The fishing method of trawling, or dragging, has long been shown to be harmful to a plethora of sea life inhabiting the world?s oceans and inland waterways. Fishing nets scour the seabed, disturbing everything in their path, while usually in search of only one type of bottom-dwelling species. Impacts to the seafloor include a removal of topographic features, disturbance of the upper sediment layers, including deep furrows, as well as physical and chemical changes to sediment morphology. While biological organisms and communities can potentially recover from this destruction, archaeological data cannot. Fishermen have been raising important artifacts in their nets for over a century. These finds have helped archaeologists locate significant sites, but they also have the adverse effect of irreparably damaging these sites. This thesis explores the impacts of bottom trawling on underwater cultural heritage. The methods and gear used by trawlers and their documented effects upon the sea floor are identified. Examples of the types of damage shipwreck sites receive after being impacted by trawling are presented. Instances where fishermen have raised prehistoric artifacts from inundated land sites are also introduced. The fishing and archaeological communities must cooperate to limit further damage to underwater cultural heritage around the globe.

The purpose of this study was to develop a methodology to analyse ROV video and scanning-sonar data to document the abundance and distribution of epi-benthic megafauna on the continental slope off Vancouver Island and to quantify the impact of trawling on these megafaunal assemblages. Impacts of bottom trawling on deep-sea ecosystems vary depending on habitat types and species present. Environmental factors such as depth, dissolved oxygen concentration, substratum type, and bottom roughness also affect the diversity and composition of benthic communities. We studied two transects (30km and 12km long) on the upper continental slope off Vancouver Island, BC, Canada, that included areas of seafloor with visible trawl marks. Our study area was also located in an oxygen minimum zone with very low bottom water dissolved oxygen concentrations in its core (600m-1000m). The main target for bottom trawling fisheries in this area is the longspine thornyhead (Sebastolobus altivelis). Field data were collected using the ROV ROPOS equipped with a 3CCD video camera and high-resolution scanning sonar. Megafaunal composition/abundance and bottom characteristic information were extracted from video imagery and assembled using a custom-designed MS Access database. The same database was used to compile information on trawl-door marks detected in recorded sonar imagery. The sonar surveyed a 50m radius around the submersible during transects, providing a broader view of evidence of trawling in the area than video. This thesis reports on relationships between environmental variables and faunal abundance, diversity and species distribution. Following the video and sonar analysis, diversity patterns and general species distribution for both transects were determined. Relationships of community structure to depth and trawling intensity were investigated using the hierarchical clusters technique to identify similarities in the megafauna assemblages between stations . Finally, spatial structures in the megafaunal community and their associated environmental variables were examined using the Principal Coordinates Neighbour Matrices (PCNM) and redundancy analysis tests. Differences in total abundance, species composition and distribution, and species diversity were detected between the high and low trawling intensity areas. One of the main highlights of our results was the dominance of ophiuroids and holothurians along most of the transect, except for the highly trawled area. Spatial structures were identified in the megafaunal community, showing a strong influence of bottom trawling intensity and, to a lesser extent, depth. Nearby water column measurements of dissolved oxygen concentrations suggest that depth might be associated with dissolved oxygen levels, but in situ oxygen data were not available during the ROV surveys. A deeper understanding of in situ oxygen levels would help clarify the role of this factor in shaping megafauna assemblages and its interaction with trawling. / Graduate

Graduation date: 2006 / Structure-forming invertebrates belong to a polyphyletic group of primarily sessile and sedentary megafauna that can significantly enhance the complexity of physical habitats. A number of these organisms, including cold-water corals and sponges, are known to be slow growing and vulnerable to physical disturbance. In addition, as filter feeders, these invertebrates can indicate areas of consistently favorable conditions for feeding and growth. This study provides the first quantitative analysis of structure-forming invertebrate communities in many areas along the continental margin of Oregon and Washington. Geological surveys during 1992-95, using the occupied submersible, Delta, sampled an extensive area in this region, primarily on and around rock outcrops. The videos from these surveys were analyzed to inventory and catalog sessile structure-forming invertebrates and to document their associations with geological habitat types. Detailed data on geological substrate, invertebrate diversity, abundance, and density were compiled and analyzed. It was found that geological substrate and depth were reliable indicators of suitable habitat for most species included in the study. Gorgonian corals tended to concentrate in high densities in depths between 200-250m, at the southern edges of submerged rocky banks, and where hard rocky substrate was covered with a thick layer of sediment. Because of recent fishery regulation changes, this information can be used as baseline data for future studies on the effectiveness of closed areas on the recovery of structure-forming invertebrates from disturbance, particularly bottom trawling.

L'étude de la variabilité spatio-temporelle des flux sédimentaires sous l'influence des forçages naturels et des activités de chalutage a été entreprise à l'échelle du plateau continental du Golfe de Gascogne, en associant des données in situ et une modélisation numérique 3D déterministe. Deux campagnes en mer spécifiques ont été menées pour quantifier les impacts physiques induits par un chalut professionnel au niveau de la Grande-Vasière, en termes de remise en suspension (panache turbide), et de perturbation de la structure et de la nature du sédiment superficiel. Ces données ont permis d'estimer à 0.13 kg.m-2 le taux d'érosion moyen. Leur croisement avec des données d'effort de pêche a conduit à une cartographie mensuelle du flux d'érosion par chalutage. D'autre part, un modèle hydro-sédimentaire 3D réaliste a été mis en place et calibré à partir de mesures au point fixe. Une attention particulière a été accordée au paramétrage de l'érosion naturelle sous l'influence combinée des vagues et des courants. Une nouvelle formulation de la loi d'érosion adaptée aux mélanges de sable fin et de vase classiquement rencontrés sur les plateaux continentaux a été proposée et a permis d'optimiser significativement la réponse du modèle en termes de turbidité. Deux simulations de 5 ans ont été réalisées en incluant ou non l'influence du chalutage de fond, dans le but de quantifier et comparer les contributions relatives des forçages naturels et anthropique sur les flux verticaux (érosion) et horizontaux (transport solide) de sédiments. La variabilité temporelle des flux est décrite en une succession de régimes caractéristiques répondant à divers forçages (e.g. marée, vent, vagues, chalutage), et les flux résiduels saisonniers et annuels sont commentés : sans tenir compte des apports fluviaux, le flux de matériel vaseux a été estimé à 1.6 Mt/an sortant par le nord (au droit de La Pointe du Raz) et à 0.62 Mt/an vers le talus continental (au niveau de l'isobathe 180 m). / The spatio-temporal variability of sediment fluxes under the influence of natural forcings and trawling activities was assessed at the scale of the Bay of Biscay shelf, from in situ data and a 3D process-based numerical modelling. Two sea trials were carried out to quantify physical impacts induced by a professional trawling gear over an intensively trawled area of the shelf, the "Grande-Vasière", in terms of resuspension (turbid plume) and alteration of the surficial sediment nature and structure. These data enabled to estimate an average trawling-induced erosion rate of 0.13 kg.m-2. Their combination with fishing effort data led to monthly spatial distributions of trawling-induced erosion fluxes.Besides, a 3D realistic hydro-sedimentary model has been set up and calibrated from measurements acquired at a mooring station. The calibration task mainly consisted in assessing the natural erosion law setting under the influence of waves and currents. A new formulation of the erosion law has been proposed to describe the erosion of any mixture of mud and fine sand (sediment facies classically encountered on continental shelves) and led to a noteworthy improvement of the model response in terms of turbidity. Two 5-year simulations were performed accounting for natural forcings only or both natural and anthropogenic forcings in order to quantify and compare their respective contributions to sediment fluxes (vertical and horizontal sediment dynamics). The temporal variability of sediment fluxes is described in a succession of typical regimes occurring in response to various conditions of forcings (e.g. tide, wind, wave, trawling), and residual fluxes are assessed at seasonal and annual scales: without accounting for riverine sediment inputs, the mud flux is estimated to 1.6 Mt/yr outflowing northward (at the latitude of the Pointe du Raz) and to 0.62 Mt/yr toward the continental slope (through the 180 m isobath).