A drag coefficient of 0. For the other part and the tension relief cords the drag coefficient of 1. To estimate the total hydrodynamic drag of a gear, we sum the contributions from each of the individual gear components. Thus, we implicitly assume that there is no hydrodynamic interaction between components and that their contribution is additive. To assess the validity of this approach, we compare our hydrodynamic drag estimates with in situ measurements of beam trawl drag.
Blom , studied the drag of a tickler chain beam trawl and estimated the separate contribution of the shoe tickler chains and net, whereas Fonteyne and co-workers in Paschen et al. For the conventional beam trawls TBT, TBC , data were from the years to before the transition to pulse trawling.
For the pulse beam trawls data were from to after the transition Poos et al. Silt fractions were taken from Wilson et al. Annual swept area ratio SAR of the conventional tickler chain beam trawl TBT, top left and chain mat beam trawl TBC, top right in the period — and pulse trawl PUL, bottom left in the period — and the mud percentage bottom right according Wilson et al.
The components were grouped into four major categories: i ground gear; ii bottom net panels; iii shoes of the beam trawl or nose of the Sumwing; iv tickler chains, chain mat or electrodes, and tension relief cords. The hydrodynamic drag of the individual gear components was estimated by applying the equations described above to the dimensions of the bottom gear components summarized in the Supplementary material SM1.
To obtain an estimate of the variability from the gear dimensions, the hydrodynamic drag of each component was bootstrapped times from a normal distribution based on the mean and standard deviation of the dimensions of the sampled gear components. For each D c , the mean m c was estimated by randomly drawing silt fractions from the observed frequency distribution of silt fraction by gear type.
The sediment mobilization of the different beam trawl types were combined to estimate the sediment mobilization of the beam trawl fleet of the Netherlands when using either the conventional beam trawl or innovative pulse beam trawl by taking account of the proportion of each gear type in the fleet Table 2.
Gear types are depicted in Figure 1. Large vessels tow their TBT at an average speed of 3. The difference in towing speed between gear types is less in small vessels: 2. The mean towing speed of PUL trawlers does not differ between rigging types.
The estimated hydrodynamic drag of the different gear components that are in close contact with the seafloor, and therefore relevant when assessing the impact on the mobilization of sediment, is presented in Table 3. For small vessels, net panels have the largest contribution for all gear types 1.
For large vessels, the net panels have the largest contribution for the pulse trawl types 1. The largest difference in hydrodynamic drag between gear types was observed for the gear component that plays a role in the stimulation of flatfish. Significant differences were also observed for the ground gear drag. For small vessels, the estimated hydrodynamic drag of 4. The fleet estimates take account of the number of vessels using a particular gear type Table 2 and the silt content where the gear types operate.
The comparison of modelled hydrodynamic drag with experimental measurements in the literature is presented in detail in Supplementary material SM2. Although the experimental values are of combined hydrodynamic and geotechnical drag, we isolate measurements for tickler chains, chain mats and the gear netting at higher speeds where we assume the contribution of geotechnical drag is minimized. The correspondence for tickler chains and chain mats is reasonably good.
The ratio between the modelled and measured drag of Blom , tickler chains is 1. The discrepancy is larger for the netting panels, and the modelled drag of Blom net is 1. The silt fraction of the fishing grounds of PUL is intermediate. Small vessels, which mainly fish within 12 nautical miles from the coast, trawl sediments with a lower silt fraction when compared to large vessels using the same gear type, except for small TBC trawlers that fish in areas with a slightly higher silt content.
Figure 4 shows the estimated sediment mobilization and the contribution of the main gear components for the gear types taking account of the modelled hydrodynamic drag and the silt fraction distribution of their fishing grounds. Small vessels mobilize between 4.
Sediment mobilization by large vessels, estimated between 4. The relative contribution of gear components to the sediment mobilization reflects the differences in hydrodynamic drag of gear components by gear type. The bars show the standard deviation of the sediment mobilization of the whole gear.
Taking account of the proportion of vessels deploying a certain beam trawl type Table 2 , the sediment mobilization of an average vessel of the Dutch beam trawl fleet is estimated at 9. For small vessels, sediment mobilization of a conventional beam trawler 4. To contextualize these values, if we assume the sediment has relative density of 2.
The application of our methodology to the Dutch beam trawl fishery in the North Sea showed that the innovative pulse trawl that replaced mechanical stimulation of sole by tickler chains or chain mats with electrical stimulation reduced the hydrodynamic drag of the gear and the amount of sediments mobilized in the wake of the trawl of large trawlers but not of small trawlers. Among the conventional beam trawlers, TBC mobilizes less sediments, despite the higher hydrodynamic drag, than TBT because this gear type is used on fishing grounds with low silt content.
Pulse trawls are more efficient to catch the target species sole Poos et al. The lower hydrodynamic drag of the pulse trawl is due to the combined effect of the replacement of tickler chains running perpendicular to the towing direction with longitudinal electrodes and the reduction in towing speed. The reduction in hydrodynamic drag is to some extent counteracted by the larger twine surface area of the pulse trawl nets and the larger surface area of the ground rope in pulse trawls.
The use of pulse gear requires a rectangular matrix of electrode arrays to generate a stable electric field, constraining the type of ground rope to be used. Three types of ground ropes evolved. The reduction in catch efficiency, however, may be compensated by the smaller disk diameter of the additional sole-rope, which is expected to better follow the bottom profile and reduce the possibility of the fish escape underneath the sole rope. The different ground rope rigging types used are related to the fishing grounds.
Vessels from the northern harbours predominantly use the U-shaped ground rope and sole-rope. In the mid-south and mid-north vessels use a rectangular or U-shaped ground rope and the vessels from the south use rectangular ground ropes. The overall hydrodynamic drag of tickler chain and chain mat beam trawls is comparable, but drag varies between individual gear components. Chains are contributing nearly equally to the overall hydrodynamic drag while the netting panels are more important in tickler chain trawls.
The ground gear is conversely causing a higher drag in chain mat beam trawls. The results of the current study are representative for the Dutch beam trawl vessels targeting sole and will likely be representative for the entire North Sea beam trawl fleet, which is largely comprised of Dutch owned vessels that were re-flagged to exploit the UK, German, and Belgian quota. We must be aware of the limitations of our approach.
The of silt fraction estimates are interpolated values from a range of data sources complied by Wilson et al. We downscaled these data to the resolution used in our study 0. There are also a number of uncertainties associated with the hydrodynamic drag estimates. For many of the gear components, their drag coefficients are estimated from experiments; i on idealized bodies that are similar to, but not exactly the same as the gear component e.
The approach further assumes that the hydrodynamic drag of the individual gear components are additive and that there is no interference between them Paschen et al. All these factors will affect the hydrodynamics, and the corresponding drag estimates and sediment mobilization estimates must be used with caution.
Indeed, the validation of our approach showed, through comparison with literature estimates, that realistic hydrodynamic drag estimates were modelled for gear components, such as beam, shoes, wings, electrodes, tension relief cords, etc. However, further research is required to study the hydrodynamic drag of components such as the beam trawl net panels. The discrepancy between the modelled and measured drags may be due to the fact that the typical beam trawl towing speeds are greater than the maximum speed of about 4 knots 2.
This differs from the otter trawl nets used by Reid , whose netting panels will have a larger angle of attack and hence will be less streamlined and in turn will have a greater drag. Chafers and dolly ropes were not included in the analysis, but may in turn increase the drag of beam trawl nets. Hence, because pulse trawlers have a lower total catch volume than conventional beam trawlers van Marlen et al. The accuracy of the model predictions will also be affected by the quality of the gear component data.
The dimension of the various beam trawls in this paper have been presented to active fishers and gear manufacturers and we are confident that they are representative of the fleet, in particular for the pulse trawls PUL and the tickler chain beam trawls TBT used by large vessels.
The sample size of the TBC and the TBT used by the small vessels was limited rendering the quantitative results for these trawls less certain. In spite of the limitations of our approach, our model-based estimates of the difference in the amount of sediment mobilized in the wake of a tickler chain beam trawl and a pulse trawl, showed good agreement with in situ field estimates Depestele et al.
Hence, although there is uncertainty associated with some of the hydrodynamic drag estimates, we expect that our methodology provides a good approximation and will be particularly useful in capturing the relative differences between gears. We are confident that our approach provides reasonable estimates of the quantity of sediment mobilized by different gears, which, as we have shown here, are particularly useful when used with information on different gear types, spatial and temporal fishing effort data and spatial sediment data, to estimate the differential impact of trawling at the fleet level.
This will provide policymakers and fisheries managers with a quantitative means to assess the physical impacts of different fishing gears and fishing methods across sediment types. It will allow the ranking of gears in terms of their impact and permit a direct comparison with the physical impact of natural events such as storms and tides and of other uses of the seabed such as mineral extraction and mining. Accordingly, it will permit a rationale and objective approach to fulfilling the requirements of the Common Fisheries Policy and the Marine Strategy Framework Directive.
Our approach could also be used to provide estimates of trawling-induced sediment mobilization for mechanistic models of biogeochemical cycles de Borger et al. Furthermore, the hydrodynamic drag estimates will provide a better understanding of the forces required to tow a trawl gear across the seabed and contribute to the development of fuel-efficient gears that will reduce CO 2 and NOx emissions by the fishing industry. Primary VMS-data and catch and effort data of the mandatory logbook are subject to confidential agreements.
An exerpt of anonimised data will be shared on reasonable request to the corresponding author. Rijnsdorp, A. Sediment mobilization by bottom trawls: a model approach applied to the Dutch North Sea beam trawl fishery. Lokker Cooperatie Westvoorn , H. Drijver and a number of individual skippers are gratefully acknowledged for providing information on gear dimensions. We tank Niels T. Hintzen for providing an updated data set with swept area ratios by grid cell for conventional tickler chain and chain mat beam trawl.
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Kaiser M. Prioritization of knowledge-needs to achieve best practices for bottom trawling in relation to seabed habitats. Fish and Fisheries , 17 : — Lambert G. Implications of using alternative methods of vessel monitoring system VMS data analysis to describe fishing activities and impacts. Lucchetti A. Impact and performance of Mediterranean fishing gear by side-scan sonar technology. Canadian Journal of Fisheries and Aquatic Sciences , 69 : — Mayer L.
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The physical impact of towed demersal fishing gears on soft sediments. Cod-end drag as a function of catch size and towing speed. Fisheries Research , 72 : — The mobilisation of sediment by demersal otter trawls. Marine Pollution Bulletin , 62 : — The hydrodynamic drag and the mobilisation of sediment into the water column of towed fishing gear components. Journal of Marine Systems , : 76 — Oberle F. What a drag: quantifying the global impact of chronic bottom trawling on continental shelf sediment.
Journal of Marine Systems , : — Palanques A. Effects of bottom trawling on the Ebro continental shelf sedimentary system NW Mediterranean. Continental Shelf Research , 72 : 83 — Paradis S. Organic matter contents and degradation in a highly trawled area during fresh particle inputs Gulf of Castellammare, southwestern Mediterranean. Biogeosciences , 16 : — Paschen M. University of Rostock, Rostock, Germany: Pitcher C.
Methods in Ecology and Evolution , 8 : — Polet H. Impact assessment of the effects of a selected range of fishing gears in the North Sea. ILVO Report. Polymenakou P. Benthic microbial abundance and activities in an intensively trawled ecosystem Thermaikos Gulf, Aegean Sea. Continental Shelf Research , 25 : — Poos J.
Efficiency changes in bottom trawling for flatfish species as a result of the replacement of mechanical stimulation by electric stimulation. Provoost P. Modelling benthic oxygen consumption and benthic-pelagic coupling at a shallow station in the southern North Sea. Estuarine, Coastal and Shelf Science , : 1 — Pusceddu A. Chronic and intensive bottom trawling impairs deep-sea biodiversity and ecosystem functioning.
Effects of bottom trawling on the quantity and biochemical composition of organic matter in coastal marine sediments Thermaikos Gulf, northwestern Aegean Sea. Reid A. A net drag formula for pelagic nets. Scottish Fisheries Research Report. Number 7. If this number increases, this means the pound is becoming stronger against the dollar — and if the price decreases, this means the dollar is becoming stronger against the pound.
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