Julie The Tickler Full Access

Hey All!I bought a new-to-me 2002 KLR 250 earlier in the summer. (Will do a little introductory post with photos soonish.)I've put around 1000 miles on it so far, bringing the mileage to 9400. Loads of fun!However, the time has come for some maintenance. I started in on the valve adjustment this afternoon. I got everything disassembled according to the instructions on the KLR 250 Information Exchange website. Which went surprisingly smoothly and ended with my peering deeper into the innards of an engine than I've ever peered before.The valves are very tight way off spec.

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I tried to loosen the locknuts so that I can adjust the valves, but the locknuts wouldn't budge. I stopped applying pressure because I was afraid any more force might cause the nut to loosen catastrophically and send my wrench or hand into the engine to do some damage.Are there any tricks, tips, or techniques you've discovered to deal with this scenario? Or should I get the locknuts off by any means necessary (e.g. Pliers or vice grips) and just replace them?Thanks!! Are you certain you rotated the crankshaft to the CORRECT TDC position?Not trying to insult your intelligence, but.

Each four-cycle engine has TWO TDC piston positions each cycle; one between compression and power strokes (THAT'S the one where you adjust your valves), and one between exhaust and intake strokes (little or no valve clearance may exist, here, even on a properly-adjusted engine).From your description, you may be at the wrong TDC position to adjust valve clearance properly.INDICATORS: ALL VALVES are COMPLETELY CLOSED, at TDC between compression and power strokes. Removing the spark plug, and rotating the engine in the direction of operation produces air gushing from the spark plug hole on the compression stroke. You need a torque wrench to tighten things up when you're done. I suggest that you use a 6 point socket and the torque wrench, set for the appropriate torque setting, to loosen the lock nuts. When the wrench clicks you should need only a little more pressure to loosen them.

It's long enough to give you good leverage and the click will give you confidence in wrenching it hard enough.Ditto on the above comments by XDragRacer. Another common error is to turn the crankshaft the wrong way. Follow the instructions E X A C T L Y. I like the looks of the cycleracks klr250 rack but wanted to ask for feedback from people who have one.I want to put a top box of some kind (action packer or tough tote) on the bike because I use it to commute and don't like wearing a backpack all the time.Any problems with fit? It looks like it touches the plastic side panels on the KLR250, which I guess just means that you have to loosen the rack to get to the battery or air filter.How much weight have you put on it or do you figure it can handle?Anyone else prefer another rack?Thanks,rube. Are you certain you rotated the crankshaft to the CORRECT TDC position?Not trying to insult your intelligence, but. Each four-cycle engine has TWO TDC piston positions each cycle; one between compression and power strokes (THAT'S the one where you adjust your valves), and one between exhaust and intake strokes (little or no valve clearance may exist, here, even on a properly-adjusted engine).From your description, you may be at the wrong TDC position to adjust valve clearance properly.INDICATORS: ALL VALVES are COMPLETELY CLOSED, at TDC between compression and power strokes.

Removing the spark plug, and rotating the engine in the direction of operation produces air gushing from the spark plug hole on the compression stroke. Click to expand. Au contraire, Dangerous Johnson, thank YOU for acknowledging the value of the advice!Glad you got things sorted out.

Lock nuts should be easier to loosen with no stress on the adjusting screws.A couple of confessions/tips; I don't use a torque wrench when cinching down the lock nuts; I do it by the 'Braile' method, by feel, taking care not to strip out anything. That's my confession, now the tip: Check the clearance AFTER you tighten the lock nuts. Sometimes the screws have a tendency to follow, slightly, the lock nut as it is tightened, reducing clearance. When this occurs, you may have to adjust the clearance slightly greater than your feeler gauge tells you to, so when the assembly is tightened, you have the final clearance you desire.You have my respect for performing your own tuning; you're learning life-long skills!

AbstractTickler-chain SumWing and electrode-fitted PulseWing trawls were compared to assess seabed impacts. Multi-beam echo sounder (MBES) bathymetry 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 showed that SumWing trawls (3.11 dB) flattened seabed roughness significantly more than PulseWing trawls (2.37 dB). Sediment Profile Imagery (SPI) showed that SumWing trawls (mean, SD) 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). The reduced PulseWing trawling impacts allowed a faster re-establishment of the oxidised layer and micro-topography.

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). Total penetration depth (mean, SD) of the SumWing trawls (4.1 cm, 0.9 cm) and PulseWing trawls (1.8 cm, 0.8 cm) was estimated by the depth of the disturbance layer and the layer of mobilized sediment (SumWing = 0.7 cm; PulseWing trawl = 0.8 cm).

PulseWing trawls reduced most of the mechanical seabed impacts compared to SumWing trawls for this substrate and area characteristics. IntroductionDemersal otter trawls and beam trawls are the most widely used fishing gears to catch bottom dwelling fish, crustaceans, and bivalves (; ) and are the most widespread source of physical disturbance to marine habitats (;; ). The development and implementation of ecosystem-based management strategies increasingly require assessments of bottom trawling impacts on the seabed. Risk-based assessments are an appropriate tool for this purpose. These and others are being developed for the implementation of the European Marine Strategy Framework Directive, Descriptor 6 “Sea-floor integrity” (; ).The risk of a significant adverse impact on the sea-floor (seabed) depends on (i) the likelihood of exposure and (ii) sensitivity of the seabed to fishing activities. The likelihood of exposure relates to the overlap of distribution of habitat types and fishing effort , while “sensitivity” depends on the ability to withstand fishing pressure and recover from the damage imposed (; ).

Quantifying impact is difficult but approaches are emerging that express impact as a function of mortality and the recovery rate. Mortality is defined as the proportion of seabed biota killed by a single trawl pass. Mortality is very difficult to estimate due to the high spatial variability of benthic organisms (;;;; ).

Measuring the trawling penetration depth is likely to be a cost-effective alternative to estimate the direct mortality imposed by bottom trawling, and allows benthic impacts across fishing gears to be compared (;;; ). These novel insights set the baseline for assessing trawling impact, but require more detail at the level of different gear types to enable implementation in fisheries management. Different demersal gear types are designed to have different levels of seabed contact or penetration, depending on the target species, their catching stimulus and seabed type. These factors contribute to different penetration depths, but have until now only been assessed for generic gear designs (; ). Different gear configurations and the quantification of their potential benefits for mitigation of seabed impacts, cannot be ignored any longer and were identified as one of the top 10 knowledge priorities for managing seabed impact.The flatfish-directed trawler fleet in the North Sea has evolved over the last decade and used different gear configurations.

Conventional tickler-chain trawls tow a number of chains over the seabed to chase flatfish out of the seabed. The net is opened horizontally by a steel bar (the beam) supported by two trawl shoes at each end to maintain the beam at a constant height above the seabed and maintain the vertical opening of the net.

In pulse trawls, the tickler chains are replaced by electrodes that induce a cramp response that bends the fish into a U-shape, thus allowing them to be scooped up by the ground gear (; ). The pulse trawls are towed at a lower speed, and may catch sole more selectively and reduce discards of benthos.

The beam and the two trawl shoes of the conventional tickler-chain trawl and the pulse trawl may be replaced by a wing-shaped foil with a “nose” in the centre. This wing-shaped foil was designed to reduce drag in the water and on the seabed. A tickler-chain trawl using a foil is called a “SumWing” trawl, while a trawl using the foil in combination with electric pulses is called a “PulseWing” trawl. In 2010, 30% of the Dutch flatfish-directed trawler effort was represented by beam trawls using the SumWing with tickler chains, 8% using electric pulses and 62% using the conventional beam trawl.

In 2016, 12% of the effort came from SumWing trawls, 83% came from pulse trawls, while only 5% was exerted by conventional beam trawls (last accessed 7 May 2018). In 2016, 19% of the pulse trawls used a steel bar with shoes as opposed to the wing-shaped foil (PulseWing trawl) to open the net.In this study, we examined differences in seabed impacts between two gear configurations used to target flatfish Dover sole ( Solea solea) in particular.

We compared the mechanical impact on the seabed of a SumWing trawl with a PulseWing trawl, with a particular focus on the comparison of penetration depths. The penetration depth of a trawl is difficult to quantify. The passage of a trawl disturbs the top layer of the sediment, which can (i) remain in the same location, (ii) be compressed or compacted, (iii) be laterally displaced (; ), or (iv) be mobilized and carried away from the area to a distance dependent on the particle size and bottom currents (; ). Sediment reworking, mobilization and transport leads to sediment erosion (;; ), altered seabed morphology (;; ), and changes in the lithological and geochemical characteristics of the seabed (;; ). We carried out a field experiment using complementary sampling approaches to improve our understanding of the acute changes to the seabed by two commercial trawl types in the North Sea. Material and methods Background to this studyIn we compared the seabed impact of bottom trawls using tickler chains vs. Electric pulses to catch flatfish in the North Sea.

This study elaborates on the previous findings in 2 main ways. First, focused on one branch of the flatfish-directed trawler fleet, i.e. “euro-cutter” vessels with engine power below 300 HP (≤221 kW), and access to coastal waters between 3 and 12 nautical miles, including the Plaice Box (;; ). This study focused on the other branch, the large trawler fleet with engine power 300 HP, operating in offshore waters with heavier and larger trawls. The main differences in gear parameters and location characteristics of two fleets are reflected in both case studies (; ). Site.Time interval.Description of time intervals.Time lapse (h).Backscatter values (dB).Statistical differences at.Mean.SD.Lower CI.Upper CI.p.

Site.Time interval.Description of time intervals.Time lapse (h).Backscatter values (dB).Statistical differences at.Mean.SD.Lower CI.Upper CI.p. Site.Time interval.Description of time intervals.Time lapse (h).Backscatter values (dB).Statistical differences at.Mean.SD.Lower CI.Upper CI.p.

Site.Time interval.Description of time intervals.Time lapse (h).Backscatter values (dB).Statistical differences at.Mean.SD.Lower CI.Upper CI.p. SD, standard deviation; CI, 95% confidence interval.The other differences, despite the analogous modelling approaches in both case studies, were experimental design, sampling equipment, and studied parameters differed ( ). Focused on bathymetrical changes using the multi-beam echo sounder (MBES), but could not directly compare the effects of tickler-chain and pulse trawling due to differences in trawling intensities at the experimental sites. In that study, we measured sediment mobilization in situ using the LISST-100X, which was not deployed in this study. In this study, we used MBES bathymetry data to directly compare one passage of a SumWing trawl vs.

A PulseWing trawl. We additionally analysed the MBES backscatter data and collected ground truthing data. Sediment samples were collected using a box corer and were analysed to quantify the changes in sediment sorting. The depth of disturbance and biogeochemical changes to cross-sections of the seabed were estimated using Sediment Profile Imagery (SPI) after trawling at the same intensities (;,).

Study areaThe study area was located between 29 and 33 m depth in the south-western part of the Frisian Front (southern North Sea, between 53.56°N and 4.2664–4.2999°E). The Frisian Front is a transitional zone in the southern North Sea, located between the shallow, sandy Southern Bight and the deeper, muddy Oyster Grounds. The seabed in this area consists of fine sand with median grain sizes in the range of 154–163 μm and silt fractions between 12 and 17% (; see “Particle size distributions and depth of sediment reworking” section). Fine sediment particles settle in this area because tidal currents drop below the critical water velocity (; ). Deposition consists of particulate matter that is transported through the East Anglian turbidity plume and from locally produced phytodetritus and results in a sediment with elevated concentrations of silt, organic carbon, and phytopigments.

The study area was located in the southwestern part of the Frisian Front. Experimental sites were designated for PulseWing trawling (north), SumWing trawling (south), and control (no trawling). Each experimental site was sampled before and after trawling using a multi-beam echo sounder (total area), a box corer (circles; N = 5), and a sediment profile imager (crosses; N = 20). Fishing gearThe impact of a 12 m SumWing trawl and of a 12 m PulseWing trawl was studied. The SumWing trawls were deployed from the FV “Helena Elisabeth” (TX 29) and the PulseWing trawls from the FV “Biem van der Vis” (TX43).

Both fishing vessels had a length overall (LOA) of ∼40 m and a main engine power of approximately 1 500 kW. The vessels deployed a pair of 12 m wide trawls from the outrigger booms, which are kept open by a wing-shaped foil with a “nose” in the centre instead of the conventional cylindrical beam with two trawl shoes ( ).

The dimensions of the wing-shaped foils did not differ.The main differences between the gears were related to the stimuli to catch the fish (tickler chains vs. Electrodes), the geometry of the net opening of the trawl, the ground gears and the nets used ( and ). Both trawls had a cod-end with 80 mm stretched diamond-shaped mesh opening as used in the commercial sole fishery (; ), but the SumWing trawl net used during the experiment was lighter than most trawl nets used in the fleet (M. Drijver, skipper of TX29, pers.

The catching process of the SumWing trawl was based on mechanical disturbance by the tickler chains, which are rigged in the V-shaped net opening, perpendicular to the towing direction. The SumWing trawls were towed at speeds of ∼6 kn with a scope ratio of 3 (ratio of warp length to water depth). The total gear weighted nearly 3.1 t in air or 1.6 t in water. Eight tickler chains with a chain link diameter of between 18 and 24 mm and a total length between 18.6 and 26 m were attached to the wing. In addition, 9 tickler chains were attached to the middle part of the ground gear. Their length varied between 6 and 14 m, with a chain link diameter was between 13 and 16 mm. Three shorter tickler chains (4.5–5 m) with a diameter of 16 mm were attached to the aft part of the ground gear.

The ground gear consisted of a 37 m long chain with rubber discs with a diameter between 18 and 28 cm covering the chain over a 7.8 m centre section. The electrodes of the PulseWing trawl were rigged in a longitudinal direction into the square-shaped mouth opening of the trawl net. The PulseWing trawls were towed at fishing speeds of ∼5 kn with a scope ratio of 3. The total gear weighted 2.8 t in air or 1.4 t in water. A total of 27 electrode modules were attached to the wing-shaped foil and the ground gear.

The commercial electrodes ( HFK Engineering) have a diameter of 3.3 cm and produce a 60 Hz pulsed bipolar current at 45–50 V with a 0.36 µ s pulse duration (; ). A disc-protected rope (of diameter 8–10 cm) is rigged alongside each electrode to withstand the tension from towing the gear over the seabed (hereafter called “tension relief cords”).

The mouth of the PulseWing trawl had a square-shaped opening, resulting from two disc-protected chains that were running parallel to the towing direction at the sides of the mouth opening and from two ground ropes, that are both running perpendicular to the towing direction and rigged just in front of the trawl net mouth opening ( and ). The tension relief cords were attached to the first rubber disc ground gear (diameter = 12 cm), while the net was attached to the second rubber disc ground gear (diameter = 20 cm) ( ). Gear components of the SumWing trawl (upper panel) and PulseWing trawl (lower panel). Tickler chains with a chain link diameter between 18 and 24 mm are attached to the SumWing, whereas the tickler chains (13–16 mm) are attached to the ground gear. The electrode modules consist of tension relief cords (dashed line, d = 8–10 cm) and an electrode (dashed-dotted line, d = 3.3cm) ( ). Tension relief cords are attached to the first ground rope, while electrodes are attached to the second ground rope to prevent damage. N indicates the number of tickler chains or electrode modules.

Gear components of the SumWing trawl (upper panel) and PulseWing trawl (lower panel). Tickler chains with a chain link diameter between 18 and 24 mm are attached to the SumWing, whereas the tickler chains (13–16 mm) are attached to the ground gear. The electrode modules consist of tension relief cords (dashed line, d = 8–10 cm) and an electrode (dashed-dotted line, d = 3.3cm) ( ). Tension relief cords are attached to the first ground rope, while electrodes are attached to the second ground rope to prevent damage. N indicates the number of tickler chains or electrode modules. Experimental fishing and experimental sitesAcute fishing disturbance by each trawl type was evaluated in a controlled experimental design. We chose 3 sites of 200 × 2 800 m (0.56 km 2) each, located 250 m apart.

The PulseWing trawl was fishing in the northern site and the SumWing trawl in the southern site. No fishing took place in the central site, which was used as a control. Samples were taken from this site to measure the influence of factors other than fishing such as waves and currents. The experimental sites were located on a gentle slope with median grain sizes of 154, 162, and 169 μm in the PulseWing, control and SumWing trawl sites, respectively. Most particle sizes (down to 10 cm depth) classified as fine sand (125–250 μm): 59% in the PulseWing site, 63% in the control and 70% in the SumWing site, with a respective silt fraction of 17, 13, and 12%. The fine sand fraction in the top layers was similar to the deeper layers, with a slight decrease (5%) in the SumWing site. The mean silt fraction was lower in the top layers than in the deeper layers but its relationship with depth differed between sites.

Multibeam echo sounder (full line arrows), Sediment Profile Imagery (dashed arrows), and box corer (dotted arrows) measurements took place before fishing (grey rectangle) and at various times T after fishing with the SumWing trawl (dark grey arrows) or the PulseWing trawl (black arrows). Light grey arrows indicate sampling in the control site where no fishing took place. Mean (± SD) wind speed (dashed line) and significant wave height (dotted line) indicate a gentle breeze (4.9 ± 2.0 m/s; 16.0 ± 12.0 m) with excellent sampling conditions, particularly after fishing. Multibeam echo sounder (full line arrows), Sediment Profile Imagery (dashed arrows), and box corer (dotted arrows) measurements took place before fishing (grey rectangle) and at various times T after fishing with the SumWing trawl (dark grey arrows) or the PulseWing trawl (black arrows). Light grey arrows indicate sampling in the control site where no fishing took place. Mean (± SD) wind speed (dashed line) and significant wave height (dotted line) indicate a gentle breeze (4.9 ± 2.0 m/s; 16.0 ± 12.0 m) with excellent sampling conditions, particularly after fishing.Six hauls of varying haul duration took place on 10 June 2014 (PulseWing trawl between 8:50 and 15:00; SumWing trawl between 9:26 and 14:24). Fishing operations were carried out as similarly as possible, resulting in 13 passages along the length of each site that represented an equal swept area of 0.872 km 2 and a fishing intensity of 156% (= 0.872/0.56 km 2) for each gear in their respective experimental site.

Observations from the Research Vessel (RV) during the entire experimental period (9–12 June 2014) ensured that no fishing had taken place in the experimental sites other than experimental trawling. Previous trawling disturbances in the experimental sites were limited, as evaluated from prior inspection using the multi-beam echo sounder. Historic disturbance by bottom-contacting gears in the area was also low, varying between once every 10 years to once every 2 years (;,). Data collection methodsThe effects on the seabed were measured using three observation techniques: (i) the multi-beam echo sounder (MBES) for assessing changes in the sedimentary interface using both bathymetrical and backscatter strength data, (ii) Sediment Profile Imagery (SPI) for identifying geochemical changes, and (iii) box corer sampling for particle size analysis. These measurements were collected in all experimental sites before and after fishing during mild weather conditions (significant wave heights 25 measurements inside the track and 40 outside the track. The bathymetric profile of each cross-section was corrected for its slope using ordinary least squares regression. The slope-corrected depth measurements inside and outside the track were then compared to a non-parametric Friedman rank sum test following a single factor (water depth) within subject (cross-section) design.

Statistical differences between water depths inside and outside the trawl tracks were tested for the SumWing and PulseWing trawls at T0 and T1. The deepening of the trawl track was then assessed by subtracting the cumulative depth distribution after fishing from the cumulative depth distribution before fishing. In other words, we first tested whether trawling was conducted on a “flat” surface, i.e. The locations to be trawled were not positioned shallower or deeper than their surroundings. We then assumed that any differences in water depths found in the trawl track locations at T1 were due to the passage of the trawl. Seabed backscatter strengthA BS mosaic of 0.5 by 0.5 m resolution was computed for each MBES line. An angular compensation was applied using the mean BS level—incident angle curves computed independently for each line.

Only BS values derived from oblique incident angle, inside the angular interval of 30–50°, have been considered. The resulting BS mosaics (by line compensated) were merged by experimental site (PulseWing trawl, control, and SumWing trawl) and time interval (T0, T2, T3, and T4). BS values from these mosaics were randomly sampled without replacement to increase computational efficiency in further analysis and eliminated as outliers (1.3% of the data) when outside 1.5 times the interquartile (25–75%) range (1.3% of the data). This procedure yielded a dataset of 1 700 backscatter values per site and time interval. A linear model was applied to the backscatter values with site, time interval and their interaction as fixed effects. Visual inspection of the histogram and QQ-plot indicated normality of the residuals and a plot of the residuals vs.

Fitted values confirmed homogeneity of the variances, which allowed ANOVA type III analysis of the model. Significant factors ( p 2. The relationship with depth was first evaluated between experimental sites before fishing (T0) and between experimental sites without fishing disturbance (T0 of each site and T1 of the control site).

The different relationships of the silt fraction with depth for each experimental site and time step were then compared to the experimental sites after trawling (T1). Numerical modelling of penetration depthThree-dimensional numerical modelling based on the finite element method ABAQUS with Explicit solution was used to simulate interaction processes between the trawls and the seabed (,). The trawl–seabed interactions were implemented using the Coupled Eulerian Lagrangian (CEL) method with Eulerian mesh based on the volume of fluid method. The flow of the material through the mesh was tracked by computing its Eulerian volume fraction (EVF). The value of EVF represents the portion of material filled; EVF = 1 indicates that the element is completely filled with the material and EVF= 0 indicates the element is devoid of the material. The seabed was modelled as elastoplastic, obeying the cap Mohr–Coulomb model criterion, having the following parameters: specific weight of 19.5 kN m −3, Young’s modulus of 10 MPa, Poisson’s ratio of 0.3, cohesion intercept of 0.01 kPa, angle of internal friction, ϕ = 32° and a dilatation angle of 1°.

The triaxial test, the shear box test and the one-dimensional compaction test were performed in the laboratory to obtain these parameters. A penalty friction formulation based on Coulomb friction law was used as contact property representing the frictional behaviour between contacting bodies.

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The interaction of the trawls and the seabed was modelled from a reference configuration represented by a cuboid, consisting of two regions: the initial seabed material and a void region. Both regions were discretized using eight-node linear multi-material Eulerian bricks with reduced integration and hourglass control.

The trawls were modelled as elastic bodies specified by elastic constants leading to large elastic stiffness. The Lagrangian (trawl) elements were discretized using four-node quadrilaterals. Their mass and rotary inertia was specified at the centre, and they were given a linear velocity in the x-direction, ramping smoothly from zero velocity to a constant value in the next step. In a simulation, the void region was initially empty but filled up with the Lagrangian (trawl) elements and material flowing into the mesh within the Eulerian domain once the passage of a trawl element is simulated.

The simulations ran sufficiently long to reach quasi-static condition and were conducted for the nose of the wing-shaped foil of the SumWing and the PulseWing trawl, one single electrode and one single tickler chain with chain link diameter of 24 mm. Modelling of sediment mobilizationThe approach of was applied to estimate the quantity of sediment mobilized in the wake of a gear component. Their model estimates the amount of sediment mobilized immediately behind a towed gear component in terms of the hydrodynamic drag of the gear component and the silt fraction of the sediment.

It does not predict the fate of the sediment and whether it falls in the track of the component (as is likely for the larger particles) or whether it goes into suspension and is diffused or transported away by ambient currents (as is likely the case for the smaller particle sizes). In we applied our model to a 4 m beam trawl and found a good agreement between the model predictions and field measurements.Here, we have applied a similar methodology to measure the amount of sediment mobilized by each of the trawls of our experiments.

We calculated the drag of the noses of the wing-shaped foil, the electrodes and the ground gear from experiments on similar shaped objects (; ), the drag of the chains from the numerical estimates of, and the drag of the netting panels in the lower half of the trawl from the empirical model of. The silt fraction at each experimental site was estimated from the upper 2 cm sediment layer of the box corer samples taken at T0. Results Seabed bathymetryThere was no difference at T0 between the water depths inside and outside the trawl tracks at either site SumWing trawling location: χ 2(df = 1)=3.6, p = 0.06; PulseWing trawling location: χ 2(df = 1)=0.08, p = 0.78. After trawling (T1), the mean ( SD) track depths were significantly deeper and were 15.1 (0.9) and 9.1 (1.6) mm for the SumWing and PulseWing trawls, respectively SumWing trawling location: χ 2(df = 1)=93.63, p. Relative depth of the trawled track (cm) before and after trawling (left and middle panels) and its deepening (right panel).

The relative depth of the trawled track was calculated from the differences in water depths inside and outside the trawled track. These relative depths were calculated after trawling (T1, solid lines) and also before trawling (T0, dashed lines) using the locations of the trawled track from T1. Negative relative depths indicate that the trawled track was higher than its surroundings. The deepening of the trawled tracks was based on the comparison of the relative depths in the trawled track before (T0, dashed) and after (T1, solid) SumWing trawling (black lines) and PulseWing trawling (grey lines). Deepening is illustrated by the shaded areas. Please note that deepening differs from penetration depth. Relative depth of the trawled track (cm) before and after trawling (left and middle panels) and its deepening (right panel).

Negative relative depths indicate that the trawled track was higher than its surroundings. The deepening of the trawled tracks was based on the comparison of the relative depths in the trawled track before (T0, dashed) and after (T1, solid) SumWing trawling (black lines) and PulseWing trawling (grey lines). Deepening is illustrated by the shaded areas. Please note that deepening differs from penetration depth. Seabed backscatter strengthSeabed BS was statistically different across the experimental sites and time intervals ( F 6, 22 031=126.7, p. Absorption of sound (acoustic measurement, i.e.

Backscatter values in dB) of three experimental sites (PulseWing trawl, SumWing trawl, and control: no fishing) at various time intervals before and after trawling (; ). Letters denote statistical differences ( p.

Parameter of seabed impact.Assessment technique.SumWing trawl.PulseWing trawl.Deepening of seabed bathymetryMBES1.5 (0.9)0.9 (1.6)Depth of disturbanceSPI3.4 (0.9)1.0 (0.8)Depth of sediment reworkingBox corer samples. Parameter of seabed impact.Assessment technique.SumWing trawl.PulseWing trawl.Deepening of seabed bathymetryMBES1.5 (0.9)0.9 (1.6)Depth of disturbanceSPI3.4 (0.9)1.0 (0.8)Depth of sediment reworkingBox corer samples. Parameter of seabed impact.Assessment technique.SumWing trawl.PulseWing trawl.Deepening of seabed bathymetryMBES1.5 (0.9)0.9 (1.6)Depth of disturbanceSPI3.4 (0.9)1.0 (0.8)Depth of sediment reworkingBox corer samples. Parameter of seabed impact.Assessment technique.SumWing trawl.PulseWing trawl.Deepening of seabed bathymetryMBES1.5 (0.9)0.9 (1.6)Depth of disturbanceSPI3.4 (0.9)1.0 (0.8)Depth of sediment reworkingBox corer samples.

Depth of disturbance following SumWing trawling is deeper than following PulseWing trawling based on the assessment of the SPI images.Depending on the trawl penetration, a remnant of the brown oxidized layer sometimes remained visible below this homogenized layer. Over time (1–2 days) the brown colour of these particles faded to grey. Also over time the homogenized layer consolidated and the oxic part of the sediment was set up again (within hours of the T2 and T3 images) and the redox clines and biological mixing were likely to be re-established. This is shown in the appearance of the iron oxidized surface layers and smoothing of the homogenized layer boundaries in T2 and T3. The oxidized layer in T0 was significantly different following SumWing trawling in T1, T2, and T3 ( F 3, 251 = 33.7, p.

Particle size distributions at the top 1 cm of the seabed (upper row) and between 1 and 4 cm depth (lower row) before (T0, dashed line) and after (T1, solid line) experimental fishing in the control, SumWing trawl, and PulseWing trawl site. Depth categories were based on the mean depth of disturbance after SumWing and PulsWing trawling (; ).The modelling exercises of 2 primary sediment fractions showed similar patterns of depth of sediment reworking. SumWing and PulseWing trawling decreased the silt fraction in the top layers, but in contrast to PulseWing trawling, the SumWing trawl also caused an increase in the mean silt fraction in the deeper layers (T1 in, upper rows). Similar trends were reflected in the fine sand fraction. Both PulseWing and SumWing trawling increased the fine sand fraction in the top layer, but only SumWing trawling caused a decrease in fine sand in the deeper layers ( ). Predicted mean (upper panels) and SD (lower panels) of the silt fraction (expressed as a percentage) in function of depth of sampling in the seabed in (a) control, (b) SumWing, and (c) PulseWing trawl sites before and after fishing (T0, T1), based on Generalized Additive Models. Grey shaded areas delineate 95% confidence intervals.The depth to which both trawling methods affected the seabed was also reflected in the variability of the particle sizes before and after trawling.

The variability of the silt and fine sand fraction was low in the control site before and after trawling ( and ). The variability in silt and fine sand fractions before fishing was higher in the experimentally fished sites (T0), and was increased by trawling (T1). SumWing trawling increased the variability to a lesser degree (silt fraction: threefold, fine sand fraction: twofold), but its effect also occurred in deeper sediment layers (.

Deformation of the seabed through penetration of the nose of the wing-shaped foil used by the SumWing and the PulseWing trawl. Deformation was measured as the EVF, which tracks the flow of material through the Eulerian mesh. The value of EVF represents the portion of material filled; EVF = 1 indicates that the element is completely filled with the material and EVF = 0 indicates the element is devoid of the material. The wing-shaped foil can move through the Eulerian mesh without any resistance if its volume fraction is zero.

The value of EVF represents the portion of material filled; EVF = 1 indicates that the element is completely filled with the material and EVF = 0 indicates the element is devoid of the material. The wing-shaped foil can move through the Eulerian mesh without any resistance if its volume fraction is zero. Modelled penetration depth (cm) of the nose of the wing-shaped foil as function of their respective weights in water. Penetration of the nose was 2.3 cm for the SumWing trawl, and 1.9 cm for the PulseWing trawl. Modelled sediment mobilizationThe amount of silt mobilized is estimated at 10.6 kg m 2 and 13.1 kg m 2 for SumWing trawl and PulseWing trawl, respectively. This corresponds to a mobilized sediment layer of 6.6 and 8.2 mm, respectively (assuming a porosity of 0.4).

This difference is caused by the different silt fractions of the study sites. The hydrodynamic drag of both gears is very similar although the contribution of the different gear components differs. Even though the SumWing trawl is towed at a higher speed (6 kn vs.

5), the hydrodynamic drag of the lower netting panels is less than that of the PulseWing trawl. This is a result of the single twine used in the SumWing trawl and the higher number of panels fitted in the belly section of the PulseWing trawl ( ). The hydrodynamic drag of the SumWing gear (ground gear and tickler chains) is greater than that of the PulseWing trawl gear (ground gear and electrodes).

The differences between the drag of the 2 types of noses of the wing-shaped foil may be directly attributed to their different towing speeds. Total penetration depthThe total penetration depth of the gears is given by the sum of the erosion due to sediment mobilization (see “Modelled sediment mobilization” section, ) and the mean depth of disturbance (see “Oxidized layer and depth of disturbance” section). Total penetration depth was estimated at 4.1 cm ( SD= 0.9 cm) and 1.8 cm ( SD= 0.8 cm) for the SumWing and the PulseWing trawl, respectively. The variability ( SD) in penetration depth is similar (see also penetration profiles in ), but occurs at different disturbance depths in the seabed. DiscussionOur study has shown that the use of PulseWing trawls instead of SumWing trawls reduced most of the observed mechanical trawling impacts on the seabed for this substrate and area characteristics. The mobilization of sediment into the water column was comparable between both gears due to lighter trawl nets used in SumWing trawls, but penetration of the seabed by the PulseWing trawl was reduced by more than 50% in comparison to the SumWing trawl. Both trawling types caused tracks and homogenized the seabed topography without full recovery within 48 h, but seabed impacts of SumWing trawling were consistently higher than those of PulseWing trawling.

The SumWing trawl penetrated into the deeper layers (subsurface layer; 2 cm) and thereby consistently flattened the surface boundary and consistently deepened the seabed along the trawled track. The seabed penetration by the PulseWing trawl varied between skimming off the top mm and penetrating into the subsurface layers, which resulted in a reduced impact on surface boundary roughness and a reduced and highly variable deepening of the seabed. Pulse trawling allowed recovery of the oxidized layer within 48 h, while this recovery was not observed within 48 h after SumWing trawling. The lower impact of the pulse trawl is mainly due to the use of electrodes instead of tickler chains. The observed reduction in penetration depth applies to the sediment characteristics of our study area; it may be less in coarser sediments and more in finer sediments.

Our results corroborate an earlier study carried out in shallow fine sand habitat in the coastal zone of the southern North Sea , where the modelled penetration depths and resultant trawl tracks by euro-cutter vessels were slightly shallower (see ). Further work comparing the mechanical, electrical, chemical, and biological effects of gears on varying substrates, habitats, and hydrographic conditions and associated effects on seabed status and functions will build a more integrated view of gear effects on the seabed overall. Sediment mobilizationThe sediment mobilization model predicts that the SumWing trawl mobilized about 1.6 mm less sediment into the water column than the pulse trawl. These results were supported by analysis of particle sizes in the top layers of the seabed, where a loss of fine particles was lower following SumWing vs.

PulseWing trawling (2.2% in the top 2 cm layer after passage of the SumWing as opposed to 2.8% following PulseWing trawling). The limited differences in sediment mobilization can be attributed to the higher median grain size and lower silt content in the SumWing site than in the PulseWing site and to the use of different netting material and twine thickness. The reduced sediment mobilization of the SumWing trawl net was compensated by the higher hydrodynamic drag and sediment mobilization resulting from the ground gear assemblage and the tickler chains. Found no differences in sediment mobilization between tickler-chain and PulseWing trawling.

While sediment mobilization was comparable for trawls using tickler chains or pulse electrodes in both case studies, these results should not be directly extrapolated to the tickler-chain or pulse trawler fleet without prior knowledge of gear and operational characteristics of the fleet, such as twine thickness and towing speed.Sediment mobilization also occurs naturally in the Frisian Front. A drop in current velocities below a critical level along the slope of the Frisian Front causes deposition of silt and clay particles and creates a muddy area located to the northeast of the experimental sites. The tidal ellipses in the Frisian Front area are mostly oriented along a W to E–NE line with a maximum velocity.