Periodic Reporting for period 1 - TrawledSeas (BOTTOM TRAWLING AS A DRIVER OF DEEP SEASCAPE TRANSFORMATION)
Reporting period: 2020-12-16 to 2022-12-15
Bottom trawling is one of the most widespread fishing practices in the world’s oceans. It involves towing of nets along the seafloor to harvest benthic and demersal living resources for human consumption or industrial uses. The dragging of trawling gears along the seafloor results in scraping and ploughing of the seabed, which leads to the formation of turbid plumes of resuspended sediments and modifications of their fluxes and budgets, impacting on the seabed integrity and submarine geomorphology and affecting the ecosystem functioning. The widespread and intensive use of this fishing practice on the continental margin has raised concerns in the European Union about the sustainability of this practice. Traditionally, most of demersal fishing activity has been undertaken on the continental shelf. Over the last decades, however, a general exhaustion of many traditional shallow stocks, combined with advances in fishing technologies, have pushed commercial fishing into deeper waters (>200 m). As fishing pressure on the deep sea intensifies, there is a growing need to quantify, monitor and mitigate the impacts of bottom trawling on the deep seafloor.
This project, entitled Bottom trawling as a driver of deep seascape transformation (TrawledSeas), aims to investigate deep sea regions that have been intensively trawled during decades. The main objective is to quantitatively characterise the contribution of bottom trawling on the morphology of different geological and climatic deep seafloor settings over a range of spatial scales, from fine (m–dam) to mesoscale (5–100 km). Particularly, TrawledSeas will address the following specific objectives:
- Develop a methodological approach to quantify the morphological signature of bottom trawling at fine and mesoscale by comparing the morphology of trawled and untrawled areas; and
- Characterise the differential impact of trawling on the seafloor, in terms of extent, rates and volume change in different geologic and climatic settings.
This project, entitled Bottom trawling as a driver of deep seascape transformation (TrawledSeas), aims to investigate deep sea regions that have been intensively trawled during decades. The main objective is to quantitatively characterise the contribution of bottom trawling on the morphology of different geological and climatic deep seafloor settings over a range of spatial scales, from fine (m–dam) to mesoscale (5–100 km). Particularly, TrawledSeas will address the following specific objectives:
- Develop a methodological approach to quantify the morphological signature of bottom trawling at fine and mesoscale by comparing the morphology of trawled and untrawled areas; and
- Characterise the differential impact of trawling on the seafloor, in terms of extent, rates and volume change in different geologic and climatic settings.
During the first year of TrawledSeas, the research focused primarily on the first objective of the project "the identification and quantification of the impact of bottom trawling on the seafloor morphology at different spatial scales". For this purpose, available data in eight study areas were collected. These include: The Catalan margin, the Malta Plateau and the NW Sicilian margin in the Western Mediterranean, the Norwegian margin in the Norwegian Sea, the Canterbury margin in the S Pacific, the Patagonian margin in the SW Atlantic, the Barkley Canyon in the NE Pacific and the Ireland margin in the NE Atlantic. Data were provided by the host, the secondment and external collaborators, or derived from European and global databases of open geospatial data. Data have been compiled into a geodatabase implemented in a Geographic Information System (GIS). The spatial geodatabase contains mostly high-resolution multibeam data, but also additional geophysical and field data, such as side scan sonographs, surficial sediment samples, sediment cores, seafloor imagery and vessel tracking data. The geodatabase was built using ArcGIS, a software commonly used in the geoscience community, and has been updated by incorporating new data acquired in different oceanographic cruises as well as the results obtained during the first year of the project.
Based on the analysis of bathymetric data in conjunction with acoustic backscatter and side scan sonar data, a new methodology for the quantitatively characterization of trawling impacts on the seafloor morphology at fine and mesoscale has been implemented. At fine scale, the methodological approach focuses on the automatic detection and spatial quantification of trawl-marks (trawl door scars) in backscatter and sidescan sonar images through image processing and analysis. Combining this information with multibeam bathymetric data, the volume of sediment remobilised during the passage of trawl doors was quantified. The application of this approach at large spatial scales has allowed to characterise quantitatively the large-scale morphologies generated by recurrent bottom trawling and assess the volume of sediment remobilised by these activities. The methodology has been developed using a free open source programming language (Python) and GIS software (ArcGIS and QGIS), both widely used and supported, thus improving the transferability of the geomorphometric methodology.
Based on the analysis of bathymetric data in conjunction with acoustic backscatter and side scan sonar data, a new methodology for the quantitatively characterization of trawling impacts on the seafloor morphology at fine and mesoscale has been implemented. At fine scale, the methodological approach focuses on the automatic detection and spatial quantification of trawl-marks (trawl door scars) in backscatter and sidescan sonar images through image processing and analysis. Combining this information with multibeam bathymetric data, the volume of sediment remobilised during the passage of trawl doors was quantified. The application of this approach at large spatial scales has allowed to characterise quantitatively the large-scale morphologies generated by recurrent bottom trawling and assess the volume of sediment remobilised by these activities. The methodology has been developed using a free open source programming language (Python) and GIS software (ArcGIS and QGIS), both widely used and supported, thus improving the transferability of the geomorphometric methodology.
One of the main findings of the first year of this project was the identification and the quantitatively characterization of the long-term impact of bottom trawling over large spatial scales. The impact of commercial bottom trawling on shallow areas has been extensively studied in the last decades; however, the long-term effect of these activities on the deep seafloor remained poorly known. While many research efforts have focused upon the direct biological effects, only a few studies have quantified the physical effects of bottom trawling; most of them focused on the physical alteration of the seabed following the passage of towed gears, rather than on the long-term impact of chronic bottom trawling over larger spatial scales. New high-resolution multlibeam bathymetric data acquired during this project, in conjunction with vessel tracking data, has provided new insights on the impact of recurrent bottom trawling on the seabed over large spatial scales. Results revealed that the repeated passage of the otter boards during successive fishing hauls of the trawling fleet that operated in the same fishing ground causes the scraping and ploughing of the seabed at fine scale but also erodes the seafloor generating large-scale morphologies with sufficient relief to be measured using different morphometric techniques.
The long-term impact of bottom trawling has important ecological and economic implications. One of the objectives of this project was to interact with fishermen's guilds and stakeholders of the fishing sector in an attempt to try to minimise the impact of bottom trawling on the seafloor. The cooperation during this year with both stakeholders and fishermen's guilds has been highly positive. Fishermen provided valuable information to the project on the different fishing methods and strategies used in commercial trawling. At the same time, the main results of TrawledSeas have been shared with both fishermen and local citizens through the production of posters and brochures, talks and the publicartion of the information on the fishermen’s guild website.
The long-term impact of bottom trawling has important ecological and economic implications. One of the objectives of this project was to interact with fishermen's guilds and stakeholders of the fishing sector in an attempt to try to minimise the impact of bottom trawling on the seafloor. The cooperation during this year with both stakeholders and fishermen's guilds has been highly positive. Fishermen provided valuable information to the project on the different fishing methods and strategies used in commercial trawling. At the same time, the main results of TrawledSeas have been shared with both fishermen and local citizens through the production of posters and brochures, talks and the publicartion of the information on the fishermen’s guild website.