Sandbanks and fisheries effects in relation to EU’s fishery and environmental policy

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Abstract

Objectives, effort, and deliverables
The primary project objectives and outcomes are organized within three overarching components, which have also formed the backbone of the work packages and associated tasks of the project.

Fishing footprint:
i) Development of a new methodology to estimate the exact spatial extent and gear footprint of Danish anchor seines.
ii) The development of a hierarchical method that combines different sources of fisheries monitoring data (VMS, AIS, and Black Box data) to produce high-precision fishing pressure maps (grid cell resolution of 100x100m) of Danish demersal fisheries.

Gear specific depletion rates:
i) Development of a new methodology that theoretically derives gear-specific depletion rates, based on gear design and component-specific penetration depths, for ten different commercial fishing gears.
ii) Quantification of the benthic impact (benthic fauna depletion) of the Danish Seine fishing gear based on in-project sea trials and experimental data.
iii) Quantification of the impact on the benthic fauna community from a standard and a modified sandeel otter trawl based on experimental data from sea trials previously conducted by DTU Aqua.
iv) Quantification of the impact on the benthic fauna community from a light-weight mussel dredge based on experimental data from sea trials previously conducted by DTU Aqua.

Fishery specific assessment of benthic impact:
i) Development of a new methodology to assess the amount of sediment mobilized by demersal fishing gears, based on the spatial distribution of both fishing activity and fine sediments, combined with gear-specific drag coefficients.
ii) Integration of project results and external science-based methods and results into a framework, tailored to assess the benthic impact from the different Danish fisheries with Mobile Bottom Contacting Gears (MBCGs) on sandbanks in the North Sea, and applied to the Danish North Sea sandbank fisheries for the period of 2018 – 2020.

Within these three overarching project components, a number of key deliverables have been produced – some in collaboration with other institutions and projects - during the project period:
- High resolution fishing pressure data and maps (100x100 meter grid cells) based on hierarchical integration of VMS, AIS, and BB data with logbook data
- Development of a predictive model of the geometry and dynamics of the Danish seine gear footprint.
- Development of a modelling tool to estimate the amount of sediment mobilization from fishing with different mobile bottom-contacting gears.
- Quantification of the depletion of benthic fauna associated with a gear passage of a Danish seine, a sandeel otter trawl (both conventional and modified), and a light mussel dredge.
- Integration of project deliverables (e.g., high resolution fishing pressure data, modelled data for natural physical disturbance, and gear specific depletion rates) into a gear-specific benthic Impact assessment of the Danish Fishery on North Sea sandbanks.

Key results and conclusions
From our application of ICES’ (International Council for the Exploration of the Sea) standard assessment framework with improved input data (high-resolution fishing pressure data in 100x100 meter grid cells, an experimentally determined Danish seine depletion rate of 0.0069, and theoretically derived, gear-specific depletion rates for the other sandbank gears) it can be concluded that the sandbank habitats in the Greater North Sea are generally subject to a low impact from the Danish demersal fisheries that use them. The Relative Benthic State (RBS; ranging from 0 (highly impacted) to 1(not impacted)) of the sandbank habitats, considering only the pressure from Danish fishing effort, has an overall average grid cell value > 0.9. This low impact is a result of a combination of factors:
i. Most gears deployed on sandbank habitats by the Danish fleet, such as Danish seines and Sandeel otter trawls, are relatively light gears and have a low associated depletion rate.
ii. The natural disturbance of sandbank habitats is generally high and consequently, the local benthic community is adapted to physical disturbances similar to the physical disturbance from demersal fishing.
iii. The total fishing intensity (total fishery footprint and associated Swept Area Ratio (SAR) values) is generally low within sandbank habitats compared to e.g., the intensity of bottom trawl fishing on softer sediments in Skagerrak and Kattegat.

Fishing impacts are standardized with the landing values of individual fisheries to enable comparisons between fishing gears. The brown shrimp fishery with beam trawls has a relatively higher impact (reduction in RBS value per DKK of the landings) than the sandeel fishery with otter trawls, and the plaice fishery with Danish seines. In addition, Danish fishing impact in general (across all fisheries) is lower in the North Sea than in the Skagerrak and Kattegat. The results presented here are very much in line with earlier findings, but in comparison to these other analyses we have (i) applied the methodology to fishing data at much higher spatial resolutions, (ii) used a novel, empirically derived depletion estimate for Danish seines, and (iii) focused specifically on sandbank habitats.
Whereas all the three steps above increase the estimation accuracy of sandbank impact and status assessment relating to fishing, they may cause false accuracy as well. Most other input data for the ICES RBS-assessment framework is not available at nearly the same high resolution as the fishing pressure data; more specifically, the North Sea wide estimates of local habitat type and benthic community composition, which serve as input in the determination of habitat sensitivity to fisheries, are based very much on extrapolations and have large uncertainties. This type of underwater data is very resource demanding to sample, but a better resolution in habitat maps is a prerequisite for fully utilizing the high-resolution fishing pressure data developed in this project and the methodological advances within benthic impact assessment in the wider scientific community.

Implementation and dissemination
With the above-described deliverables the key project objectives have been met, and all national stakeholders will profit from the project outputs, e.g., improved fishing pressure maps and impact assessments as a basis for future environmental and fisheries management and industry and NGO initiated conservation efforts.

Internationally, the outputs have been and will be disseminated mainly through the high number of scientific papers and manuscripts produced with contributions from this project (4 published scientific papers, 2 scientific manuscripts, 1 published scientific report). Furthermore, the project has involved leading European experts in several phases and deliverables of the project and the project coordinator has acted as co-chair of ICES-WGFBIT (ICES Working Group on Fisheries Benthic Impacts and Trade-offs) for the duration of the project, which has ensured knowledge transfer and synergy both ways.
Original languageEnglish
Place of PublicationKgs. Lyngby, Denmark
PublisherDTU Aqua
Number of pages49
ISBN (Electronic)978-87-7481-337-8
Publication statusPublished - 2022
SeriesDTU Aqua-rapport
Number409-2022
ISSN1395-8216

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