Abstract
Ecosystem-based management strategies increasingly require assessments of bottom trawling impacts on benthic habitats at the detailed level of different gear types. Two types of bottom trawls for catching flatfish (tickler-chain SumWing and electrode-fitted PulseWing trawls) were compared by combining several observational and modelling techniques to assess the changes in trawl penetration and associated effects on the seabed texture and sediment sorting. Bathymetrical measurements using a multi-beam echo sounder (MBES) confirmed that the SumWing trawl tracks were consistently and uniformly deepened to 1.5 cm depth in contrast to 0.7 cm following PulseWing trawling. MBES backscatter strength analysis indicated that SumWing trawls (3.11 dB) also flattened seabed roughness significantly more than PulseWing trawls (2.37 dB). Sediment Profile Imagery (SPI) showed that SumWing trawls (mean, standard deviation) homogenised the sediment deeper (3.4 cm, 0.9 cm) and removed more of the oxidised layer than PulseWing trawls (1 cm, 0.8 cm). SPI imagery showed that the reduced PulseWing trawling impacts allowed a faster re-establishment of the oxidised layer and micro-topography in contrast to SumWing trawling. Particle size analysis suggested that SumWing trawls injected finer particles into the deeper sediment layers (~4 cm depth), while PulseWing trawling only caused coarsening of the top layers (winnowing effect). This is in agreement with numerical modelling, which predicted that SumWing trawls would penetrate deeper into the sediment than PulseWing trawls. The total penetration depth (mean, standard deviation) of the SumWing trawls (4.1 cm, 0.9 cm) and PulseWing trawls (1.8 cm, 0.8 cm) was estimated by measuring the depth of the disturbance layer and by modelling the erosion of the surficial sediments due to sediment mobilisation in the wake of the gear (SumWing = 0.7 cm; PulseWing trawl = 0.8 cm). Our study has shown that PulseWing trawls reduced most of the mechanical trawling impacts on the seabed compared to SumWing trawls for this substrate and area characteristics.
| Original language | English |
|---|---|
| Pages (from-to) | 312-329 |
| Number of pages | 18 |
| Journal | ICES Journal of Marine Science |
| Volume | 76 |
| Issue number | 1 |
| Early online date | 20 Sept 2018 |
| DOIs | |
| Publication status | Published - 1 Jan 2019 |
Bibliographical note
This study was part-funded by the EU FP 7 project BENTHIS (grant no. 312088). It does not necessarily reflect the views of the European Commission and does not anticipate the Commission’s future policy in this area. We are grateful for the logistic support of VLIZ, the fishermen of TX43 and TX29 and crew members of RV ISIS and RV Simon Stevin during the sea trials and NIOZ for the use of their box corer. ADR and LRT were partly supported by the project “Impact assessment pulsvisserij”. We are indebted to the skippers and Eddy Buyvoets for drawing the net plans of the trawls. We thank John Aldridge for his insights in sediment transport in relation to natural dynamics; Bavo De Witte for conducting the particle size analysis; Daniel Benden for assisting SPI analyses; Miriam Levenson for English-language editing and Julie Bremner and Stefan Bolam for their critical review. We also wish to thank 3 anonymous reviewers for their constructive comments on earlier drafts of this manuscript.Keywords
- beam trawl
- biogeochemistry
- habitat impacts
- particle size distribution
- penetration depth
- pulse trawl
- seafloor integrity
- sediment resuspension
- Seafloor integrity
- Pulse trawl
- Sediment resuspension
- Penetration depth
- Biogeochemistry
- Particle size distribution
- Habitat impacts
- Beam trawl