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Beam trawl fisheries for sole are characterised by large amounts of unwanted bycatch (i.e. discards) consisting of fish below the Minimum Conservation Reference Size, unwanted fish due to low commercial value or lack of quota, and benthic invertebrates. In order to reduce the quantity of discards, a substantial part of the Dutch beam trawl fleet was allowed to replace the conventional tickler chain beam trawl (BT) with the pulse trawl (PT) on an experimental basis. The PT used electrical stimulation to immobilise and capture fish. Here we study whether pulse trawling reduced the amount of discards by comparing catch rates of landings and discards of BT and PT in the period 2009–2018 for a wide range of species. The PT caught (kg.km-2) significantly more marketable sized sole (Solea solea, 48 %), turbot (Psetta maxima, 8 %), brill (Scophthalmus rhombus, 28 %) and whiting (Merlangius merlangus, 95 %), and significantly less marketable sized plaice (Pleuronectes platessa, −16 %), cod (Gadus morhua, −32 %) and gurnards (−12 %). No significant difference was found for dab (Limanda limanda), gadoids, or rays and sharks. Among discards, the PT caught more undersized sole (27 %) and whiting (42 %) but less undersized plaice (−21 %), dab (−19 %) and grey gurnard (Eutrigla gurnardus, −31 %). The observed differences in species selectivity are discussed in relation to the response of fish to bottom trawl gear and the effects of pulse stimulation. For the benthic invertebrates Ophiuroidae, bivalves and crabs the PT caught fewer individuals (between −38 % and −57 %). No significant difference was observed in sea urchins and sea stars. Overall, this study shows that the transition from the BT to PT resulted in a 36 % decrease (95 % prediction interval: 31–42 %) in discards (kg.hour-1).

Networks play a key role in the functioning of socioecological fishery systems. Most network studies among fish harvesters examining fishing success utilize interviews and questionnaires. Though insightful, such studies are resource and time-intensive and thus unlikely to be replicated frequently through time. Alternatively, commercial landings records and vessel monitoring systems (VMS) provide continuous sources of information that can be used to examine variation in vessel networks through time. We used VMS data to define association networks among vessels. Relationships were found between common network metrics and annual performance based on landings data. Associations between vessels were more closely examined as a function of annual activity, performance, favoured species, and landing port using temporal exponential random graph models. We examined network dynamics across 4 consecutive years. Changes in vessel associations were clearly related to performance, landing port, and species targeted. Network structure could affect the relationship between catch and nominal effort, influencing stock assessments and responses to management actions. Our methodology provides a means to follow network change, identifying situations where more detailed study is warranted.

Continental shelves around the world are subject to intensive bottom trawling. Demersal fish assemblages inhabiting these shelves account for one-fourth of landed wild marine species. Increasing spatial claims for nature protection and wind farm energy suppresses, however, the area available to fisheries. In this marine spatial planning discussion, it is essential to understand what defines suitable fishing grounds for bottom trawlers. We developed a statistical methodology to study the habitat preference of a fishery, accounting for spatial correlation naturally present in fisheries data using high-resolution location data of fishing vessels and environmental variables. We focused on two types of beam trawls to target sole using mechanical or electrical stimulation. Although results indicated only subtle differences in habitat preference between the two gear types, a clear difference in spatial distribution of the two gears was predicted. We argue that this change is driven by both changes in habitat preference as well as a change in target species distribution. We discuss modelling of fisheries’ habitat preference in light of marine spatial planning and as support in benthic impact assessments.

This study investigates the effect of water temperature on the development rate of eggs and larvae, the duration of the endogenous feeding period and its consequences for recruitment of smelt (Osmerus eperlanus ) in Dutch lakes IJsselmeer and Markermeer. This study measured temperature‐dependent egg and larval development rates as well as mortality rates from fertilization till the moment of absorption of the yolk‐sac and from yolk‐sac depletion onwards in temperature‐controlled indoor experiments. Using multinomial modelling the authors found significant differences in development time of egg development stages under different temperature regimes. Based on historic water temperatures, the model predicted that the larval endogenous feeding period has advanced at a rate of about 2.9 days per decade in a more than 50 year period since 1961, yet there was no change in the duration of the endogenous feeding period. As zooplankton is more responsive to daylight than water temperature cues, a mismatch between the peak of the onset of exogenous feeding of smelt and the peak of zooplankton blooms could lead to high mortality and therefore low recruitment of smelt. Such a mismatch might contribute to a decline in the smelt population in Lake IJsselmeer and Lake Markermeer.

Although fishing with electricity is illegal in the European Union, a number of temporary licences allowed converting beam trawlers to pulse trawling. To analyse how the adaption of pulse trawling changed this fishery, we studied fishing speeds and landings per unit effort as proxies for catch efficiencies for the main target species. Compared to conventional tickler chain beam trawls, pulse trawls were towed at lower speeds (small vessels −10%, large vessels −23%). Large vessels that switched from conventional beam trawls to pulse trawls at the end of 2009 gradually increased catch efficiency for sole over the period of almost 1 year. While pulse trawling was found to have higher catch rates (kg/h) for sole (small vessels +74%, large vessels +17%), lower catch rates were observed for plaice (small vessels −31%, large vessels −32%). Vessels that switched later achieved immediate gains in catch efficiency for sole. The change in catch efficiency is likely due to the difference in cramp response between the species.

Significance We estimated the biological state of seabed sedimentary habitats, with specified uncertainty, in 24 trawled regions worldwide. Seabed status differed greatly among regions (from 0.25 to 0.999, relative to an untrawled state of 1); 15 regions had average status > 0.9. Two-thirds of all assessed seabed area was untrawled with status = 1, 93% had status > 0.8, but 1.5% had status = 0. The total area swept by trawling was a strong driver of regional status, providing a relationship to predict status from the regional estimated total amount of trawling. Seabed status is high in regions where fisheries are exploited sustainably—emphasizing that good fishery management contributes to better ecosystem outcomes—and, conversely, low status highlights regions needing improved management., Bottom trawling is widespread globally and impacts seabed habitats. However, risks from trawling remain unquantified at large scales in most regions. We address these issues by synthesizing evidence on the impacts of different trawl-gear types, seabed recovery rates, and spatial distributions of trawling intensity in a quantitative indicator of biotic status (relative amount of pretrawling biota) for sedimentary habitats, where most bottom-trawling occurs, in 24 regions worldwide. Regional average status relative to an untrawled state (=1) was high (>0.9) in 15 regions, but 0.8. These assessments are first order, based on parameters estimated with uncertainty from meta-analyses; we recommend regional analyses to refine parameters for local specificity. Nevertheless, our results are sufficiently robust to highlight regions needing more effective management to reduce exploitation and improve stock sustainability and seabed environmental status—while also showing seabed status was high (>0.95) in regions where catches of trawled fish stocks meet accepted benchmarks for sustainable exploitation, demonstrating that environmental benefits accrue from effective fisheries management. Furthermore, regional seabed status was related to the proportional area swept by trawling, enabling preliminary predictions of regional status when only the total amount of trawling is known. This research advances seascape-scale understanding of trawl impacts in regions around the world, enables quantitative assessment of sustainability risks, and facilitates implementation of an ecosystem approach to trawl fisheries management globally.

Electrical pulse fishing has been widely adopted by Dutch fishers as an economically viable alternative to tickler-chain trawling for common sole (Solea solea) in the North Sea. Concerns exist, however, that the use of electrical pulses may cause spinal injuries and haemorrhages, as previously shown for Atlantic cod (Gadus morhua). To find out whether other gadoids are similarly affected, we studied injury occurrences in whiting (Merlangius merlangus) catches on commercial vessels. To distinguish mechanically and electrically-induced injuries, we compared (1) injuries for pulse gears with electrical pulses either turned on or off and (2) injuries from pulse-trawl catches with those in tickler-chain trawling. Spinal injuries were visualised with X-radiography and internal haemorrhages with subsequent dissection. Injuries were categorised on a severity scale and their location was quantified along the anteroposterior fish axis. Major spinal injury occurrence in (1) pulses-on and pulses-off samples were lower than 1% and not significantly different between catch methods. Major spinal injury occurrence was slightly higher in (2) tickler-chain catches (2.4%) than in pulses-on samples (1.1%). Major haemorrhage occurrences were also low. The slightly higher occurrences of these haemorrhages in pulses-on samples (1.8%) compared to fish caught with tickler chains (0.3%) and their locations suggest that they may be partly related to electrical-pulse exposure. Overall, our results indicate that injuries in whiting are rare and primarily due to mechanical impact. These findings suggest that pulse trawling is unlikely to impose increased mortality on whiting populations compared to the tickler-chain technique.

Electric stimulation was used in the North Sea beam trawl fishery for common sole to reduce its environmental impact. Because electrical stimulation may cause internal injuries in fish, a laboratory experiment was conducted to study the effect of pulse exposure on lesser sandeel (Ammodytes tobianus) and greater sandeel (Hyperoplus lanceolatus), important mid-trophic species in the North Sea ecosystem. We exposed 244 sandeels between two electrodes to a pulsed bipolar current for 2 s in an experimental cage with 5 cm sediment; 221 control fish were handled similarly but not exposed. The occurrence of spinal injuries and internal haemorrhages were scored using X-radiography and dissection. None of the sandeels exposed to a field strength of up to 600 V m–1 showed spinal injury or haemorrhage. Equal numbers of minor spinal abnormalities were found in exposed and control fish. In the absence of spinal injuries, we estimated by bootstrapping the field strength below which spinal injuries are unlikely to occur, i.e. the lower limit threshold, and the corresponding limit dose–response relationship between field strength and injury probability. We conclude that it is unlikely that pulse trawl fishery will have an ecologically significant adverse effect on the population abundance of sandeels, because of the low probabilities of exposure and injury.

Sexual size dimorphism (SSD) is caused by differences in selection pressures and life-history trade-offs faced by males and females. Proximate causes of SSD may involve sex-specific mortality, energy acquisition, and energy expenditure for maintenance, reproductive tissues, and reproductive behavior. Using a quantitative, individual-based, eco-genetic model parameterized for North Sea plaice, we explore the importance of these mechanisms for female-biased SSD, under which males are smaller and reach sexual maturity earlier than females (common among fish, but also arising in arthropods and mammals). We consider two mechanisms potentially serving as ultimate causes: (a) Male investments in male reproductive behavior might evolve to detract energy resources that would otherwise be available for somatic growth, and (b) diminishing returns on male reproductive investments might evolve to reduce energy acquisition. In general, both of these can bring about smaller male body sizes. We report the following findings. First, higher investments in male reproductive behavior alone cannot explain the North Sea plaice SSD. This is because such higher reproductive investments require increased energy acquisition, which would cause a delay in maturation, leading to male-biased SSD contrary to observations. When accounting for the observed differential (lower) male mortality, maturation is postponed even further, leading to even larger males. Second, diminishing returns on male reproductive investments alone can qualitatively account for the North Sea plaice SSD, even though the quantitative match is imperfect. Third, both mechanisms can be reconciled with, and thus provide a mechanistic basis for, the previously advanced Ghiselin–Reiss hypothesis, according to which smaller males will evolve if their reproductive success is dominated by scramble competition for fertilizing females, as males would consequently invest more in reproduction than growth, potentially implying lower survival rates, and thus relaxing male–male competition. Fourth, a good quantitative fit with the North Sea plaice SSD is achieved by combining both mechanisms while accounting for sex-specific costs males incur during their spawning season. Fifth, evolution caused by fishing is likely to have modified the North Sea plaice SSD.

Bottom trawls impact the seafloor and benthic ecosystem. One of the direct physical impacts is the mobilization of sediment in the wake of trawl gear components that are in contact with or are close to the seabed. The quantity of sediment mobilized is related to the hydrodynamic drag of the gear components and the type of sediment over which they are trawled. Here we present a methodology to estimate the sediment mobilization from hydrodynamic drag. The hydrodynamic drag of individual gear components is estimated using empirical measurements of similarly shaped objects, including cylinders, cubes, and nets. The method is applied to beam trawls used in the Dutch North Sea flatfish fishery and validated using measurements of beam trawl drag from the literature. Netting contributes most to the hydrodynamic drag of pulse trawls, while the tickler chains and chain mat comprise most of the hydrodynamic drag of conventional beam trawls. Taking account of the silt content of the areas trawled and the number of different beam trawl types used by the fleet, sediment mobilization is estimated as 9.2 and 5.3 kg m−2 for conventional 12 m beam and pulse trawls, respectively, and 4.2 and 4.3 kg m−2 for conventional 4.5 m beam and pulse trawls.

Conflicts of interests between economic and nature conservation stakeholders are increasingly common in coastal seas, inducing a growing need for evidence-based marine spatial planning. This requires accurate, high-resolution habitat maps showing the spatial distribution of benthic assemblages and enabling intersections of habitats and anthropogenic activities. However, such detailed maps are often not available because relevant biological data are scarce or poorly integrated. Instead, physiotope maps, solely based on abiotic variables, are now often used in marine spatial planning. Here, we investigated how pointwise, relatively sparse biological data can be integrated with gridded, high-resolution environmental data into informative habitat maps, using the intensively used southern North Sea as a case-study. We first conducted hierarchical clustering to identify discrete biological assemblages for three faunal groups: demersal fish, epifauna, and endobenthos. Using Random Forest models with high-resolution abiotic predictors, we then interpolated the distribution of these assemblages to high resolution grids. Finally, we quantified different anthropogenic pressures for each habitat. Habitat maps comprised a different number of habitats between faunal groups (6, 13, and 10 for demersal fish, epifauna, and endobenthos respectively) but showed similar spatial patterns for each group. Several of these ‘fauna-inclusive’ habitats resembled physiotopes, but substantial differences were also observed, especially when few (6; demersal fish) or most (13; epifauna) physiotopes were delineated. Demersal fishing and offshore wind farms (OWFs) were clearly associated with specific habitats, resulting in unequal anthropogenic pressure between different habitats. Natura-2000 areas were not specifically associated with demersal fishing, but OWFs were situated mostly inside these protected areas. We thus conclude that habitat maps derived from biological datasets that cover relevant faunal groups should be included more in ecology-inclusive marine spatial planning, instead of only using physiotope maps based on abiotic variables. This allows better balancing of nature conservation and socio-economic interests in continental shelf seas.

Bottom trawl fishing is a controversial activity. It yields about a quarter of the world's wild seafood, but also has impacts on the marine environment. Recent advances have quantified and improved understanding of large‐scale impacts of trawling on the seabed. However, such information needs to be coupled with distributions of benthic invertebrates (benthos) to assess whether these populations are being sustained under current trawling regimes. This study collated data from 13 diverse regions of the globe spanning four continents. Within each region, we combined trawl intensity distributions and predicted abundance distributions of benthos groups with impact and recovery parameters for taxonomic classes in a risk assessment model to estimate benthos status. The exposure of 220 predicted benthos‐group distributions to trawling intensity (as swept area ratio) ranged between 0% and 210% (mean = 37%) of abundance. However, benthos status, an indicator of the depleted abundance under chronic trawling pressure as a proportion of untrawled state, ranged between 0.86 and 1 (mean = 0.99), with 78% of benthos groups > 0.95. Mean benthos status was lowest in regions of Europe and Africa, and for taxonomic classes Bivalvia and Gastropoda. Our results demonstrate that while spatial overlap studies can help infer general patterns of potential risk, actual risks cannot be evaluated without using an assessment model that incorporates trawl impact and recovery metrics. These quantitative outputs are essential for sustainability assessments, and together with reference points and thresholds, can help managers ensure use of the marine environment is sustainable under the ecosystem approach to management., T. Mazor was supported during her research by a CSIRO Postdoctoral Fellowship and CSIRO Ruby Payne‐Scott Award.

Bottom trawling in shelf seas can occur more than 10 times per year for a given location. This affects the benthic metabolism, through a mortality of the macrofauna, resuspension of organic matter from the sediment, and alterations of the physical sediment structure. However, the trawling impacts on organic carbon mineralization and associated processes are not well known. Using a modelling approach, the effects of increasing trawling frequencies on early diagenesis were studied in five different sedimentary environments, simulating the effects of a deeper-penetrating gear (e.g. a tickler chain beam trawl) versus a shallower, more variable penetrating gear (e.g. an electric pulse trawl). Trawling events strongly increased oxygen and nitrate concentrations in surface sediment layers and led to significantly lower amounts of ammonium (43 %–99 % reduction) and organic carbon in the top 10 cm of the sediment (62 %–96 % reduction). As a result, total mineralization rates in the sediment were decreased by up to 28 %. The effect on different mineralization processes differed both between sediment types and between trawling frequencies. The shallow-penetrating gear had a slightly smaller effect on benthic denitrification than the deeper-penetrating gear, but there were no statistically different results between gear types for all other parameters. Denitrification was reduced by 69 % in a fine sandy sediment, whereas nitrogen removal nearly doubled in a highly eutrophic mud. This suggests that even relatively low penetration depths from bottom fishing gears generate significant biogeochemical alterations. Physical organic carbon removal through trawl-induced resuspension of sediments, exacerbated by a removal of bioturbating macrofauna, was identified as the main cause of the changes in the mineralization process.

Ecosystem effects of bottom trawl fisheries are a major concern. We analysed whether the replacement of mechanical stimulation by electrical stimulation may reduce the adverse impacts on the benthic ecosystem in the beam trawl fishery for sole. Although the use of electricity is not allowed to catch fish in European Union waters, a number of beam trawlers got derogation and switched to pulse trawling to explore the potential to reduce impacts. We extended a recently developed assessment framework and showed that the switch to pulse trawling substantially reduced benthic impacts when exploiting the Total Allowable Catch of sole in the North Sea. We applied the framework to Dutch beam trawl logbook data from 2009 to 2017 and estimated that the trawling footprint decreased by 23%; the precautionary impact indicator of the benthic community decreased by 39%; the impact on median longevity decreased by 20%; the impact on benthic biomass decreased by 61%; the amount of sediment mobilised decreased by 39%. The decrease is due to the replacement of tickler chains by electrode arrays, a lower towing speed and higher catch efficiency for sole. The effort and benthic impact of the beam trawl fishery targeting plaice Pleuronectes platessa in the central North Sea increased with the recovery of the plaice stock. This study illustrates the usefulness of a standardized methodological framework to assess the differences in time trends and trawling impact between gears.

Ecosystem effects of bottom trawl fisheries are of major concern. Although it is prohibited to catch fish using electricity in European Union waters, a number of beam trawlers obtained a derogation and switched to pulse trawling to explore the potential to reduce impacts. Here we analyse whether using electrical rather than mechanical stimulation results in an overall reduction in physical disturbance of the seafloor in the beam-trawl fishery for soleSolea solea. We extend and apply a recently developed assessment framework to the Dutch beam-trawl fleet and show that the switch to pulse trawling substantially reduced benthic impacts when exploiting the total allowable catch of sole in the North Sea. Using Vessel Monitoring by Satellite and logbook data from 2009 to 2017, we estimate that the trawling footprint decreased by 23%, the precautionary impact indicator of the benthic community decreased by 39%, the impact on median longevity of the benthic community decreased by 20%, the impact on benthic biomass decreased by 61%, and the amount of sediment mobilised decreased by 39%. The decrease in impact is due to the replacement of tickler chains by electrode arrays, a lower towing speed and higher catch efficiency for sole. The effort and benthic physical disturbance of the beam-trawl fishery targeting plaicePleuronectes platessain the central North Sea increased with the recovery of the plaice stock. Our study illustrates the utility of a standardized methodological framework to assess the differences in time trends and physical disturbance between gears.

Knowledge on how fishers exploit their fisheries resources is important for understanding how fishing affects the population dynamics of the exploited species and how the fishery may affect the ecosystem. The introduction of a new gear may affect the way fishers deploy their gear in space and time. We compared the behaviour of pulse trawl vessels to the behaviour of traditional beam trawl vessels. Because the pulse logbook monitoring is still ongoing, the results are preliminary and will be updated when the complete data set will become available. The logbook data set analysed comprised catch and effort information per tow collected between 1 January 2017 and 31 December 2019. The results were compared with an analysis of logbook data of traditional beam trawl vessels collected between 2000 - 2005. This report repeated the analyses of the previous report (data from 01/01/2017-30/08/2018) and found very similar results regarding the differences between fishing patterns of the pulse trawl and the traditional beam trawl. In total, after quality filtering, data of 141,000 hauls from 75 pulse vessels were investigated. The previous study showed that pulse trawl (PT) and traditional beam trawl (BT) vessels had similar fishing patterns with alternating periods of searching, or sampling, for fishing grounds and exploitation of fishing grounds. The main differences with the results of the previous report is that a longer time series and therefore more data has been used to analyse the logbooks. Because of this similarity, the results of these analyses will not be reported in this provisional report but will be discussed in the final report. The cpue (catch per unit effort) of sole follows a clear seasonal pattern and seems to decrease slightly through time. Also the number of logbook received by fishermen clearly decreased as a result of the ban on pulse fisheries by the EU. The logbook data provide detailed information on what happens on the local fishing grounds which is fundamental to assess the impact of the pulse trawl fishery and beam trawl fishery on the fisheries resources and on the benthic ecosystem. The study of the total pulse fleet provides a unique data set to study not only the dynamics of the whole fleet, including the interactions among pulse vessels, but also provides a solid basis to study competitive interactions with other fisheries. The next step is to study the weekly fishery patterns of the fleet as a whole and to study the interaction between vessels. Kennis over hoe vissers hun visgronden exploiteren is essentieel voor het begrijpen hoe de visserij de populatie dynamica van de gevangen soorten en het ecosysteem beïnvloedt. Het introduceren van een nieuw vistuig kan invloed hebben op waar en wanneer vissers gaan vissen. Met ingang van 1 januari 2017 is een begin gemaakt met het registreren van de gedetailleerde vangstgegevens van de pulsschepen. WMR heeft de tot 31 december 2019 verzamelde gegevens geanalyseerd en het visgedrag van pulsschepen vergeleken met dat van traditionele boomkorschepen (verzameld tussen 2000-2005). Dit rapport geeft een update van de analyse van de data verzameld in de periode 1 januari 2017 en 31 december 2019. Doel van het logboekonderzoek is om inzicht te krijgen of het visgedrag van schippers is veranderd door de overgang van wekkertuigen naar pulstuigen. In totaal zijn, na het filteren van de gegevens op kwaliteit, de gegevens van 141.000 trekken van 75 pulsschepen onderzocht. De resultaten van de analyse van de logboeken over de periode 01/01/2017-30/08/2018 verschillen nauwelijks met de resultaten over de periode 01/01/2017-31/12/2019 en worden om deze reden ook niet in dit rapport bediscussieerd. De catch per unit effort (cpue) van tong volgt een duidelijk seizoenspatroon voor de pulsvisserij en lijkt met de tijd licht af te nemen. Daarnaast is ook duidelijk te zien hoe het aantal ingevulde logboeken is afgenomen als gevolg van de besluitvorming van de EU jegens de pulsvisserij. De gepresenteerde resultaten zijn voorlopig en het onderzoek wordt voortgezet. De logboekgegevens geven de gedetailleerde informatie waarmee de lokale visgronden in kaart kunnen worden gebracht en waarmee kan worden onderzocht hoe de lokale visgronden worden geëxploiteerd. Deze informatie is belangrijk om de invloed van de pulskorvisserij op de visbestanden en op het bentische ecosysteem te beoordelen. De volgende stap zal zijn om de wekelijkse visserijpatronen van de hele vloot te analyseren en de interacties tussen visserijvaartuigen te bestuderen.

Dutch demersal fisheries in the North Sea is a mixed fishery that mainly targets Dover sole (Solea solea) with plaice (Pleuronectes platessa), turbot (Scophthalmus maximus), brill (Scophthalmus rhombus) and other species as valuable bycatches. The fleet currently uses two gear types: pulse beam trawls and conventional tickler chain beam trawls. Pulse beam trawlers operate with a temporary exemption from the EU prohibition to use electric stimulation in fishing gears, of which the last exemptions will expire in June 2021.To assess the consequences of transitions between pulse and tickler chain beam trawling for discards mortality, knowledge on the discards survival probabilities as well as the amount of discards is required for both gear types. The objective of the current study was to estimate discards survival probabilities for undersized plaice, sole, turbot, brill and thornback ray discarded by tickler chain beam trawl fisheries using fish condition as a proxy for survival probability. To this end the condition and reflex impairment of undersized fish in the catches of tickler chain beam trawlers were assessed and compared to similar data collected from pulse trawl fisheries. For spotted ray we assessed fish condition in tickler chain beam trawling but could not estimate its discards survival probability because a relation between survival probability and fish condition is lacking for this species. In this study direct mortality imposed by the tickler chain beam trawling ranged between 10 and 32% in flatfish species and was between 2-4 times higher than in pulse beam trawling. Direct mortality in ray species was lowest among the investigated species (2-8%) and did not differ between the two gear types. Differences in direct mortality were reflected in the condition scores. Direct mortality of sole was higher in tickler chain beam trawling (17%) than in pulse beam trawling (8%). Brill, turbot and plaice discarded by pulse beam trawling are in better condition than when discarded by tickler chain beam trawl fisheries. For sole no effect of gear type on fish condition could be detected. We consider the lower fish condition scores of brill, plaice and turbot from tickler chain beam trawling a direct reflection of the higher mechanical impact of this gear on the fish. For thornback ray and spotted ray no effect of gear type on fish condition could be detected. The predicted survival of plaice, brill and turbot discards indicate that discards survival could indeed be lower in tickler chain beam trawl fisheries compared to pulse beam trawl fisheries. For sole and thornback ray discards we found no evidence for such difference between gear types. The discards survival probabilities for tickler chain beam trawling as presented in this study should be considered as predictions based on the currently best available information instead of definite values. Actual measurements of discards survival at sea are needed to confirm and quantify survival probabilities in tickler chain beam trawling.

Bottom trawls impact the seafloor and benthic ecosystem. To estimate the trawling impact, information is required about the dimensions of the gear that determine their footprint, sediment penetration depth and hydrodynamic drag that determines the amount of sediment mobilised in the wake of the trawl. Here we present the dimension of gear components of the different beam trawls used in the North Sea flatfish fishery including the traditional tickler chain beam trawl, chain-mat trawl and pulse trawls. The hydrodynamic drag of the gears is estimated by summing the drag of different gear components using empirical equations that describe the hydrodynamic drag of different shaped objects, including cylinders, blocks and fish nets. Netting contributes most to the hydrodynamic drag, followed by the ground rope, tickler chains and chain mat. The hydrodynamic drag of bottom components, which determines sediment mobilisation, is estimated at 6.2 and 6.3 kN.m-1 for a traditional tickler chain and chain-mat trawl, and 3.8 kN.m-1 for a pulse trawler. Drag of pulse trawls depends on their rigging and ranges between 2.8 – 3.2 kN.m-1 for the rectangular ground rope types and 4.0 – 4.1 kN.m-1 for the sole rope type. The amount of sediment mobilised in a seabed habitat with 20% silt content is 24 kg.m-2 for a large tickler chain and chain-mat trawler and between 12 – 16 kg.m-2 for a large pulse trawler.

The pulse trawl is presented as an alternative for the traditional tickler chain beam trawl. The beam trawl has a direct lethal effect on the benthic community, which is much less for the pulse trawl. However, the pulse trawl, due to its electrical field has non-lethal effects on the benthic community. The existence of such non-lethal effects have been demonstrated for different organisms and most often involve a short period of inactivity. Such a period of inactivity may have consequences at the population level as it may limit feeding. Yet the population dynamical consequences, nor food web level consequences, have been studied in experimental or field settings. To study non-lethal and lethal effects of trawling on the benthic community a model was developed which is based on a trait-based characterization of the food web. This model describes the equilibrium dynamics of predators, filter feeders and deposit feeders. The model includes direct mortality as a function of fishing mortality and a non-lethal effect, which was incorporated as a decrease of the maximum food intake rate. This approach was based on experiments with electric pulse and benthic invertebrates. We conducted a comparative analysis of lethal and non-lethal impact of trawling on the equilibrium dynamics of the benthic food web. Only lethal effects represent a tickler chain beam trawl and a combination of lethal and non-lethal effect represents a pulse trawl. We first studied the effects of lethal and non-lethal impacts on the food web separately. We then studied the trade-off between lethal and non-lethal effects for a given total impact on the community. This approach shows how much non-lethal impact is required to compensate for a decrease in lethal effects of trawling. The study shows that lethal effects need to be replaced by non-lethal effects with similar strength in order to maintain the effect of fishing intensity on the food web. Such strong non-lethal effects are unlikely to occur in the field based on experiments assessing the impact of electricity on benthic macro-invertebrates and trawling intensity. In other words, the combined non-lethal and lethal effects of the pulse trawl are unlikely to have a similar effect on the macro-invertebrate benthic community compared to the lethal effects of the traditional trawl.

Bottom trawl fisheries are the most widespread source of anthropogenic physical disturbance to seabed habitats. Development of fisheries-, conservation- and ecosystem-based management strategies requires the selection of indicators of the impact of bottom trawling on the state of benthic biota. Many indicators have been proposed, but no rigorous test of a range of candidate indicators against nine commonly agreed criteria (concreteness, theoretical basis, public awareness, cost, measurement, historical data, sensitivity, responsiveness, specificity) has been performed. Here, we collated data from 41 studies that compared the benthic biota in trawled areas with those in control locations (that were either not trawled or trawled infrequently), examining seven potential indicators (numbers and biomass for individual taxa and whole communities, evenness, Shannon–Wiener diversity and species richness) to assess their performance against the set of nine criteria. The effects of trawling were stronger on whole-community numbers and biomass than for individual taxa. Species richness was also negatively affected by trawling but other measures of diversity were not. Community numbers and biomass met all criteria, taxa numbers and biomass and species richness satisfied most criteria, but evenness and Shannon–Wiener diversity did not respond to trawling and only met few criteria, and hence are not suitable state indicators of the effect of bottom trawling. Synthesis and applications. An evaluation of each candidate indicator against a commonly agreed suite of desirable properties coupled with the outputs of our meta-analysis showed that whole-community numbers of individuals and biomass are the most suitable indicators of bottom trawling impacts as they performed well on all criteria. Strengths of these indicators are that they respond strongly to trawling, relate directly to ecosystem functioning and are straightforward to measure. Evenness and Shannon–Wiener diversity are not responsive to trawling and unsuitable for the monitoring and assessment of bottom trawl impacts. Fil: Hiddink, Jan Geert. Bangor University; Reino Unido Fil: Kaiser, Michel J.. Bangor University; Fil: Sciberras, Marija. Bangor University; Fil: McConnaughey, Robert A.. National Marine Fisheries Service; Estados Unidos Fil: Mazor, Tessa. The University of Queensland; Australia. Csiro Oceans And Atmosphere.; Australia Fil: Hilborn, Ray. University of Washington; Estados Unidos Fil: Collie, Jeremy S.. University of Rhode Island; Estados Unidos Fil: Pitcher, Clifford Roland. Csiro Oceans And Atmosphere.; Australia Fil: Parma, Ana María. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Centro Nacional Patagónico; Argentina Fil: Suuronen, Petri. Natural Resources Institute Finland; Finlandia Fil: Rijnsdorp, Adriaan D.. University of Agriculture Wageningen; Países Bajos Fil: Jennings, Simon. University of East Anglia; Reino Unido. International Council for the Exploration of the Sea; Dinamarca. Centre for Environment, Fisheries and Aquaculture Science. Lowestoft Laboratory; Reino Unido

Bottom trawling accounts for almost one quarter of global fish landings but may also have significant and unwanted impacts on seabed habitats and biota. Management measures and voluntary industry actions can reduce these impacts, helping to meet sustainability objectives for fisheries, conservation and environmental management. These include changes in gear design and operation of trawls, spatial controls, impact quotas and effort controls. We review nine different measures and actions and use published studies and a simple conceptual model to evaluate and compare their performance. The risks and benefits of these management measures depend on the extent to which the fishery is already achieving management objectives for target stocks and the characteristics of the management system that is already in place. We offer guidance on identifying best practices for trawl-fisheries management and show that best practices and their likelihood of reducing trawling impacts depend on local, national and regional management objectives and priorities, societal values and resources for implementation. There is no universal best practice, and multiple management measures and industry actions are required to meet sustainability objectives and improve trade-offs between food production and environmental protection. Fil: McConnaughey, Robert A.. National Marine Fisheries Service; Estados Unidos Fil: Hiddink, Jan G.. Bangor University; Reino Unido Fil: Jennings, Simon. University of East Anglia; Reino Unido. International Council for the Exploration of the Sea; Dinamarca Fil: Pitcher, Clifford Roland. Csiro Oceans And Atmosphere; Australia Fil: Kaiser, Michel J.. Heriot-watt University; Reino Unido Fil: Suuronen, Petri. Natural Resources Institute Finland ; Finlandia Fil: Sciberras, Marija. Bangor University; Reino Unido Fil: Rijnsdorp, Adriaan D.. Wageningen Marine Research; Países Bajos Fil: Collie, Jeremy S.. University Of Rhode Island ; Estados Unidos Fil: Mazor, Tessa. University of Queensland; Australia Fil: Amoroso, Ricardo Oscar. University Of Washington, Seattle; Estados Unidos Fil: Parma, Ana María. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Centro Nacional Patagónico. Centro para el Estudio de Sistemas Marinos; Argentina Fil: Hilborn, Ray. University Of Washington, Seattle; Estados Unidos

Bottom trawling disturbance and hypoxia are affecting marine benthic habitats worldwide. We present an approach to predict their effects on benthic communities, and use the approach to estimate the state, the biomass relative to carrying capacity, of the Baltic Sea at the local, habitat, and regional scale. Responses to both pressures are expected to depend on the longevity of fauna, which is predicted from benthic data from 1558 locations. We find that communities in low-salinity regions mostly consist of short-lived species, which are, in our model, more resilient than those of the saline areas. The model predicts that in 14% of the Baltic Sea region benthic biomass is reduced by at least 50%, whereas an additional 8% of the region has reductions of 10–50%. The effects of hypoxia occur over larger spatial scales and lead to a low state of especially deep habitats. The approach is based on a simple characterization of the benthic community, which comes with high uncertainty, but allows for the identification of benthic habitats that are at greatest risk and prioritization of management actions at the regional scale. This information supports the development of sustainable approaches to manage impact of human activities on benthic ecosystems.

High-resolution vessel monitoring (VMS) data have led to detailed estimates of the distribution of fishing in both time and space. While several studies have documented large-scale changes in fishing distribution, fine-scale patterns are still poorly documented, despite VMS data allowing for such analyses. We apply a methodology that can explain and predict effort allocation at fine spatial scales; a scale relevant to assess impact on the benthic ecosystem. This study uses VMS data to quantify the stability of fishing grounds (i.e. aggregated fishing effort) at a microscale (tens of meters). The model links effort registered at a large scale (ICES rectangle; 1° longitude × 0.5° latitude, ˜3600 km2) to fine spatial trawling intensities at a local scale (i.e. scale matching gear width, here 24 m). For the first time in the literature, the method estimates the part of an ICES rectangle that is unfavourable or inaccessible for fisheries, which is shown to be highly stable over time and suggests higher proportions of inaccessible grounds for either extremely muddy or courser substrates. The study furthermore shows high stability in aggregation of fishing, where aggregation shows a positive relationship with depth heterogeneity and a negative relationship with year-on-year variability in fishing intensity.

International audience; The performance of a combined catch quota and habitat credit system was explored to manage the sustainable exploitation of a mix of demersal fish species and reduce the benthic impacts of bottom trawl fisheries using a dynamic state variable model approach. The model was parameterised for the Eastern English Channel demersal mixed fishery using otter trawls or dredges. Target species differed in their association with habitat types. Restricting catch quota for plaice and cod had a limited effect on benthic impact, except when reduced to very low values, forcing the vessels to stay in port. Quota management had a minimal influence on fishing behaviour and hence resulted in a minimal reduction of benthic impact. Habitat credits may reduce the benthic impacts of the trawl fisheries at a minimal loss of landings and revenue, as vessels are still able to reallocate their effort to less vulnerable fishing grounds, while allowing the fishery to catch their catch quota and maintain their revenue. Only if they are reduced to extremely low levels can habitat credits potentially constrain fishing activities to levels that prevent the fisheries from using up the catch quota for the target species.

Bottom-contact fishing gears are globally the most widespread anthropogenic sources of direct disturbance to the seabed and associated biota. Managing these fishing disturbances requires quantification of gear impacts on biota and the rate of recovery following disturbance. We undertook a systematic review and meta-analysis of 122 experiments on the effects-of-bottom fishing to quantify the removal of benthos in the path of the fishing gear and to estimate rates of recovery following disturbance. A gear pass reduced benthic invertebrate abundance by 26% and species richness by 19%. The effect was strongly gear-specific, with gears that penetrate deeper into the sediment having a significantly larger impact than those that penetrate less. Sediment composition (% mud and presence of biogenic habitat) and the history of fishing disturbance prior to an experimental fishing event were also important predictors of depletion, with communities in areas that were not previously fished, predominantly muddy or biogenic habitats being more strongly affected by fishing. Sessile and low mobility biota with longer life-spans such as sponges, soft corals and bivalves took much longer to recover after fishing (>3 year) than mobile biota with shorter life-spans such as polychaetes and malacostracans (

The beam trawl fishery targeting sole is known for their substantial bycatch of flatfish below the minimum landing size. Pulse trawls were developed to improve the selectivity by replacing mechanical stimulation with electrical stimulation which immobilises fish in front of the footrope. Results are presented of an experiment on board of a commercial pulse trawler studying the effect of electrical stimulation on footrope and cod-end selectivity for three flatfish species - sole (Solea solea), plaice (Pleuronectes platessa) and dab (Limanda limanda) - in 29 paired hauls with electrical stimulation alternating between the starboard and portside gear. It was shown that electrical stimulation increased the footrope selection by a factor 2 in plaice and dab and a factor 7 in sole. The effect on sole is related to the specific response of sole which cramps into a U-shape. Footrope selection showed a small diurnal pattern with the highest selectivity of the pulse trawl during the day. Electrical stimulation and catch weight were shown to have a small effect on the cod-end selectivity retaining slightly more marketable sole. Cod-end mesh selection factors (SF) were estimated at 3.00 (se = 0.02), 2.11 (se = 0.02), and 2.3 (se = 0.1) for sole, plaice and dab, respectively. Selection ratio (SFA = selection range/mesh size) was estimated at 0.45 (se = 0.03), 0.23 (se = 0.02), and 0.41 (se = 0.04) for sole, plaice and dab, respectively. The SF and SFA of the pulse trawl is comparable to values reported for conventional beam or otter trawls.

One quarter of marine fish production is caught with bottom trawls and dredges on continental shelves around the world. Towed bottom-fishing gears typically kill 20-50 per cent of the benthic invertebrates in their path, depending on gear type, substrate and vulnerability of particular taxa. Particularly vulnerable are epifaunal species, which stabilize the sediment and provide habitat for benthic invertebrates. To identify the habitats, fisheries or target species most likely to be affected, we review evidence of the indirect effects of bottom fishing on fish production. Recent studies have found differences in the diets of certain species in relation to bottom fishing intensity, thereby linking demersal fish to their benthic habitats at spatial scales of ~10 km. Bottom fishing affects diet composition and prey quality rather than the amount of prey consumed; scavenging of discarded by-catch makes only a small contribution to yearly food intake. Flatfish may benefit from light trawling levels on sandy seabeds, while higher-intensity trawling on more vulnerable habitats has a negative effect. Models suggest that reduction in the carrying capacity of habitats by bottom fishing could lead to lower equilibrium yield and a lower level of fishing mortality to obtain maximum yield. Trawling effort is patchily distributed - small fractions of fishing grounds are heavily fished, while large fractions are lightly fished or unfished. This patchiness, coupled with the foraging behaviour of demersal fish, may mitigate the indirect effects of bottom fishing on fish productivity. Current research attempts to scale up these localized effects to the population level.

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.

The tube-building polychaete Sabellaria spinulosa (Ross worm) can form conspicuous biogenic reefs that stabilize the seabed and increase biodiversity by providing a habitat for a multitude of other species. These reefs, however, are assumed to be vulnerable to human-induced physical disturbances of the seabed. In the Greater North Sea, S. spinulosa reefs are recognized to be under threat and worthy of protection. In August 2017, three S. spinulosa reefs with a minimum extent of 1016 m2 were discovered in the Dutch Brown Bank area. This area comprises a large-scale sandbank and adjacent troughs. The reefs were found within the sandbank troughs, which have proven to be subject to high demersal fishing intensities (fished >5 times a year). Detailed bathymetry measurements showed that S. spinulosa reefs were mainly located within valleys of smaller-scaled sand waves, which have a perpendicular orientation compared to the large-scale sandbank structure of the Brown Bank. We hypothesize that the valleys in between sand waves offer suitable substrate for settlement and refuge from abrasion by fishing activities, enabling the S. spinulosa reefs to persist despite high fishing intensities. ROV footage of the reefs showed higher estimates of species abundances on the reefs compared with adjacent habitats, with some species present that are typical for hard substrate (rock gunnel, Pholisgunnellus; edible crab, Cancer pagurus; and velvet swimming crab, Necora puber). The information presented could be used for drafting management policies to protect these reefs, as Contracting Parties of the OSPAR Convention are committed to take measures and protect biodiversity.

Bottom trawling is the most widespread human activity directly affecting seabed habitats. Assessment and effective management of the effects of bottom trawling at the scale of fisheries requires an understanding of differences in sensitivity of biota to trawling. Responses to disturbance are expected to depend on the intrinsic rate of increase in populations (r), which is expected to be linearly related to the reciprocal of longevity. We examine the relationship between the longevity of benthic invertebrates and their response to bottom trawling; both in terms of the immediate mortality following a trawl pass and their subsequent rates of recovery. We collate all available data from experimental and comparative trawling studies, and test how longevity influences these aspects of sensitivity. The shortest lived organisms (1 year decreased by ~9% immediately following a trawl pass. The effect of bottom trawling in comparative studies increased with longevity, with a 2–3× larger effect on biota living >10 years than on biota living 1–3 years. We attribute this difference to the slower recovery rates of the long-lived biota. The observed relationship between the intrinsic rate of population increase (r, our metric of recovery rate) and the reciprocal of longevity matches theoretical expectation and predicts that the sensitivity of habitats to bottom trawling is higher in habitats with higher proportions of long-lived organisms. Synthesis and applications. Where the longevity of a species or the longevity distribution of a community is known or can be inferred, our estimates of depletion and intrinsic rate of increase can be combined with high-resolution maps of trawling intensity to assess trawling impacts at the scale of the fishery or other defined unit of assessment. Our estimates of r may also be used to estimate recovery times following other forms of seabed disturbance. Fil: Hiddink, Jan Geert. Fil: Jennings, Simon. Fil: Sciberras, Marija. Fil: Bolam, Stefan G.. Fil: Cambiè, Giulia. Fil: McConnaughey, Robert A.. Fil: Mazor, Tessa. Fil: Hilborn, Ray. Fil: Collie, Jeremy S.. Fil: Pitcher, Clifford Roland. Fil: Parma, Ana María. Fil: Suuronen, Petri. Fil: Kaiser, Michel J.. Fil: Rijnsdorp, Adriaan D..

There is growing evidence that fishing causes evolution in life-history traits that affect the productivity of fish stocks. Here we explore the impact of fisheries-induced evolution (FIE) on the productivity of North Sea plaice (Pleuronectes platessa) using an ecogenetic, individual-based model by comparing management scenarios with and without an evolutionary response. Under status-quo management, plaice evolve towards smaller size at age, earlier maturation, and higher reproductive investment. Current reference points of maximum sustainable yield (MSY) and corresponding fishing-mortality rate (FMSY) that ignore FIE will decrease and cannot be considered sustainable. The nature and extent of the change through FIE depend on fishing effort and selectivity. The adverse evolutionary effects can be reduced — and even reversed — by implementing a dome-shaped exploitation pattern protecting the large fish. The evolutionarily sustainable maximum yield can be obtained by combining such a dome-shaped exploitation pattern with a reduction in fishing mortality and an increase in mesh size; it is similar to the MSY that would apply if life-history traits were static. Fisheries managers will need to trade off the short-term loss in yield associated with evolutionarily informed management with the long-term loss in yield FIE causes under evolutionarily uninformed management.

In the North Sea flatfish fishery, electric pulse trawls have been introduced to replace the conventional mechanical method. Pulse trawls reduce the fuel consumption, reduce adverse impact on the ecosystem but cause injuries in gadoids. We describe the design and electrical properties of pulse trawls currently in use and study the behavioural response and injuries in cod exposed to electrical pulses under controlled conditions. Pulse trawls operate at an average power of 0.7 kW m−1 beam length and a duty cycle of ∼2%. The electric field is heterogeneous with highest field strength occurring close to the conductors. Cod were exposed to three different pulse types for a range of field strengths, frequencies, and duty cycles. Two size classes were tested representing cod that escape through the meshes (11–17 cm) and market-sized cod that are retained in the net (34–56 cm). Cod exposed to a field strength of ≥37 V m−1 responded by moderate-to-strong muscular contractions. Some of the large cod (n = 260) developed haemorrhages and fractures in the spine, and haemal and neural arches in the tail part of the body. The probability of injuries increased with field strength and decreased when frequency was increased from 100 to 180 Hz. None of the small cod (n = 132) were injured and all survived. The field strength at the lateral boundaries of the trawl was too low to inflict injuries in cod.

This study assesses the seabed pressure of towed fishing gears and models the physical impact (area and depth of seabed penetration) from trip-based information of vessel size, gear type, and catch. Traditionally fishing pressures are calculated top-down by making use of large-scale statistics such as logbook data. Here, we take a different approach starting from the gear itself (design and dimensions) to estimate the physical interactions with the seabed at the level of the individual fishing operation. We defined 14 distinct towed gear groups in European waters (eight otter trawl groups, three beam trawl groups, two demersal seine groups, and one dredge group), for which we established gear “footprints”. The footprint of a gear is defined as the relative contribution from individual larger gear components, such as trawl doors, sweeps, and groundgear, to the total area and severity of the gear's impact. An industry-based survey covering 13 countries provided the basis for estimating the relative impact-area contributions from individual gear components, whereas sediment penetration was estimated based on a literature review. For each gear group, a vessel size–gear size relationship was estimated to enable the prediction of gear footprint area and sediment penetration from vessel size. Application of these relationships with average vessel sizes and towing speeds provided hourly swept-area estimates by métier. Scottish seining has the largest overall gear footprint of ∼1.6 km 2 h −1 of which 0.08 km 2 has an impact at the subsurface level (sediment penetration ≥ 2 cm). Beam trawling for flatfish ranks low when comparing overall footprint size/hour but ranks substantially higher when comparing only impact at the subsurface level (0.19 km 2 h −1 ). These results have substantial implications for the definition, estimation, and monitoring of fishing pressure indicators, which are discussed in the context of an ecosystem approach to fisheries management.

Significance We conducted a systematic, high-resolution analysis of bottom trawl fishing footprints for 24 regions on continental shelves and slopes of five continents and New Zealand. The proportion of seabed trawled varied >200-fold among regions (from 0.4 to 80.7% of area to a depth of 1,000 m). Within 18 regions, more than two-thirds of seabed area remained untrawled during study periods of 2–6 years. Relationships between metrics of total trawling activity and footprint were strong and positive, providing a method to estimate trawling footprints for regions where high-resolution data are not available. Trawling footprints were generally smaller in regions where fisheries met targets for exploitation rates, implying collateral environmental benefits of effective fisheries management., Bottom trawlers land around 19 million tons of fish and invertebrates annually, almost one-quarter of wild marine landings. The extent of bottom trawling footprint (seabed area trawled at least once in a specified region and time period) is often contested but poorly described. We quantify footprints using high-resolution satellite vessel monitoring system (VMS) and logbook data on 24 continental shelves and slopes to 1,000-m depth over at least 2 years. Trawling footprint varied markedly among regions: from 50% in some European seas. Overall, 14% of the 7.8 million-km2 study area was trawled, and 86% was not trawled. Trawling activity was aggregated; the most intensively trawled areas accounting for 90% of activity comprised 77% of footprint on average. Regional swept area ratio (SAR; ratio of total swept area trawled annually to total area of region, a metric of trawling intensity) and footprint area were related, providing an approach to estimate regional trawling footprints when high-resolution spatial data are unavailable. If SAR was ≤0.1, as in 8 of 24 regions, there was >95% probability that >90% of seabed was not trawled. If SAR was 7.9, equal to the highest SAR recorded, there was >95% probability that >70% of seabed was trawled. Footprints were smaller and SAR was ≤0.25 in regions where fishing rates consistently met international sustainability benchmarks for fish stocks, implying collateral environmental benefits from sustainable fishing.

Based on a European union request, the International Council for the Exploration of the Sea (ICES) has explored and advised on indicators of pressure and impact of bottom trawling on the seabed, and of trade-offs in the catch and value of landings. Such assessment frameworks combine pressure (trawling intensity) with benthic habitats and their sensitivity to estimate the impacts on regional scale. The main differences between the indicators lay in their underlying scientific basis for determining sensitivity. Two approaches, the longevity and the population dynamic approach, are based on a statistical prediction of the longevity distribution of the benthic community, which is a practically very attractive way to capture sensitivity to trawling. The BH3 approach uses a categorical scoring approach for sensitivity. The BH2 relies on empirical observations to estimate changes in Margaleff diversity. In general, the use of continuous environmental variables, rather than the EUNIS habitat classes seemed promising for assessing impacts as they result in larger gradients in the sensitivity layer. Major steps forward in assessing the impact of bottom trawling were made, but improved parameterization and further ground-truthing is required, which will be further taken up in the ICES working group WGFBIT.

Historic hunting has led to severe reductions of many marine mammal species across the globe. After hunting ceased, some populations have recovered to pre-exploitation levels, and may again act as a top-down regulatory force on marine ecosystems. Also the harbour seal population in the international Wadden Sea grew at an exponential rate following a ban on seal hunting in 1960’s, and the current number ∼38,000 is close to the historic population size. Here we estimate the impact of the harbour seal predation on the fish community in the Wadden Sea and nearby coastal waters.Fish remains in faecal samples and published estimates on the seal’s daily energy requirement were used to estimate prey selection and the magnitude of seal consumption. Estimates on prey abundance were derived from demersal fish surveys, and fish growth was estimated using a Dynamic Energy Budget model. GPS tracking provided information on where seals most likely caught their prey.Harbour seals from the Dutch Wadden Sea fed predominantly on demersal fish, e.g. flatfish species (flounder, sole, plaice, dab), but also sandeel, cod and whiting. Total fish biomass in the Wadden Sea was insufficient to sustain the estimated prey consumption of the entire seal population year-round. This probably explains why seals also acquire prey further offshore in the adjacent North Sea, only spending 13% of their diving time in the Wadden Sea. Still, seal predation was estimated to cause an average annual mortality of 43% and 60% on fish in the Wadden Sea and adjacent coastal zone, respectively. There were however large sources of uncertainty in the estimate, including the migration of fish between the North Sea and Wadden Sea, and catchability estimates of the fish survey sampling gear, particularly for sandeel and other pelagic fish species.Our estimate suggested a considerable top-down control by harbour seals on demersal fish. However predation by seals may also alleviate density-dependent competition between the remaining fish, increasing fish growth, and partly compensating for the reduction in fish numbers. This study shows that recovering coastal marine mammal populations could potentially become an important component in the functioning of shallow coastal systems.

INTRODUCTION: The future protection of marine biodiversity through good conservation planning requires both the identification of key habitats with unique ecological characteristics and detailed knowledge of their human utilization through fisheries. Demersal fisheries are important disturbers of benthic habitats. They often have a heterogeneous spatial distribution, pressurizing particular habitats with high abundances of target species. For the North Sea, we quantified the commonness/rarity of habitats in relation to the environmental determinants of so-called fishing hotspots, to support better-informed conservation planning of benthic habitats in this intensively used continental shelf. METHODS: We first distinguished 9 main seascapes in the study area based on seabed morphology. Secondly, we determined average fishing intensity and fishing hotspots using VMS-data for the three dominant Dutch fisheries from 2008 to 2015: beam-trawlers targeting sole Solea solea (Beam-Sole), beam-trawlers targeting plaice Pleuronectes platessa (Beam-Plaice), and otter-trawlers targeting Norway lobster Nephrops norvegicus and demersal fish (Otter-Mix). Within the seascapes subjected to >80% of the fishing activity, nineteen environmental factors (summarized by PCA) were used to ecologically characterize fishing hotspot locations using MaxEnt response modelling. RESULTS: We found that all three fisheries target highly specific, uncommon habitats. Beam-Sole fishers targeted warmer, shallow, dynamic, nearshore habitats, and within these specifically the depressions between sand ridges. Beam-Plaice fishers mainly targeted the exposed, non-muddy flanks of the Dogger Bank and similar large-scale elevations (50-75 km) where especially the ridges of smaller sand banks are used. Otter-Mix fisheries concentrated in areas with low bed shear stress, located in muddy, relatively deeper areas. IMPLICATIONS: This study is the first to provide insight in benthic habitat types that are frequently targeted by fishers in the North Sea. We demonstrated unequal exploitation pressure between seabed habitats, with the majority of hotspots in the less common habitats. Our results hence contribute to a more effective, ecologically informed planning for the protection and monitoring of all seabed habitats and biodiversity of the North Sea.

Bottom fishing such as trawling and dredging may pose serious risks to the seabed and benthic habitats, calling for a quantitative assessment method to evaluate the impact and guide management to develop mitigation measures. We provide a method to estimate the sensitivity of benthic habitats based on the longevity composition of the invertebrate community. We hypothesize that long‐lived species are more sensitive to trawling mortality due to their lower pace of life (i.e., slower growth, late maturation). We analyze data from box‐core and grab samples taken from 401 stations in the English Channel and southern North Sea to estimate the habitat‐specific longevity composition of the benthic invertebrate community and of specific functional groups (i.e., suspension feeders and bioturbators), and examine how bottom trawling affects the longevity biomass composition. The longevity biomass composition differed between habitats governed by differences in sediment composition (gravel and mud content) and tidal bed‐shear stress. The biomass proportion of long‐lived species increased with gravel content and decreased with mud content and shear stress. Bioturbators had a higher median longevity than suspension feeders. Trawling, in particular by gears that penetrate the seabed >2 cm, shifted the community toward shorter‐lived species. Changes from bottom trawling were highest in habitats with many long‐lived species (hence increasing with gravel content, decreasing with mud content). Benthic communities in high shear stress habitats were less affected by bottom trawling. Using these relationships, we predicted the sensitivity of the benthic community from bottom trawling impact at large spatial scale (the North Sea). We derived different benthic sensitivity metrics that provide a basis to estimate indicators of trawling impact on a continuous scale for the total community and specific functional groups. In combination with high resolution data of trawling pressure, our approach can be used to monitor and assess trawling impact and seabed status at the scale of the region or broadscale habitat and to compare the environmental impact of bottom‐contacting fishing gears across fisheries.

Local adaptation is often found to be in a delicate balance with gene flow in marine species with high dispersal potential. Genotyping with mapped transcriptome-derived markers and advanced seascape statistical analyses are proven tools to uncover the genomic basis of biologically relevant traits under environmental selection. Using a panel of 426 gene-linked single nucleotide polymorphisms (SNPs), we scanned 17 samples (n¼539) of sole (Solea solea L.) from the Northeast Atlantic Ocean and applied a node-based seascape analysis. Neutral loci confirmed a clear distinction between the North Sea–Baltic Sea transition zone and the other Eastern Atlantic samples. At a more subtle level, the latter unit split in an English Channel and North Sea group, and a Bay of Biscay and Atlantic Iberian coast group. A fourth group, the Irish and Celtic Sea, was identified with 19 outlier loci. A pattern of isolation by distance (IBD) characterized the latitudinal distribution. Seascape analyses identified winter seawater temperature, food availability and coastal currents to explain a significant component of geographically distributed genetic variation, suggesting that these factors act as drivers of local adaptation. The evidence for local adaptation is in line with the current understanding on the impact of two key ecological factors, the life-history trait winter mortality and the behaviour of inshore/offshore spawning. We conclude that the subtle differentiation between two metapopulations (North Sea and Bay of Biscay) mirrors local adaptation. At least three genomic regions with strong population differentiation point to locally divergent selection. Further functional characterization of these genomic regions should help with formulating adaptive management policies.

Concern about fisheries impact on marine ecosystems has raised the interest in the reconstruction of the state of marine ecosystems and the nature of the human activities in the past. We present late 16th century information on the occurrence and relative abundance of biota in Dutch coastal and inland waters (50 marine fish, 13 diadromous or freshwater and 4 marine mammal species), as well as a description of the sea fisheries (target species, fishing grounds, gear), fish trade, export, and fish consumption in Holland as documented in the handwritten Fish Book by Adriaen Coenen (1577–1581). The species composition and abundances are compared to published trawl survey data from around 1900 and in the 1990s. Fish species that have disappeared almost completely, were already rare around 1900 and are characterised by a large body size (rays and sharks, sturgeon, ling), whereas currently abundant species were already abundant in the 16th century. Intensive fisheries for herring occurred near Orkney, Fairhill and Shetland. Coastal and freshwater fisheries provided fresh fish for local as well as export markets, but also provided bait for the massive offshore hook and line fishery for the production of salted cod, which remained largely unnoticed. Dried flatfish were exported to Germany. Consumption of fish and marine invertebrates differed between social classes. Coenen distinguished eight consumer categories, a refinement of the categories ‘rich’ and ‘poor’ used in archaeological studies.

High-grading is the decision by fishers to discard fish of low value that allows them to land more valuable fish. A literature review showed high-grading is reported in commercial and non-commercial fisheries around the world, although the number of observations is small. High-grading occurs in fisheries that are restricted to land their total catch due to management, market or physical constraints. Using the mixed flatfish fishery as a model system, a dynamic state variable model simulation showed that high-grading of certain grades occurs throughout the year when their ex-vessel price is low. High-grading increases with the degree of quota restriction, while the level of over-quota discarding is unrelated to the quota level. The size composition of the high-graded catch differs from the landed catch. Due to the differences in the seasonal variation in size specific ex-vessel price, the effect of quota restrictions on the size composition of the discarded catch is non-linear. High-grading is difficult to detect for the fishery inspection as it occurs on-board during the short period when the catch is processed. We conclude that high-grading is under-reported in fish stocks managed by restrictive quota, undermining the quality of stock assessments and sustainable management of exploited fish stocks.

Bottom trawling is the most widespread human activity affecting seabed habitats. Here, we collate all available data for experimental and comparative studies of trawling impacts on whole communities of seabed macroinvertebrates on sedimentary habitats and develop widely applicable methods to estimate depletion and recovery rates of biota after trawling. Depletion of biota and trawl penetration into the seabed are highly correlated. Otter trawls caused the least depletion, removing 6% of biota per pass and penetrating the seabed on average down to 2.4 cm, whereas hydraulic dredges caused the most depletion, removing 41% of biota and penetrating the seabed on average 16.1 cm. Median recovery times posttrawling (from 50 to 95% of unimpacted biomass) ranged between 1.9 and 6.4 y. By accounting for the effects of penetration depth, environmental variation, and uncertainty, the models explained much of the variability of depletion and recovery estimates from single studies. Coupled with large-scale, high-resolution maps of trawling frequency and habitat, our estimates of depletion and recovery rates enable the assessment of trawling impacts on unprecedented spatial scales. Fil: Hiddink, Jan Geert. Bangor University; Reino Unido Fil: Jennings, Simon. Centre for the Environment; Reino Unido. University of East Anglia; Reino Unido Fil: Sciberras, Marija. Bangor University; Reino Unido Fil: Szostek, Claire L.. Bangor University; Reino Unido Fil: Hughes, Kathryn M.. Bangor University; Reino Unido Fil: Ellis, Nick. Commonwealth Scientific and Industrial Research Organization Oceans & Atmosphere; Australia Fil: Rijnsdorp, Adriaan D.. Institute for Marine Resources and Ecosystem Studies; Países Bajos Fil: McConnaughey, Robert A.. National Oceanic and Atmospheric Administration; Estados Unidos Fil: Mazor, Tessa. Commonwealth Scientific and Industrial Research Organization Oceans & Atmosphere; Australia Fil: Hilborn, Ray. University of Washington; Estados Unidos Fil: Collie, Jeremy S.. University of Rhode Island; Estados Unidos Fil: Pitcher, C. Roland. Commonwealth Scientific and Industrial Research Organization Oceans & Atmosphere; Australia Fil: Amoroso, Ricardo Oscar. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Centro Nacional Patagónico; Argentina Fil: Parma, Ana María. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Centro Nacional Patagónico; Argentina Fil: Suuronen, Petri. Food and Agriculture Organisation of the United Nations; Italia Fil: Kaiser, Michel J.. Bangor University; Reino Unido