity (n): log₁₀ k=10.83n - 23.21. Field porosity estimates are calculated from well logs using two approaches; an empirical correlation based on sonic velocities, and a calculation using the bulk density. Porosity of the resedimented mudrock falls above the sonic-derived porosity and below the density porosity at all effective stresses. Measurements on intact core specimens display similar compression and permeability behavior to the resedimented specimens. Similar compression behavior is also observed in Ursa Basin mudrocks. Based on these similarities, resedimented Gulf of Mexico mudrock is a reasonable analog for field behavior. / text

Seagrasses are submerged marine plants that provide essential ecosystem functions, but are declining in abundance worldwide. As angiosperms, seagrasses are capable of sexual reproduction, but also propagate asexually through clonal rhizome growth. Clonal growth was traditionally considered the primary means for seagrass propagation. Recent developments in genetic techniques and an increasing number of studies examining seagrass population genetics, however, indicate that sexual reproduction is important for bed establishment and maintenance. Few studies have investigated the reproductive biology and ecology of sub-tropical seagrass species, although this information is necessary for effective management and restoration. This work investigates the influence of pore-water nutrients on flowering, water flow on seed dispersal, consumption on seed survival, and describes the reproductive phenology in Texas for the two dominant seagrass species in the Gulf of Mexico: turtle grass (Thalassia testudinum) and shoal grass (Halodule wrightii). These species exhibit distinctive reproductive seasons that span summertime months, but reproductive output varies spatially and temporally. Results of an in situ nutrient enrichment experiment indicate that turtle grass produces fewer flowers (but more somatic tissue) when exposed to high pore-water ammonium than when exposed to low pore-water ammonium, suggesting that nutrient loading has the potential to reduce seagrass reproductive output. Seed consumption may also limit reproduction and recruitment in some areas, as laboratory feeding experiments show that several local crustaceans consume shoal grass and turtle grass seeds and seedlings, which do not survive consumption. Dispersal experiments indicate that seed movement along the substrate depends on local water flow conditions, is greater for turtle grass than shoal grass, and is related to seed morphology. Under normal water flow conditions in Texas, turtle grass secondary seedling dispersal is relatively minimal (< 2.1 m d⁻¹) compared to primary dispersal, which can be on the order of kilometers, and shoal grass secondary seed dispersal can be up to 1.1 m d⁻¹, but seeds are likely retained in the parent meadow. Results from this work can be used when developing seagrass management, conservation and restoration actions and provide necessary information concerning a life history stage whose importance was historically under-recognized. / text

The Gulf of Mexico (GOM) represents a challenge for exploration and production. Most of the sediments coming from North America has bypassed the shelf margin into Deep Water. In an Attempt to attack this challenge this thesis pretends to break the GOM's false bottom, mainly comprised by diverse salt structures and growth fault families. In this attempt, geological and geophysical data are integrated to find clues to potential hydrocarbons indicator (PHI) that could be of Reservoir Quality (RQ). 3D Pre stack depth migrated data comprised of Mississippi Canyon blocks, were interpreted: Top and base of salt, leading to the identification of a PHI represented by a consistent Amplitude Anomaly (AA) below and towards a salt structure. This AA may be of RQ and feasibility evaluation for further decisions may be taken. Following the structural sequences that Govern central GOM during Oligocene through out Miocene was important to support the results.

The aim of paleoenvironmental studies is to reconstruct characteristics of the past environment from fossil assemblages preserved in sedimentary strata. Thus, studies of modern surface assemblages, quantitatively correlated to the environmental parameters, are required before reliable interpretations can be made. For this dissertation, two different techniques were applied in two case studies: a reconstruction making use of a benthic foraminiferal transfer function from the intertidal marshes in the eastern Mississippi Sound, Alabama/Mississippi, and a qualitative reconstruction of ocean current activity on the Western Australian shelf. Modern salt-marsh foraminifera were collected from Grand Bay, Pascagoula, Fowl River, and Dauphin Island across several elevation transects and different salinity regimes. Cluster analysis yielded nine dead biofacies and five live assemblages from Open Estuarine to Upland Transition. Canonical correspondence analysis indicated a strong relationship between distributions of dead biofacies and elevation. Both dissolution of calcareous species in the organic marsh sediment and the long-term accumulative nature of the dead assemblage favored the use of non-estuarine dead assemblages. A Weighted Average-Partial Least Squared transfer function was applied to the surface data and yielded a Root Mean Squared Error of Prediction (RMSEP) of 0.14 m, which represents 33% Mean Range of Tide at Grand Bay and 39% at Dauphin Island. The transfer function was applied to two sedimentary cores from Grand Bay and two from Dauphin Island, which revealed disparate developments between the regions during the last 1,900 years. While both Dauphin Island cores indicated relative sea-level trends aligned with other Gulf of Mexico studies, Grand Bay was likely impacted by a river avulsion event disconnecting Grand Bay from fluvial sediment influx, and by the erosion of a protective headland, Grand Batture Island. Sediments spanning the last ~100 years contained increased abundances of low marsh foraminifera likely associated with coastline erosion, which was most prominently displayed by a lithology shift towards grey silt in the Dauphin Island cores. Surface carbonate sediments from Western Australia’s Northwestern Shelf and Carnarvon Ramp were collected from 127–264 m water depth. Foraminiferal assemblages changed between 127 m and 145–264 m due to rapidly decreasing water temperature in the thermocline, and loss of sufficient light for support of “larger” benthic foraminifera. Latitudinal differences were likely caused by three factors: (1) limited influence of the warm Leeuwin Current to support tropical taxa at the sampled depths, (2) reduced habitat diversity on the narrow Carnarvon Ramp compared to surrounding shelves, and (3) differing water-mass characteristics. The gathered information was used to interpret the assemblages from a Carnarvon Ramp core (total depth 300 m), providing insight into the activity of the warm, surficial Leeuwin Current for the last 3.54 My (Pliocene). Abundant infaunal taxa were inferred to indicate low oxygenation, increased supply of organic matter, and high sea-surface productivity during the absence of the Leeuwin Current above the coring site. Dominance of epifaunal species signified higher oxygenation at the sediment-water interface when upwelling of nutrient-rich waters was effectively suppressed by the Leeuwin Current. Around 1.14 Ma, waning of hypoxic conditions was initiated until a more substantial change was marked at 0.91 Ma. Suspension-feeding sponges became common sediment constituents during a Leeuwin Current flow optimum at ~0.6 Ma. The epifaunal taxa dominance persisted on the modern shelf, yet short episodes of infaunal peaks were likely caused by lateral shifts and fluctuating influence of the Leeuwin Current during more intense glacial cycles.

Density, species diversity, species richness, and evenness of bivalve mollusks were measured in the deep (0.2km to 3.7km) northern Gulf of Mexico to describe the community structure of benthic bivalve mollusks. Density decreased gradually from shallow continental slope depths, with remarkably high values in the Mississippi canyon, to the deepest sites. Diversity of bivalve mollusks increased from shallow continental slope depths, with low values in the Mississippi canyon, to a maximum at intermediate depths (1-2km), followed by a decrease down to the deepest locations (3.7km). Nine distinct groups were formed on the basis of the similarity in species composition. The pattern varied more abruptly on the slope compared to the deeper depths, possibly due to steeper gradients in physical variables. ANOVA indicated that the density of bivalve mollusks was not significantly different at different depths, was not significantly different on different transects, was not significantly different between basin and non-basin, but was significantly different in canyon and non-canyon locations. Similar distinctions were observed in diversity, except that basins were lower than non-basins. The patterns observed reflect the intense elevated input of terrigenous sediments accompanied by high surface-water plankton production from the Mississippi River to the north central gulf.

The hypoxic area in the Gulf of Mexico is the second largest in the world, which has received extensive scientific study and management interest. Previous modeling studies have concluded that the increased hypoxic area in the Gulf of Mexico was caused by the increased anthropogenic nitrogen loading of the Mississippi River; however, the nitrogen-area relationship is complicated by many other factors, such as wind, river discharge, and the ratio of Mississippi to Atchafalaya River flow. These factors are related to large-scale climate variability, and thus will not be affected by regional nitrogen reduction efforts. In the research presented here, both statistical (regression) and numerical models are used to study the influence of anthropogenic and climate factors on the hypoxic area variability in the Gulf of Mexico. The numerical model is a three-dimensional, coupled hydrological-biogeochemical model (ROMS-Fennel). Results include: (1) the west wind duration during the summer explain 55% of the hypoxic area variability since 1993. Combined wind duration and nitrogen loading explain over 70% of the variability, and combined wind duration and river discharge explain over 85% of the variability. (2) The numerical model captures the temporal variability, but overestimates the bottom oxygen concentrations. The model shows that the simulated hypoxic area is in agreement with the observations from the year 1991, as long as hypoxia is defined as oxygen concentrations below 3 mg/L rather than below 2 mg/L. (3) The first three modes from an Empirical Orthogonal Function (EOF) analysis of the numerical model output results explain 62%, 8.1% and 4.9% of the variability of the hypoxic area. The Principle Component time series is cross-correlated with wind, dissolved inorganic nitrogen concentration and river discharge. (4) Scenario experiments with the same nitrogen loading, but different duration of upwelling favorable wind, indicate that the upwelling favorable wind is important for hypoxic area development. However, a long duration of upwelling wind decreases the area. (5) Scenario experiments with the same nitrogen loading, but different discharges, indicate that increasing river discharge by 50% increases the area by 42%. Additionally, scenario experiments with the same river discharge, but different nitrogen concentrations, indicate that reducing the nitrogen concentration by 50% decreases the area by 75%. (6) Scenario experiments with the same nitrogen loading, but different flow diver- sions, indicate that if the Atchafalaya River discharges increased to 66.7%, the total hypoxic area increases the hypoxic area by 30%, and most of the hypoxic area moved from east to west Louisiana shelf. Additionally, if the Atchafalaya River discharge decreased to zero, the total hypoxic area increases by 13%. (7) Scenario experiments with the same nitrogen loading, but different nitrogen forms, indicate that if all the nitrogen was in the inorganic forms, the hypoxic area increases by 15%. These results have multiple implications for understanding the mechanisms that control the oxygen dynamics, reevaluating management strategies, and improving the observational methods.

The Gulf of Mexico is home to two of the world?s ten busiest ports by cargo volume, the Port of New Orleans and the Port of Houston; and in 2008, these ports hosted a combined 14,000 ships, a number which is likely only to increase. Past research shows that this increase in shipping worldwide has historically lead to an increase in ambient noise level of 3-5dB per decade. Sperm whales in the Gulf of Mexico are considered a genetically distinct, resident population. They have a preference for the Louisiana-Mississippi Shelf region which directly overlaps with the entrance to the Mississippi and the Port of New Orleans. Disruptions from vessel noise could influence feeding and breeding patterns essential to the health of the stock. Data used in this analysis were collected continuously over 36 days in the summer of 2001 from bottom moored Navy Environmental Acoustic Recording System (EARS) buoys. Results showed a significant difference (P<0.05) in noise level between hours with ships passing and hours without. Metrics for 56 ship passages were analyzed to compare duration of ship passage with duration of maximum received level (MRL) during ship passage. Results of that analysis showed an average ship passage of 29 minutes with average MRL lasting 23% of the ship passage and an average increase of 40dB. Lastly, click counts were made with the Pamguard. Click counts for ship passages were completed for 35 min and 17.5 min before and after the estimated closest point of approach (CPA) for each ship. Results showed a 36% decrease in the number of detectable clicks as a ship approaches when comparing clicks detected at intervals of both 35 minutes before and 17 minutes before the CPA; additionally, 22% fewer clicks were counted 30 min after the ship than 30 min before (results significant at the P=0.01 level). These results indicate a potential change in sperm whale behavior when exposed to large class size vessel traffic (e.g. tankers and container ships) from major shipping lanes. Recommendations for addressing this issue are discussed.