Molluscan diversity caught by trawling fisheries: a case study in southern Portugal

Abstract  The effects of commercial trawling on the malacological communities (except for the Cephalopoda) were examined, based on a study undertaken between 1996 and 2000 on the continental shelf and slope of southern Portugal. More than 50% of species caught by trawling in southern Portugal were discarded, with molluscs representing about 19%. Forty-four species of molluscs (15 bivalves, 28 gastropods and one polyplacophoran) were identified from the discarded specimens. Crustacean trawlers accounted for 34 molluscs species, and fish trawlers for 24. Twenty species were only caught by the crustacean trawl, compared with 10 species by the fish trawl, and 14 species were common to both trawls. The bivalve Venus nux Gmelin was the most numerous species discarded in the fish trawl, accounting for 42.0% of the total number of individuals, followed by the gastropods Ampulla priamus (Meuschen) (7.8%) and Ranella olearium (L.) (7.3%). In the crustacean trawl, the most numerous species discarded were the bivalve Anadara diluvii (Lamarck) (19.4%), the gastropod Calliostoma granulatum (Born) (15.5%), and the bivalve V. nux (15.1%). The third most discarded species from fish trawls in Algarve waters, the gastropod species R. olearium , is a species listed in Annex II of the Bern Convention. The difficulties of managing the real impact of fisheries on the molluscan populations and in defining a conservation strategy are discussed.     

A large scale BACI experiment to test the effects of prawn trawling on seabed biota in a closed area of the Great Barrier Reef Marine Park, Australia

A multiple Before-After-Control-Impact experiment was conducted to estimate the large scale effects of a single coverage by prawn trawlers on an offshore inter-reef area in the northern Great Barrier Reef that was closed to fishing. Prospective power and cost-benefit analyses facilitated the design and optimization of the experiment, the effect size of which was guided by prior publications at the time that had indicated impacts of 10–100×. The optimal design, given the number of factors to be tested and the constraint of available ship time (100 days) and resources needed to process samples, was for 12 control and 12 impact plots each measuring 2.8 km × 1.2 km. The final design specification was capable of detecting an average −80% effect size (5×). The experiment was divided over two seasons, wet and dry. During the experimental trawl impact phase, a total of 32 t of benthic biota was removed from six shallow plots (15–25 m depth) and 6 t from six deep plots (30–50 m depth). Sampling before and 6 months after impact was conducted by an epibenthic dredge, a prawn trawl and a fish trawl. Analysis of means before and after impact, relative to controls, showed very few significant differences. This indicated that the impact of a single prawn trawl was less than the design specification. The lower than expected impact, compared to previous studies, may have been partly because this study included all benthic groups, not just the most sensitive as earlier studies had focussed on. Comparison of catch rates from the prawn trawl and the dredge indicated the overall impact on total biomass was around −3% but ranged from close to 0% for some species to around −20% for sensitive sessile species. A review of effect sizes in 30 other recent trawl experiments indicated that this result was not exceptional; i.e. the reported effects of single trawls generally were not large, and prawn trawls appeared to have smaller effect sizes than fish trawls, beam trawls and scallop dredges—also, several studies reported recovery within ca. 6 months. Analysis of fishery effort data indicated that this result was appropriate for the majority of trawl fishing grounds, where effort is sparse and infrequent. Many published experiments had confounded designs and most had not used a priori power analyses or pre-specified effect sizes. Nevertheless, recurrent trawling can be expected to have cumulative impacts on benthos, as has been demonstrated by repeat-trawl depletion experiments. On the other hand, the spatial extent of fishing grounds trawled as intensively as these depletion experiments is quite limited.     

Strategies for improving fuel efficiency in the Portuguese trawl fishery

The recent rise in oil prices has brought renewed attention to energy savings in the fishing industry, and particularly in trawling. Coastal trawlers spend most of their time on fishing grounds near the coast. In such cases, the most successful energy-saving modifications ought to result from changes in the fishing gear and towing conditions. The purpose of this study was to identify the energy-economy potential for Portuguese fish trawlers after altering a vessel's operating conditions and improving its trawl gear performance. Two trawlers, named Tricana de Aveiro and João Macedo were selected as subjects in this project. Both vessels work with gear of similar design and size. Experimental sea trials were carried out to elucidate the actual vessel and gear performance. A model trawl was then built and tested in a flume tank, which provided the basis for improving the gear design. Full-scale trials were then carried out with both vessels using the modified trawls in order to assess changes in gear performance. The new trawls maintained their previous ability to catch species of different ecological groups and consumed less fuel at the same commercial trawling speed. An economic study showed potential increases in the net cash flow (NCF) of up to 27% over the range of operational navigation and trawling speeds. Having demonstrated the performance of the new trawls, the skippers of both vessels subsequently adopted the new design for commercial fishing.     

离底拖网的游泳动物群落特征——以兄弟岛渔场为例

为探究离底拖网对游泳动物的群落结构以及捕捞量的影响,于2019年9月和2020年9月在福建海域兄弟岛渔场开展了36网次的底拖网专项试验,通过调整曳纲长度控制底拖网作业过程中的网位变动,分别保持底拖网作业在贴底或离底1 m的状态,以物种组成、渔获量对数均值、优势度、多度谱、多样性等参数,分析和比较了贴底拖和离底拖的游泳动物群落结构变化。结果显示:贴底拖和离底拖游泳动物的种类数分别为126种和79种,贴底拖游泳动物的总尾数和总体质量均为离底拖的3倍,贴底拖的渔获量对数均值比离底拖要高0.36,表明贴底拖网的渔获物以栖息于底层的游泳动物为主;贴底拖的游泳动物多样性、均匀度和丰富度分别为2.81、0.65和10.01,均高于离底拖(2.29、0.60和8.13),经济幼鱼的比例更高,说明贴底拖会对游泳动物的多样性以及幼鱼造成更大的损害;离底拖游泳动物的优势度指数为0.41,高于贴底拖的0.29,其多度谱曲线更陡,说明近底层游泳动物群落优势种分布更集中、数量分布不均匀。研究结果可以为底拖网渔业的科学管理提供依据。     

Comparing and modifying penaeid beam and otter trawls to improve ecological efficiencies

In an attempt to improve the selectivity and engineering performances of generic penaeid trawls, three established and one novel spreading‐mechanism configurations were assessed: otter boards attached (1) with and (2) without 3.15‐m sweeps to a 7.35‐m headline trawl, and a beam rigged directly to a 9.19‐m trawl (3) with and (4) without a horizontal wire and plastic streamers. Despite more surface area (7.5 vs 6.0 m²), both beam‐trawl configurations had significantly lower drag than the otter trawls (≤30%). When catches were standardised to per ha, the otter trawl with sweeps retained significantly more (1.3–2.4 times) school prawns, Metapenaeus macleayi (Haswell), than the other three configurations. Within systems, removing sweeps or adding a horizontal wire significantly reduced the unwanted catches of a key teleost (southern herring, Herklotsichthys castelnaui Ogilby) by 41 and 48%. The results illustrate the utility of simple anterior modifications for independently addressing penaeid‐trawling environmental issues.     

聚乙烯编织线替代普通聚乙烯捻线对拖网性能的影响

聚乙烯编织网线比普通聚乙烯捻线强度高,用编织线来替代目前的普通捻线,可降低拖网渔具的阻力从而达到节能降耗的目的。本文通过实际取样测试,比较了两种网线的拉伸性能,在不降低原拖网渔具的单线结强力的前提下,用聚乙烯编织线代替原普通捻线,并通过模型试验研究其对拖网渔具性能的影响和节能效果。结果表明:单线结强力相同的两种网线,编织线的直径可比普通网线的直径下降15.17%以上。本试验编织线拖网的平均网线直径比原普通捻线拖网下降16.00%,网具阻力面积(线面积)从304.57 m2下降到256.47 m2,下降了15.79%。同等试验条件下,编织线拖网的阻力比普通捻线拖网下降17.25%左右,而网口高度的变化则不明显。能耗系数的计算结果为编织线拖网过滤同体积的水体所消耗的能量比普通捻线拖网下降16.67%。功率消耗的分析表明,消耗同样的功率,440 kW的双拖渔船使用编织线拖网,拖速可比普通捻线拖网提高8.33%,如采用和普通捻线拖网同样的拖速作业,则去掉起、放网所消耗的时间,以一天正常拖网作业18 h计算,可节约燃油77 L左右。     

Adaptive bycatch reduction in penaeid trawls via rapid adjustments to headline height

Penaeid trawling is among the world's least selective fishing methods; a characteristic that has evoked spatial closures being implemented in some fisheries if certain bycatch limits are exceeded. For decades, considerable work has been done to develop modifications to penaeid trawls that reduce unwanted bycatches, with most focussed at the posterior section (i.e. codend). More recently, efforts have examined ways to prevent bycatch entry into trawls entirely—via modifications to anterior components. This study assessed the utility of proactively lowering the headlines of Australian penaeid trawls, using clips at the otter boards, to 68% and 54% of their conventional height, and demonstrated mean total bycatch reductions (by weight) of 69% and 79%, respectively, with no effects on the targeted Metapenaeus macleayi (Haswell). The results provide insights into the location and behaviour of various species in the water column preceding capture, and support a simple and easy method for regional fishers to use in situ to avoid excessive bycatch and associated fishing closures. More broadly, the data support ongoing efforts in other penaeid‐trawl fisheries to reduce bycatches via similar, rapid adjustments to anterior components, depending on species‐specific behaviours during capture.     

Effects of a research trawl on a hard-bottom assemblage of sponges and corals

The effects of a research trawl on several sponge and coral species was assessed in a shallow-water, hard-bottom area located southeast of Savannah, Georgia. The study entailed a census of the numerically dominant species in replicate 25-m² quadrats located along five transects established across a trawling alley. The density of undamaged sponges and corals was assessed in trawled and non-trawled (control) portions of each transect immediately before, immediately after, and 12 months after a 40/54 roller-rigged trawl was dragged through the alley once. Some damage to individuals of all target species was observed immediately after trawling, but only the density of barrel sponges (Cliona spp.) was significantly reduced. The extent of damage to the other sponges (Ircinia campana, Haliclona oculata), octocorals (Leptogorgia virgulata, Lophogorgia hebes, Titanideum frauenfeldii) and hard corals (Oculina varicosa) varied depending on the species, but changes in density were not statistically significant. Twelve months after trawling, the abundance of specimens counted in the trawled quadrats had increased to pre-trawl densities or greater, and damage to the sponges and corals could no longer be detected due to healing and growth. Trawl damage observed in this study was less severe than the damage reported for a similar habitat in a previous study. Differences between the two studies are attributed to (1) differences in the rollerrig design of the trawls used, and (2) differences in the number of times the same bottom was trawled.     

Trawl fishing impacts on the status of seabed fauna in diverse regions of the globe

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.     

大网目底中层拖网模型试验研究

利用1:22.82比例的2张网具模型,在拖曳水槽中进行底中层拖网的网具性能测试,得出网具阻力公式为:F=0.121(d/a)LCV~(1.62),网口高度的公式为:H=0.225CV~(-1.316),2张网具的作业性能良好,GM_2的垂直张开性能优良,拖速3节时的垂直扩张系数已接近7％,能耗系数在拖速3节时为0.2kw.h/10~4m~3,在同类渔具中较优;GM_1网的能耗系数比GM_2网低,在拖速3节时低于9％左右,作为中层拖网时的总体性能优良。     

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.     

The efficiency of three fry trawls for sampling the freshwater pelagic fry community

Avoidance reactions of young-of-the-year fish assemblages were investigated in the offshore zone of three reservoirs in the Czech Republic during late summer. Pikeperch (Stizostedion lucioperca) strongly dominated in the Lipno Reservoir in 2003, bream (Abramis brama) and bleak (Alburnus alburnus) were the most abundant species in the Slapy Reservoir in 2004 and roach (Rutilus rutilus) and bleak dominated in the Želivka Reservoir in 2004 and 2005. Densities of fry ranged between 1.09 ind./100 m³ in the Lipno Reservoir and 2.95 ind./100 m³ in the Slapy Reservoir. In each reservoir, three trawl nets of different widths (dimensions 1.5 m × 3 m, 3 m × 3 m, 6 m × 3 m) were used in the pelagic area at night. For each trawl the estimated fish density was determined as the catch per 100 m³ of water and was compared between all three trawls. The fish length–frequency distribution was also compared between all the trawls. Day catches were much lower compared with those taken at night. Adult fish showed very clear avoidance of fry trawls. At night, no important differences in estimated fry density, obtained by these trawls, were observed in any locality for any fish species, nor in the length distribution of important species between trawls. According to these results, the avoidance reactions of fry in the late summer (August, September), at night, are unimportant. At this time of year, nighttime pelagic trawling appears to be an effective method for quantitative fry sampling. We recommend the 3 m × 3 m trawl for quantitative night sampling of the fry community as a good compromise between assured efficiency and relatively easy handling.     

Effect of trawling with traditional and ‘T90’ trawl codends on fish size and on different quality parameters of cod <Emphasis Type="Italic">Gadus morhua</Emphasis> and haddock <Emphasis Type="Italic">Melanogrammus aeglefinus</Emphasis>

The effect of trawling on fish size and on different quality parameters of cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) was evaluated after conducting 16 valid hauls using two trawls in a double rig fitted with a traditional and a novel ‘T90’ codend, respectively. The total catch volume during the fishing period was 47.6 metric tons, with an average catch per codend of 1.5 (range 0.5–2.9) tons. The mean haul duration was 5 h. The catch was assessed according to fish size, mortality, external damage, initial white muscle pH and development of rigor mortis. Fillet quality (colour, blood spots, gaping) was assessed after 1 week of freeze-storage. Our results showed there was no difference between the two types of nets in terms of catch volume, but significantly slightly bigger fish were caught with T90 than with the traditional trawl net (p < 0.05). Haddock caught with the traditional trawl net had more external injuries related to the trawl gear than haddock caught with the T90 gear (p < 0.05). The gaping frequency for cod caught with the traditional trawl net tended to be higher than cod caught with the T90 gear, but the difference was not significant (p = 0.07). No other differences in fish quality between fish caught in the trawl nets were observed.     

基于正交试验方法的四片式虾拖网结构参数优化

针对规格为40.3 m×(20.8~16.2)m的我国过洋性虾拖渔业四片式虾拖网,基于L_9(3~4)正交表,设计了网身长度(L_b)、下网袖长度(L_w)、侧网宽度(W_S)3种因素各3个水平的正交试验,并利用方差分析检测3种试验因素对网具阻力、能耗系数、L/R(袖端间距与阻力之比)系数影响的显著性,旨在对此种网具的网身长度与网口周长之比(L_b/C)、下网袖长度与网口周长之比(L_w/C)、侧网宽度与上网片宽度之比(W_S/W_U)这3种结构参数进行优化。结果表明,对网具阻力的影响因素由主到次依次是下网袖长度、网身长度、侧网宽度。对能耗系数的影响因素在水平扩张L/S(袖端间距/下纲长度)=0.45时由主到次依次是侧网宽度、网身长度、下网袖长度;但在水平扩张L(袖端间距)=10 m时,下网袖长度为主影响因素。对L/R系数的主影响因素为下网袖长度。此类型网具的L_b/C应比小网目底层拖网略小,但在较大的范围内(0.26~0.38)变化亦不会对网具水动力性能有显著影响。L_w/C=0.31为试验的最佳水平,下网袖长度为3种因素中对网具性能影响最显著的因素。W_S/W_U=0.19为试验的最佳水平,侧网宽度应优先满足虾拖网作业对网口高度的要求,在此基础上,尽量缩小W_S/W_U值,不必拘泥于W_S/W_U结构参数值。     

Proximity of Pacific cod to the sea floor: Using archival tags to estimate fish availability to research bottom trawls

The percentage of Pacific cod available to Alaskan bottom trawl surveys was estimated from the proximity of tagged cod to the seafloor. Archival tags recorded time and depth data at 15- or 30-min intervals. The distance of a cod above the bottom was computed by subtracting tag depth from bottom depth, estimated as the maximum depth recorded during each 24-h day. These estimates of bottom depth are based on the assumption that cod approach the seafloor at least once a day, and do not undergo migrations up or down bottom gradients. To ensure that migrations over variable bottom gradients did not confound our estimates of bottom depth, we narrowed our analysis to 11 tagged cod that were recaptured in areas of flat bathymetry, and only analyzed data recorded within one month prior to recapture (N = 29,462 depth recordings). Pacific cod maintained short distances above the seafloor that often differed between day and night. Over 95% of the daytime tag recordings occurred within 10 m of the seafloor. Average effective headrope heights for survey bottom trawls currently used in the eastern Bering Sea and Alaska Gulf/Aleutian Islands groundfish surveys are approximately 2.5 and 7 m, respectively. In the absence of any behavior responses to an approaching trawl, we would expect 47.3% of the cod within the water column are available to the trawl used on the eastern Bering Sea survey and 91.6% are available to the trawl used on the Gulf/Aleutian Islands surveys. This study demonstrated that Pacific cod are highly demersal, and current values of trawl survey catchability (Q) used in current stock assessments are consistent with estimates of cod availability to the trawl gear.     

Choosing best practices for managing impacts of trawl fishing on seabed habitats and biota

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.     

Efficiency of the bottom trawl used for the Mediterranean international trawl survey (MEDITS)

The aim of the work presented in this paper was to assess the relative efficiency of the GOC 73 sampling trawl used for the Mediterranean international trawl survey programme (MEDITS) compared with that of a typical Italian commercial trawl. The latter was chosen because it is commonly used by fishermen in the central Adriatic, where the experiment was conducted, and therefore appeared as the best possible sampler of the commercial species present in those areas. Moreover, this trawl is similar to the various trawls used for the Italian national survey programme (GRUND). Four fishing trips were conducted between 1996 and 1997 in different areas and seasons to sample different species. Each trip was conducted in the same fishing area. A codend cover mounted on the commercial trawl ensured that the codends of the two nets had the same mesh opening. The two trawls were alternated daily. Net geometry (horizontal and vertical net opening) was measured during all tows. Catch data were converted into abundance per swept area before comparing the trawls. Efficiency comparisons were performed on ten fish species, one crustacean and four molluscs belonging to the MEDITS list of main reference species. The MEDITS trawl was significantly less efficient in terms of both weight and numbers of individuals fished for hake (Merluccius merluccius), common sole (Solea vulgaris) and Norway lobster (Nephrops norvegicus). A highly significant difference in favour of the commercial trawl was found in the weight, but not the numbers, of common pandora (Pagellus erythrinus). Efficiency differences were negligible for red mullet (Mullus barbatus), while the MEDITS trawl was significantly more efficient for the numbers of Atlantic horse mackerel (Trachurus trachurus). For all the other main reference species differences were not significant. The commercial trawl was more efficient for large individuals of some species. The relative efficiency of the MEDITS trawl was especially low for small-size classes of N. norvegicus. The results confirm the lower efficiency of the MEDITS compared with the commercial Italian trawl for benthic species, and its greater efficiency for some others released from the bottom as well as for the pelagic ones. These data allow the results of the MEDITS surveys to be compared with those of the GRUND programme and with commercial-fleet catches in areas where the commercial trawls used are similar to the one studied here.     

Origins of the bottom trawling controversy in the British Isles: 19th century witness testimonies reveal evidence of early fishery declines

Bottom trawling (nets towed along the seabed) spread around the British Isles from the 1820s, yet the collection of national fisheries statistics did not begin until 1886. Consequently, analysis of the impacts of trawling on fish stocks and habitats during this early period is difficult, yet without this information, we risk underestimating the extent of changes that have occurred as a result of trawling activities. We examined witness testimonies recorded during two Royal Commissions of Enquiry (1863–66 and 1883–85). These enquiries interviewed hundreds of fishers about the early effects of sail trawling and the changes they were witnessing to fish stocks, habitats and fishing practises during this time. We converted all quantitative statements of perceived change in fish stocks and fishing practices to relative change. Witnesses from the north‐east of England interviewed during 1863 revealed an average perceived decline in whitefish of 64% during their careers, which many blamed upon trawling. Between 1867 and 1892, trawl‐landing records from the same location suggest that this trajectory continued, with fish availability declining by 66% during the period. Fishers adapted to these declines by increasing distances travelled to fishing grounds and increasing gear size and quantity. However, inshore declines continued and by the early 1880s even trawl owners were calling for closures of territorial waters to trawling in order to protect fish nursery and spawning grounds. Until now, these testimonies have been largely forgotten, yet they reveal that alterations to near‐shore habitats as a result of trawling began long before official data collection was initiated.     

Distribution and intensity of bottom trawl fisheries in the Patagonian Shelf Large Marine Ecosystem and its relationship with marine fronts

Trawling is a major concern worldwide and there is considerable debate about its impact on marine ecosystems. The Patagonian Shelf Large Marine Ecosystem (PSLME) is an important fishing area in the Southwest Atlantic where bottom trawling is the dominant fishing method. We investigated the distribution of bottom trawl fishing within this region, defining the areas of highest trawling intensity (hotspots) and evaluating their relationship with marine fronts. We focused on the three main oceanographic fronts, the shelf‐break front, the southern Patagonia front and the mid‐shelf front. To estimate fishing effort and trawled areas, we used VMS data from 2006 to 2012. Despite being almost a fully trawlable shelf, we found that the spatial distribution of trawling activity is patchy and trawling hotspots were small, comprising annually <5% of the shelf extension or <7% of the total trawlable area. Contrary to what is believed worldwide, our findings suggest that over the PSLME the magnitude of habitat effects as a result of bottom trawling is relatively small. Regarding the three frontal systems studied, only the shelf‐break front showed a positive relationship with trawl fishing activity. Although trawling hotspots did not overlap with marine fronts, the shelf‐break front receives more trawling effort than expected. We hypothesize that this pattern is due to aggregation of species near or at the front taking advantage of the opportunities provided by this area.     

Benthic disturbance by fishing gear in the Irish Sea: a comparison of beam trawling and scallop dredging

1. The distribution of effort for the most frequently used mobile demersal gears in the Irish Sea was examined and their potential to disturb different benthic communities calculated. Fishing effort data, expressed as the number of days fished, was collated for all fleets operating in the Irish Sea in 1994. For each gear, the percentage of the seabed swept by those parts of the gear that penetrate the seabed was calculated. 2. For all gears, the majority of fishing effort was concentrated in the northern Irish Sea. Effort was concentrated in three main locations: on the muddy sediments between Northern Ireland and the Isle of Man (otter and Nephrops trawling); off the north Wales, Lancashire and Cumbrian coast (beam trawling); the area surrounding the Isle of Man (scallop dredging). 3. In some areas, e.g. between Anglesey and the Isle of Man, the use of scallop dredges and beam trawls was coincident. A comparative experimental study revealed that scallop dredges caught much less by-catch than beam trawls. Multivariate analysis revealed that both gears modified the benthic community in a similar manner, causing a reduction in the abundance of most epifaunal species. 4. Although beam trawling disturbed the greatest area of seabed in 1994, the majority of effort occurred on grounds which supported communities that are exposed to high levels of natural disturbance. Scallop dredging, Nephrops and otter trawling were concentrated in areas that either have long-lived or poorly studied communities. The latter highlights the need for more detailed knowledge of the distribution of sublittoral communities that are vulnerable to fishing disturbance. ©British Crown Copyright 1996.