Trawl Tracks Research Articles

The influence of seabed geology on the nature and preservation of bottom trawl marks in the Bornholm Basin, southern Baltic Sea

This study investigates six areas in a historically heavily trawled region of the southern Baltic Sea. Using acoustic geophysical mapping data and sediment cores from three field campaigns (2019, 2020, 2023), we evaluate and quantify the cumulative physical impacts from bottom trawling and the influence of seabed geology on mapped trawl tracks. The results are compared with fishing intensity data over three periods; 2012–2016, 2017–2019 and after the fishery closed. A correlation between fishing intensity and density of mapped trawl tracks exists in the soft sediments of the northern part of the area, while this link is weak in the less trawled southern part, where the seabed is characterized by more consolidated glacial clays and the high density of mapped trawl tracks reflects the preservation of tracks >8 years old. Four years after the closure of the fishery there were no signs of trawl-track degradation in any of the areas. In summary, mapped track densities alone are not a suitable measure of trawling intensity, considering the influence of seabed geology and the persistence of trawl tracks over time. Sediment deformation, observed by CT-scanning, indicates extensive remoulding and coarsening of the upper 20–40 cm of sediments in the trawled areas.

Behavioural response thresholds of marine fish species for pulsed electric fields

Electrical pulse trawling is an alternative to conventional beam trawling for common sole (Solea solea), with substantially less discards, lower fuel consumption, and reduced impact on the benthic ecosystem. Pulsed electric fields are used to drive the fish from the seafloor and immobilise them in front of the nets. Concerns exist, however, that the electric fields may affect fishes outside the trawl track. Here, we address these concerns by measuring amplitude thresholds for behavioural responses and by comparing these response thresholds to simulated field strengths around fishing gear. Electroreceptive small-spotted catshark (Scyliorhinus canicula) and thornback ray (Raja clavata) as well as non-electroreceptive European seabass (Dicentrarchus labrax), turbot (Scophthalmus maximus), and common sole were, one at the time, placed in a ø2.5 m circular tank with seven, individually controlled, evenly spaced electrode pairs, spanning the diameter of the tank. Behavioural responses were assessed from camera recordings for different pulse amplitudes and for different fish positions relative to the stimulating electrodes. Electrical stimulation consisted of a Pulsed Bipolar Current at 45 Hz and 0.3 ms pulse width, similar to that used in commercial gears. Computer simulations of the electric field, verified with in situ measurements, were used to determine the field strength at the location of the animal. Thresholds for different species varied between 6.0 and 9.8 V m–1, with no significant difference between electroreceptive and non-electroreceptive species. The thresholds correspond to a distance of maximally 80 cm from the electrode arrays in simulated electric fields around commercial fishing gears. These findings suggest that electrical pulses as used in pulse trawling are unlikely to elicit a behavioural response outside the nets that surround the electrode arrays.

Sediment suspended by bottom trawling can reduce reproductive success in a broadcast spawning fish

Suspended sediment adheres to pelagic fish eggs, affecting their buoyancy. In the stratified southern Baltic Sea, eggs of the Eastern cod depend on neutral buoyancy in the “reproductive volume” (RV) (approx. >11 salinity and >2 ml O2/L) for successful hatching. With increased suspended sediment concentrations (SSC), eggs risk sinking out of the RV into deeper, unfavourable conditions. Bottom trawling, which increases SSC, has been intense around the Eastern cod spawning ground. We modelled the transport of sediment suspended from trawling at this spawning ground to estimate the degree to which eggs could be affected by increased SSC. SSC >1 mg/L above background levels was found 3 km away, one trawl track subjecting a water volume equivalent to 0.01% of the RV to this excess SSC for >12 h. At this excess SSC, it would take c. 6 d for an egg to sink out into unfavourable conditions; insufficient time for it to become a larva. Extrapolating to real bottom trawling intensities in the area of the RV where suspension is highest showed that a water volume equivalent to half the RV experiences excess turbidity of >1 mg/L for c. 24 h during a year. However, fishing effort is heterogeneous; spatio-temporal overlap between trawling and the RV will enhance the duration and/or frequency of turbidity in the spawning area, affecting a higher fraction of the eggs than the model predicts. We conclude that bottom trawling at this spawning ground could decrease cod's reproductive success through increased SSC. Such effects are likely in populations of other fish with pelagic eggs that spawn at trawling grounds.

A Transdisciplinary Approach Towards Studying Direct Mortality Among Demersal Fish and Benthic Invertebrates in the Wake of Pulse Trawling

Between 2009 and 2020, beam trawlers in the North Sea switched to electrical stimulation to target sole (Solea solea). The transition to pulse trawling raised widespread concern about possible adverse effects of electrical stimulation on marine organisms. Environmental NGO’s and small scale fishers claimed that it would electrocute marine life and create a ‘graveyard’ in the wake of pulse trawlers. This paper uses realistic field experiments to investigate the ‘graveyard’ hypothesis. In cooperation with fishers, a field experiment was designed where we simultaneously sampled marine organisms in the wake of pulse trawlers and in untrawled control areas. The impact was quantified by estimating the direct mortality among three dominant fish species and four dominant invertebrate species. In total, nine experimental tows were conducted in two years. Direct mortality among fish and invertebrates was low (0-10%) and did not differ between the pulse trawl track and the untrawled controls. Equally, no impact of the pulse trawl was found on external damages and vitality scores. The limited effects observed are likely due to the mechanical impact of the pulse and the sampling gear. The results of experiment do not support the claim that pulse trawling results in mass mortality among marine organisms in the trawl track. Throughout the research period, the concerns of small-scale fishers on pulse fishing shifted from being focused on biological effects to political and managerial issues. This can partly be attributed to the engagement in and the results of our research and has increased its credibility and salience. By integrating fishers’ knowledge and examining their perceptions through transdisciplinary research, we were able to show the importance of untangling the intricate relation between perceived knowledge gaps and political or management related concerns.

Physical Disturbance by Bottom Trawling Suspends Particulate Matter and Alters Biogeochemical Processes on and Near the Seafloor

Bottom trawling is known to affect benthic faunal communities but its effects on sediment suspension and seabed biogeochemistry are less well described. In addition, few studies have been carried out in the Baltic Sea, despite decades of trawling in this unique brackish environment and the frequent occurrence of trawling in areas where hypoxia and low and variable salinity already act as ecosystem stressors. We measured the physical and biogeochemical impacts of an otter trawl on a muddy Baltic seabed. Multibeam bathymetry revealed a 36 m-wide trawl track, comprising parallel furrows and sediment piles caused by the trawl doors and shallower grooves from the groundgear, that displaced 1,000 m3 (500 t) sediment and suspended 9.5 t sediment per km of track. The trawl doors had less effect than the rest of the gear in terms of total sediment mass but per m2 the doors had 5× the displacement and 2× the suspension effect, due to their greater penetration and hydrodynamic drag. The suspended sediment spread >1 km away over the following 3–4 days, creating a 5–10 m thick layer of turbid bottom water. Turbidity reached 4.3 NTU (7 mgDW L–1), 550 m from the track, 20 h post-trawling. Particulate Al, Ti, Fe, P, and Mn were correlated with the spatio-temporal pattern of suspension. There was a pulse of dissolved N, P, and Mn to a height of 10 m above the seabed within a few hundred meters of the track, 2 h post-trawling. Dissolved methane concentrations were elevated in the water for at least 20 h. Sediment biogeochemistry in the door track was still perturbed after 48 h, with a decreased oxygen penetration depth and nutrient and oxygen fluxes across the sediment-water interface. These results clearly show the physical effects of bottom trawling, both on seabed topography (on the scale of km and years) and on sediment and particle suspension (on the scale of km and days-weeks). Alterations to biogeochemical processes suggest that, where bottom trawling is frequent, sediment biogeochemistry may not have time to recover between disturbance events and elevated turbidity may persist, even outside the trawled area.

Recent advances in understanding the environmental footprint of trawling on the seabed

Bottom trawling accounts for nearly a quarter of wild-capture seafood production, but it is associated with physical disturbance of the seabed leading to changes in benthic abundance, habitat structure, and biogeochemical processes. Understanding the processes of benthic depletion and recovery in relation to different types of fishing gears, and in different seabed types, is an important pre-requisite to inform appropriate management measures to limit or reduce the effects of trawling on the seabed. The combined approaches of meta-analysis and modelling that link fishing-gear penetration of the seabed to benthic depletion, and recovery to taxon longevity, have enabled the development of a modelling framework to estimate relative benthic status in areas subject to trawling. Such estimations are highly sensitive to the spatial resolution at which fishing footprint (trawl track) data are aggregated, and this leads to overinflated estimates of fishing impacts on benthos when coarse-level aggregation is applied. These approaches present a framework into which other “sustainability” criteria can be added, e.g., the consideration of carbon footprints of fishing activities.

Fish species identification using a convolutional neural network trained on synthetic data

Abstract Acoustic-trawl surveys are an important tool for marine stock management and environmental monitoring of marine life. Correctly assigning the acoustic signal to species or species groups is a challenge, and recently trawl camera systems have been developed to support interpretation of acoustic data. Examining images from known positions in the trawl track provides high resolution ground truth for the presence of species. Here, we develop and deploy a deep learning neural network to automate the classification of species present in images from the Deep Vision trawl camera system. To remedy the scarcity of training data, we developed a novel training regime based on realistic simulation of Deep Vision images. We achieved a classification accuracy of 94% for blue whiting, Atlantic herring, and Atlantic mackerel, showing that automatic species classification is a viable and efficient approach, and further that using synthetic data can effectively mitigate the all too common lack of training data.

Comparison of mechanical disturbance in soft sediments due to tickler-chain SumWing trawl vs. electro-fitted PulseWing trawl

Abstract Tickler-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.

Cold‐water coral Madrepora oculata in the eastern Ligurian Sea (NW Mediterranean): Historical and recent findings

Abstract Cold‐water coral (CWC) ecosystems are long‐lived, slow‐growing and fragile, which makes them especially vulnerable to physical damage. In recent decades, CWCs have been severely threatened by fisheries, hydrocarbon extraction, pollution and other human activities. In the Mediterranean Sea, some investigations have been carried out on CWC ecosystems, mostly focused on their distributions within the central and eastern basins. Historical reports and fishermen's maps for the eastern Ligurian Sea (NW Mediterranean) from the 1960s document the occurrence of extensive banks of living CWC, mostly Madrepora oculata, between depths of 200 and 500 m. In 2013/2014, multibeam, side scan sonar (SSS) and remotely operated vehicle (ROV) surveys were carried out in that area, specifically in the Levante Canyon, to assess the occurrence, distribution and conservation status of CWC. The SSS and ROV showed numerous trawl tracks and small (10 cm high), dead, buried colonies at 300–500 m. Deeper, between 525 and 575 m, dense populations of living, 1 m high colonies of Madrepora oculata were found on the flanks of Levante Canyon. The deep sites showed colonies overturned or entangled by long‐line fishing activities. The discovery of new CWC banks not yet heavily damaged by fishing activities, suggests that urgent measures for conservation should be taken in the Mediterranean and worldwide. The present limitation of trawl‐fishing to above 1000 m depth, established by the General Fisheries Commission for the Mediterranean (GFCM) in 2005, seems to be ineffective, since CWCs are mostly located at less than 1000 m depth in the Ligurian Sea. A network of high‐seas/deep‐sea marine protected areas (MPAs) would favour a better strategy for protecting substantial areas of CWCs.

Feeding ecology and trophic segregation of two sympatric mesopredatory sharks in the heavily exploited coastal ecosystem of the Adriatic Sea.

The ecological roles and trophic interactions of two commercially important mesopredatory shark species, Squalus acanthias and Mustelus punctulatus that co-occur on the continental shelf of the north-central Adriatic Sea were investigated. Both shark species are dietary specialists, with a significant dietary overlap recorded only during the spring season. They showed different patterns of feeding as they grew: S. acanthias extended its trophic niche with an increase in size, while M. punctulatus developed a more specialized diet. These two sharks partition food resources and reduce niche overlap by foraging at different trophic levels. Mustelus punctulatus is a crustacean feeder, specialized in foraging on scavenging malacostracans frequently found along trawl tracks or on discards in the Adriatic fishing zone. Conversely, S. acanthias prefers small pelagic fishes, which are commercially exploited and in decline. The different foraging strategies adopted by these two species suggest that they should be managed separately. Dietary specialization, direct competition with humans for prey and their higher intrinsic vulnerability make S. acanthias particularly susceptible to the effects of anthropogenic perturbations.

An interaction index to predict turtle bycatch in a Mediterranean bottom trawl fishery

Incidental catch is the major threat to the survival of loggerhead turtles (Caretta caretta) in the Mediterranean and the main reason for their decline. More than 100,000 turtles are estimated to be caught annually due to fishing practices; 10–50% die. Bottom trawls are among the fishing gears exerting the worst impact on sea turtle populations, especially in the coastal waters of Tunisia and Libya, northern Adriatic Sea, the Mediterranean coastal areas of Turkey and Egypt, where the continental shelf is large and turtles in the demersal stage are commonly found also in winter. Seven adult-sized loggerhead turtles captured incidentally by bottom trawls in the central-northern Adriatic Sea and treated in rescue centres were tagged with satellite transmitters before release. In this paper Italian bottom trawl track data obtained from vessel monitoring systems (VMSs) were analysed by VMSbase R suite to identify the areas of maximum fishing effort in the Adriatic. These data were combined with satellite information on turtle distribution to provide an interaction index enabling prediction of potential trawl–turtle interaction hotspots and periods. The present pilot study can be considered as a risk-analysis approach directed at identifying the areas and times of possible trawling–turtle interactions in a Mediterranean trawl fishery. By identifying the areas and seasons at highest risk of turtle bycatch, the index here developed has the potential to provide key information to design and implement mitigation strategies. Vessel monitoring system is actually in force in several countries, therefore the approach here studied might have a worldwide application.

The substantial first impact of bottom fishing on rare biodiversity hotspots: a dilemma for evidence-based conservation.

This study describes the impact of the first passage of two types of bottom-towed fishing gear on rare protected shellfish-reefs formed by the horse mussel Modiolus modiolus (L.). One of the study sites was trawled and the other was scallop-dredged. Divers collected HD video imagery of epifauna from quadrats at the two study sites and directed infaunal samples from one site.The total number of epifaunal organisms was significantly reduced following a single pass of a trawl (90%) or scallop dredge (59%), as was the diversity of the associated community and the total number of M. modiolus at the trawled site. At both sites declines in anthozoans, hydrozoans, bivalves, echinoderms and ascidians accounted for most of the change. A year later, no recovery was evident at the trawled site and significantly fewer infaunal taxa (polychaetes, malacostracans, bivalves and ophuroids) were recorded in the trawl track.The severity of the two types of impact reflected the undisturbed status of the habitats compared to previous studies. As a ‘priority habitat’ the nature of the impacts described on M. modiolus communities are important to the development of conservation management policy and indicators of condition in Marine Protected Areas (EU Habitats Directive) as well as indicators of ‘Good Environmental Status’ under the European Union Marine Strategy Framework Directive.Conservation managers are under pressure to support decisions with good quality evidence. Elsewhere, indirect studies have shown declines of M. modiolus biogenic communities in fishing grounds. However, given the protected status of the rare habitat, premeditated demonstration of direct impact is unethical or illegal in Marine Protected Areas. This study therefore provides a unique opportunity to investigate the impact from fishing gear whilst at the same time reflecting on the dilemma of evidence-based conservation management.

The reactions of shoaling adult cod to a pelagic trawl: implications for commercial trawling

AbstractRosen, S., Engås, A., Fernö, A., and Jörgensen, T. 2012. The reactions of shoaling adult cod to a pelagic trawl: implications for commercial trawling. – ICES Journal of Marine Science, 69: 303–312. The reactions of shoaling adult Atlantic cod to a pelagic trawl were measured during fishing off the north coast of Norway. Cod remaining in the trawl track dived at rates as fast as 0.35 m s−1 following vessel passage and swam away from the vessel, in the direction of the approaching trawl, at an average rate of 0.6 m s−1. They did not attempt to swim ahead of the trawl as documented previously, but passed into the lower half of the trawl entrance and swam slowly in the direction of trawling at a rate of 0.2–0.5 m s−1 as the trawl's greater speed through water carried them deeper into the trawl. Shoals compressed vertically once inside the trawl, suggesting that packing density increased at least fourfold. Fish remained in the lower part of the trawl as they moved through its tapered section towards the codend, with little to no clearance above the bottom panel, but significant clearance beneath the top panel. Catches were sufficient to support commercial harvest, and the behaviour observed suggests that changes in trawl design and fishing strategy might improve fuel economy and species selectivity.

New insights in interpolating fishing tracks from VMS data for different métiers

The Vessel Monitoring by satellite System (VMS) is a powerful tool in fishery management, since it allows for high resolution analyses of fishing activity and quantitative evaluations of fishing effort at both spatial and temporal scale. Given that the main VMS limit is represented by the temporal resolution (generally 2 h) of signals, a series of approach has been developed to interpolate vessels positions. The newest and most powerful method in this framework is based on cubic Hermite splines (cHs), which have been efficiently tested against the conventional straight line interpolation over a dataset representing fishing activity by beam trawl. However, this method has never been applied on other different gears and/or métiers. Here we propose a new approach (CRm), which is a modification of the Catmull–Rom algorithm (CR). This new method takes into account for the different aspects involved in vessel navigation, such as the combined actions of human control and drift by sea current and wind (if present). The drift component is not observed, but is estimated within the method, using the VMS data. This method has been developed in order to model the behaviour of vessels that operate using different gear types. The CRm method was compared to the cHs method, using three reference datasets (each containing VMS signals at intervals of 20 min) corresponding to three different métiers largely used in Mediterranean fisheries: bottom otter trawl for demersal species (OTB), trammel nets for demersal species (GTR), and purse seine for small pelagic fish (PS), which differ each other for the dynamic aspects connected to the use of fishing gears, and represent an archetype of the three groups actually used to classify fishing gears (namely towed, active and passive). The comparison was carried out both analyzing the error affecting interpolation of single tracks and converting the interpolated tracks into gridded data to be used for computation of ecological indicators of fishing pressure. All the results coherently evidences that the CRm algorithm performs better in interpolating trawl tracks (OTB) and that, moreover, it is able to capture the complex behaviour characterizing the trajectories of vessels performing the other two inspected métiers (GTR and PS). Finally, CRm allows a better estimation of fishing effort, as measured by ecological indicators. These findings support the idea that the conceptual formulation of CRm method is appropriate to model whatever fishing tracks presently generated by fishery vessels and could be efficiently applied in order to obtain better estimation of fishing pressure and, if sensitivity data are available, of fishing impacts.

Improved estimation of trawling tracks using cubic Hermite spline interpolation of position registration data

For control and enforcement purposes, all fishing vessels operating in European waters are equipped with satellite-based Vessel Monitoring by Satellite systems (VMS) recording their position at regular time intervals. VMS data are increasingly used by scientists to study spatial and temporal patterns of fishing activity and thus fishing impact (e.g. surface of sea bed trawled during a fishing trip). However, due to their low resolution (2 h basis), these data may provide a biased perception of fishing impact. We present here a method aiming at interpolating vessel trajectories from VMS data points to obtain higher-resolution data on vessel trajectories which in turn should provide improved estimates of the spatial and temporal patterns of fishing activity and hence fishing impact. This method is based on a spline interpolation technique, the cubic Hermite spline (cHs), using position, heading and speed to interpolate the trawl track of a vessel between two succeeding VMS data points. To take uncertainty of the interpolated track into account, the method also determines a confidence interval, which represents the spatial distribution of vessel presence probability between two successive VMS positions. The cHs method was compared to the straight line interpolation technique using a reference data set with intervals of 6 min which was assumed to represent the real trawl tracks. The results showed that the cHs method approximates the real trawl track markedly better than a straight line interpolation. The cHs method should therefore be preferred to the conventional straight line approach to interpolate vessel tracks in studies aiming at estimating fishing impact from VMS data.

Patterns in the Local Distribution of the Sea Whip, Halipteris willemoesi, in an Area Impacted by Mobile Fishing Gear

While conducting a larger project along the continental shelf off central California in June 2006, we encountered a large patch of sea whips (Halipteris willemoesi) in an area that was actively fished by vessels using otter trawls. A total of 10 transects were conducted using a remotely operated vehicle (ROV) to collect video imagery of seafloor communities. Video records allowed us to quantify sea whip density and to calculate the densities of upright and damaged or broken sea whips. Though the transects were sited within a dense aggregation of trawl tracks, we recorded significant variability in sea whip densities across transects. While subtle differences in water depth among transects may have contributed to the variability in sea whip density, we suggest that the distribution of trawling effort is a more likely explanation.

Can vessel monitoring system data also be used to study trawling intensity and population depletion? The example of Australia's northern prawn fishery

We explore the potential of using data from Australia's northern prawn fishery (NPF) vessel monitoring system(s) (VMS) to examine trawl track, trawling intensity, and stock depletion due to trawling. We simulate VMS data by subsampling global positioning system (GPS) fixes from the NPF fishing vessels at different polling intervals to examine their accuracy in describing trawl tracks. The results of the simulations suggest that VMS data with polling intervals longer than 30 min cannot accurately estimate trawl tracks. The analysis of high-polling-frequency VMS data collected in four (later reduced to three) 6 nautical mile × 6 nautical mile grids that historically received high levels of fishing effort showed that trawling was not random and some areas were trawled up to 28 times in the tiger prawn fishing season and the impact varied among years. The results of a catch-depletion analysis suggest that fishery catch-per-unit-effort and cumulative catch may not be proportional to overall target-species biomass in areas with highly aggregated trawl effort. The VMS data also showed a large number of trawls can occur in productive areas and that trawling impacts on benthos may be quite marked.

Survey gear calibration independent of spatial fish distribution

Trawl surveys provide important information for evaluation of relative stock abundance fluctuations over time. Therefore, when survey gears or vessels are changed, it is important to compare the efficiency and selectivity of old and new gears and vessels. A method for estimation of conversion factors is developed based on a survey design where paired hauls are taken in the same trawl track line. The method explicitly accounts for changes in fish density caused by trawling disturbance. A generalized linear model for paired hauls catches is analytically derived and the gear conversion and disturbance parameters with their precision are obtained using standard software. Simulation studies carried out additionally showed that the estimated conversion factors were practically unbiased. Because of the independence of the spatial fish distribution, the new method is preferable to the traditional paired hauls design for which it is generally not possible to obtain the statistical properties of the estimated conversion factors. The paper is concluded with suggestions on how to optimize survey design. The method was used to estimate conversion factors for Atlantic cod (Gadus morhua) from Danish gear calibration experiments in the Baltic Sea.

Impact of experimental trawling on the benthic assemblage along the Tuscany coast (north Tyrrhenian Sea, Italy)

Abstract The impact of repeated experimental otter trawling, in the north Tyrrhenian Sea (Mediterranean) was investigated using a spatially replicated sampling design. Macroscopic modifications of the seabed morphology were assessed by sidescan sonar. Alterations of the sediment texture and changes in macrobenthic infauna were assessed using a box-corer. The most obvious modifications of the seabed were trawl tracks caused by the passage of trawl doors through the sediments. Ephemeral but significant changes in the sediment composition were observed. Changes in the benthic assemblage were detected only 48 h after experimental trawling. The clearest changes were detected in the molluscan component. The present study suggests that recovery from trawling may take place within one month.

Effects of demersal trawling on marine sedimentary habitats analysed by sediment profile imagery

Demersal trawling causes one of the most widespread physical and biological changes in marine shallow and shelf sedimentary habitats: trawl otter boards may create furrows in the sediment surface, while trawl nets and attached weights scrape the sediment surface. As a consequence, benthic animals are disturbed or killed, and resuspension of particles increase. The impact of trawling on benthic animals has traditionally been analysed by changes in species composition and abundance, whereas frequency and distribution of trawl tracks are frequently analysed by side-scan sonar. We have used sediment profile images (SPIs) (30×22 cm) and observed furrows and other physical disturbances on the sediment surface that we attribute to trawling. In a manipulative experimental trawl study in Sweden (BACI design), significant impacts were found in trawled benthic habitats (73–93 m deep) compared with pre-trawling conditions and with reference areas. In particular, furrows from trawl boards had a severe ecological impact. In the Gulf of Lions (northwest Mediterranean), similar patterns were observed in the vast majority of 76 images taken at random at depths between 35 and 88 m in four different areas. Epifauna and polychaete tubes were generally either rare or not observed at all on trawled sediment surfaces. Burrows and feeding voids were, however, frequently present in some trawled areas and seemed to be comparatively less affected. Such biogenic structures in the sediment were generally associated with rather deep (3–4 cm) mean apparent redox profile discontinuities (aRPDs), which were measured digitally as the visible division zone between oxidised (sub-oxic) and reduced sediments. Increased roughness caused by the trawl boards acting on the sediment surface, e.g. depressions and protrusions, could have effects on sediment solute fluxes.