Quantifying the impact of bottom trawling on soft-bottom megafauna communities using video and scanning-sonar data on the continental slope off Vancouver Island, British Columbia
Date
2012-05-04
Authors
Gauthier, Maeva
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Abstract
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.
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Keywords
bottom trawling, fisheries, continental slope, British Columbia, oxygen, ROV, video, sonar, megafauna