Trawl Gear Research Articles

A Theoretical Approach to Determine the Total Resistance on Different Trawl Gears Used in Turkey

The study aims to theoretically determine the total resistance forces acting on bottom trawls used in the commercial trawl fishery around the Turkish Peninsula. For this purpose, a total of 14 (2 for Black Sea, 2 for Sea of Marmara, 6 for Aegean Sea and 4 Mediterranean Sea) trawl gears were examined during the study representing all types of the bottom trawl fishery (e.g., fish targeted, shrimp targeted and mix) in Turkey. The calculation of the hydrodynamic forces of the gears was based on the determination of each gear component; net, floaters, sweep, door, warp and the friction force caused by footrope, sweep and door. The total resistance of trawl gears was calculated according to different towing speeds and warp lengths. Average resistance values of the examined nets with a 500 m warp length indicated that the frictional force of the trawl door composed more than 41.3% of the total resistance, followed by net (39.4%), both doors (6.3%), footrope friction (6.1%), sweep frictions (3.1%) and the rest belonged to sweeps, warps and floaters (totally 3.8%). It was determined that resistance caused by friction including sweeps, footrope and doors constituted more than 50% of the total resistance. Among the examined gears, the greatest resistance values were determined for the 600 meshes tailored-traditional Mediterranean type trawl gear. 

Quantifying the resuspension of nutrients and sediment by demersal trawling

Demersal fisheries trawling is widely acknowledged as one of the most intense forms of widespread benthic disturbance, resuspending extensive plumes of sediments and dissolved nutrients. However, difficulties associated with sampling within trawl plumes have hitherto limited our quantitative understanding of these widespread phenomena. This lack of knowledge hinders our ability to understand the broader consequences of demersal trawling and the development of new fishing gears to limit benthic disturbance. Here we present data from a series of novel in situ experiments using a specially designed trawl sled to quantitatively examine how trawl gear-induced drag and pressure influence the height and concentrations of resuspended sediments and nutrients within a trawl plume. Our data demonstrate that the composition of resuspended particles and sampled nutrients are both influenced by sampling height above the seafloor and the amount of drag exerted by the trawl gear (p < 0.001 in all cases), although the relative importance of these factors differed between the response variables examined. These differences likely reflect that sediment particles are more influenced by gravity than dissolved nutrients are. Our results demonstrate that trawl gear specification strongly influences the amount of dissolved and particulate material resuspended, suggesting that their design could be modified to reduce impacts on benthic ecosystems.

International megabenthic long-term monitoring of a changing arctic ecosystem: Baseline results

The sustainable development and environmental protection of the Arctic ecosystem is on the agenda globally. The Convention of Biological Diversity (CBD) and the UN Sustainable Development Goals call for conserving at least 10 per cent of coastal and marine areas globally. Management tools to achieve this goal include marine protected areas (MPAs) and “other effective area-based conservation measures” (OECMs) of structural megabenthic organisms (e.g. corals, sea pens, sponges, anemones, etc.). But large areas of the ocean are lacking information about seabed communities. Here we show that this data gap can potentially be filled by collecting data on megabenthic organisms that are “bycatch” (not the target species) on government research vessels monitoring commercial fish and shellfish. For this paper, several Arctic and sub-arctic nations contributed megabenthos data from a total of 12.569 fish assessment trawls and associated bottom water temperature data. The latter outline areas of warm sub-Arctic inflow versus colder Arctic waters, which we align with temperature affinities of community. We also found that maximum levels of shared taxa were higher between Atlantic and Eurasian Arctic Seas than with Pacific Arctic Seas. Areas of high standardized species richness generally, but not everywhere, coincided with areas of high standardized biomass and/or high current velocity and in transition zones between water masses. We did not find that standardized taxon richness declined with latitude (from 60 to 81°N) as has been previously hypothesized. High biomass was generally associated with Arctic outflow shelves and/ or (within-region) colder water masses. We identify areas with high proportions of sessile and upright taxa that may be susceptible to damage by bottom trawl gear, taxa with calcareous skeletons that may be susceptible to ocean acidification, and 'cold-water' taxa that may be most vulnerable to ocean warming. Our results demonstrate the feasibility and value of international collaboration and cooperation in understanding large-scale patterns of Arctic megabenthic communities and providing scientific advice for management of human activities in the global Arctic ecosystem.

Deep-sea benthic habitats and the impacts of trawling on them in the offshore Greenland halibut fishery, Davis Strait, west Greenland

Abstract The offshore Greenland halibut (Reinhardtius hippoglossoides) fishery, west Greenland, employs demersal trawl gear at depths of 800–1400 m. In contrast to many deep-sea fisheries, the target stock appears stable and the fishery is of significant economic importance. Recent Marine Stewardship Council certification of this fishery highlighted the paucity of knowledge of benthic habitats and trawling impacts, which this study aimed to address using a towed benthic video sled. The spatially discrete northern and southern areas of the fishery were found to be distinct in terms of the communities present, which non-metric multidimensional scaling suggests is primarily driven by temperature. Extensive physical evidence of trawling was observed. Trawling effort was significantly linked with community composition, with a negative association between trawling effort and abundance of some taxa, including some vulnerable marine ecosystem (VME) indicator species. Three potential VMEs are identified: (i) Flabellum alabastrum cup coral meadows; (ii) a Halipteris finmarchica sea pen field; and (iii) areas exhibiting mixed assemblages of VME indicators. Of immediate conservation concern is a H. finmarchica field, which seems to be at least regionally rare, is situated within the fringes of existing trawling effort and is currently afforded no protection by management measures.

First observation and effect of fishery of seabed litter on sea bed by trawl survey Korea waters

Fisheries activities for supplying marine productions were excessively overwhelming. Furthermore in the competitive industrialization, the impact on marine ecosystems and fisheries resources are severe due to the increase of various seabed litter such as plastic materials, styrofoam and plastic bottles and the deterioration of the marine environment. Despite these seriously situation, very few studies of some sea of Korea have been carried out on sedimentary waste fishing. Also some fisheries and there are few reports on plastics present on the seabed of around Korea. This study investigated the distribution of seabed litter collected from the seabed and characteristics of each area by using the trawl gear of the survey vessel from February to November 2018. The weight of all seabed litter collected during the investigation was 62,541.5 kg km–2, with a range of 0.2–15,019.7 kg km–2. The most amount of seabed litter was collected from sea block 106 in the South Sea, followed by sea blocks 76 and 82 in the East Sea. Through this study, which was conducted for the first time in all sea in around Korea, it can be used as basic data to understand the current status of seabed litter in the seabed and to establish effective policies at the led by the government.

Discards reduction of non-commercial benthic species from a simple net modification

The high discards rates of non-commercial benthic species is one of the main problems of the Mediterranean bottom trawl fishery developed on the continental shelf. The present work analyzes data from 22 bottom trawls, carried out in parallel by two commercial trawlers between 70 and 90 m depth on maërl beds off eastern Menorca (Balearic Islands). One vessel used an experimental trawl gear (EXP) equipped with a lighter and smaller groundrope in the mouth of the net, leaving a gap without net between the main groundrope and the secondary one. The other vessel used a trawl gear traditionally used by the fleet operating in the area (TRA), without that modification. Catches, discards and commercial yields between both gears were compared. Discarded biomass of non-commercial benthic species with EXP was reduced by half when compared to TRA, particularly for rhodoliths (54%) and ascidians (51%). The reduction of discarded rhodoliths is especially relevant in the Balearic Islands where sensitive habitats, such as maërl beds, are an important part of the benthic communities. The modification of the net, at the same time, does not imply a reduction of yields of commercial captures and can easily be implemented in any bottom trawl net. It is concluded that this could be an appropriate and plausible technical measure to improve the ecological efficiency and the sustainability of the Mediterranean bottom trawl fishery.

Sediment mobilization by bottom trawls: a model approach applied to the Dutch North Sea beam trawl fishery

Abstract 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.

An Assessment on a Subsea Pipeline Subject to a Diagonal Trawl Impact

In oil and gas offshore areas, flowlines in fishing waters are at risk of trawl gear impacts. In this regard, DNV-RP-F111 recommends a beam and spring-mass (BSM) model to assess the overtrawlability of pipelines. The BSM model is commonly employed to assess perpendicular trawl impacts on pipelines. However, in the case of a subsea pipeline under a non-perpendicular trawl impact, the BSM model does not incorporate the full diagonal load; the tangential component of the impact load is disregarded. Furthermore, the BSM model does not account for the progression of damage in a pipe (e.g., the plastic damage in a pipe that is imposed by a non-perpendicular trawl impact and is translated along the pipe during the impact). In this regard, a recent study showed that damage progression could decrease the structural resistance of a pipeline. Therefore, as the BSM model does not consider the damage progression effect, it could underestimate the resulting dent depth in a pipe. The present study uses an experimental investigation to examine the damage progression effect in a cylindrical specimen. Furthermore, using a hybrid shell-beam finite element model, an enhanced version of the BSM model, a scenario of a pipe subject to a diagonal impact is investigated. The experimental results show that the dent depth in a 5-inch pipe under a perpendicular 150 kN load could increase by 52%, where the imposed damage is translated for 300 mm along the pipe. In addition, the results obtained from the finite element analyses show that the dent depth in 5-inch and 14-inch pipelines is 20% higher under a full diagonal trawl impact with 30° angle from the pipe normal, compared to applying only the normal component of the diagonal impact. In conclusion, the resulting dent size in a pipe under a diagonal trawl impact is dependent on both normal and tangential components of the impact. Furthermore, the hybrid shell-beam model could be a suitable alternative to the BSM model for scenarios where a pipe is subject to a diagonal trawl impact.

Evaluation of impacts of bottom fishing on demersal habitats: A case study off the Pacific coast of north-eastern Japan

Physical disturbance of the seafloor induced by bottom trawling is of increasing concern from the viewpoints of ecosystem conservation and sustainable fishery. We developed a method for evaluating the impacts of several kinds of trawl gears on the different seafloor habitats using bathymetry and sediment data and scientific survey data off the Pacific coast of north-eastern Japan. Ten types of habitats were classified by the cluster analysis applied to seafloor bathymetry, topography, and sediment hardness over the study area. Our mapping of these habitats demonstrated that the intensity of bottom fishing activities on the seafloor was different among habitat types and related to the abundance of commercial species living in each habitat. The ecological influences of fishing on these habitats were examined by focusing on the changes in demersal species diversity and biomass detected through scientific surveys. A decline in species diversity was more evident in the habitat types with complex topography and hard sediment in the intensely trawled areas. Comparison of fishing gear suggested that the otter trawl may be the most influential owing to the significant change in the demersal ecosystem.

Numerical simulation of bottom trawl with large-size mesh in steady flow field

The physical model experiment and numerical model calculation of bottom trawl gear with large-size mesh were carried out for verifying the application of numerical method in bottom trawl, which provides an alternative numerical analysis method for the design and optimization of bottom trawl gear. The mathematical models of bottom trawl gear with large-size mesh were constructed by using the finite element method to calculate the resistance, the expansion of net mouth and changes of fishing gear shape under steady flow conditions. In order to improve the computational efficiency of numerical trawl model, a new kind of mesh group method was proposed in this paper, and results of two different models of different computational units were compared to find a better group number of large trawl.The physical model was designed according to the Tani similarity criterion, which was tested in the static flume, and the large scale ratio was 1∶35. After mesh grouping in numerical calculation, two specifications of model-1 with 6 513 virtual elements and model-2 with 1 787 virtual elements were established, which were respectively calculated and compared with the experimental results of physical model in water tank. Results showed that the morphological variation trend of model-1 and model-2 was basically consistent under the same flow velocity. The resistance calculation results of model-1 was closer to that of the physical model test than those of model-2, and there was a relative error of 16%-26% between the calculation results of model-2 and the physical model. The calculated results of vertical and horizontal expansion of the two numerical models were close to those of physical model test, and the maximum relative error was only 5%-10%. The state of bottom trawl with large-size mesh was presented in the steady flow field successfully through numerical simulation, which should be able to replace the traditional flume model experiment to some extent.

A study on towing power of trawlers and a proper scale of trawl gears

A study on towing power of trawlers and a proper scale of trawl gears

Increasing value through gear flexibility: a case study of US west coast sablefish

This paper explores the potential economic gains of allowing additional flexibility in gear choice, within rights-based management programs. A case study of US west coast sablefish (Anoplopoma fimbria) provides an example of a commercially important species where gear-switching is currently occurring within the individual fishing quota (IFQ) program, allowing us to isolate the economic potential of gear flexibility along two important margins: size and quality. We conduct a hedonic price analysis of ex-vessel prices using panel fish ticket data and linear mixed-effect econometric models to examine the influences of gear, size, condition, fishing sector, port group, landing month, and year on the price of sablefish. We generate a counter-factual scenario that represents the IFQ fishery where the use of fixed gear is prohibited by predicting what the size composition of catch would have been if the sablefish had been caught with trawl gear. We find that the flexibility of targeting sablefish with fixed gear between 2011 and 2016 generated an annual mean 10.45% increase in total revenue, or US$1.17 million, compared with the trawl-only scenario. These results show sablefish value increases through implementing gear flexibility, which contributes to a broader conversation of allocative efficiency.

How Fishery Collapses: The Case of Lepidopus caudatus (Pisces: Trichiuridae) in the Strait of Sicily (Central Mediterranean)

The silver scabbardfish Lepidopus caudatus is a mesopelagic species living on the shelf and slope down to 600 m in temperate seas all around the world. In the Mediterranean, the species is caught mainly by longlines with a marked seasonality. In the early 90s in the Strait of Sicily (Central Mediterranean Sea), a new fishery targeting L. caudatus was developed. This fishery uses an ad hoc pelagic trawl gear called “spatolara.” Vessels using spatolara have increased from 1 in 1993 to 10 in 2007 with a growth of catches of up to 1,200 tons in 2011. Development of this fishery was not regulated by any specific management measures and, due to the progressive reduction of catch to 169 tons, only one vessel was active in 2018. The availability of catch and biomass indices from trawl survey since the beginning of trawling exploitation allowed providing the first assessment of the state of L. caudatus stock in the Central Mediterranean (GFCM Geographical Sub-Area 16) by using data-limited methods. Catch-Maximum Sustainable Yield (CMSY) and Bayesian State Space Schaefer model (BSM) were fitted to landings and abundance indices (2004–2018). The Abundance-Maximum Sustainable Yield model (AMSY) was also applied to survey data from 1994 (1 year after the start of the spatolara fishery) to 2018 to further corroborate the results. BSM prediction of biomass levels was just above 50% of BMSY, whereas AMSY estimated the current stock levels below 50% of BMSY. The BSM was used for forecasting B/BMSY and catches under different fishing scenarios. Although current exploitation was very close to FMSY, more than a decade would be needed to rebuild the stock to biomass levels producing MSY. A faster rebuilding could be achieved by fishing at least 80% of FMSY, with minimal loss in yield over the next 5–8 years. Following the development of a new fishery since the beginning, the study provides a further example of how unregulated exploitation leads to a heavy overfished state of stock and collapse of fishing activities.

Observations on Bycatch Rate for Vulnerable Fish Species on the Romanian Coast

"Living marine resources obviously contribute to the survival of humanity. Unfortunately, numerous studies highlighted that marine biological diversity faces various threats related to overfishing and illegal fishing, pollution (marine and terrestrial), invasive species, marine litter and climate change. In this context, the incidental catch of vulnerable species (i.e., elasmobranchs, marine mammals, seabirds) is considered one of the main threats to good environmental condition of the marine fauna.The aim of the study conducted in 2020, was to highlight the impact of fishing activities on aquatic resources, especially on vulnerable fish species as picked dogfish Squalus acanthias), thornback ray (Raja clavata) and common stingray (Dasyatis pastinaca), marine mammals and seabirds. The background information was collected by observations from scientific trawl surveys by pelagic and demersal trawl gears, observations from stationary fishing points, observations from commercial beam-trawl fishing operations. The analysis of logbooks of several economic operators and data collected by observers on board vessels, highlighted the low bycatch rate for vulnerable species in most fishing gear. Key Words: vulnerable species, by-catch, fishing gear "

Bottom trawling noise: Are fishing vessels polluting to deeper acoustic habitats?

The impact of bottom trawling noise was quantified on two surrounding marine acoustic habitats using fixed mooring acoustic recorders. Noise during trawling activity is shown to be considerably louder than ambient noise and a nearby underway research vessel. Estimated source levels were above cetacean damage thresholds. Measurements at a submarine canyon indicated potential noise focussing, inferring a role for such features to enhance down slope noise propagation at continental margin sites. Modelled sound propagates more efficiently when sourced from trawling gear dragging along the seabed relative to the vessel as a surface source. Results are contextualised with respect to marine mammal harm, to other anthropogenic ocean noise sources, topography and seasons. Noise energy emitted by bottom trawling activity is a source of pollution that requires further consideration, in line with other pervasive trawling pressures on marine species and seabed habitats, especially in areas of heightened ecological susceptibility.

The relationships between fish size and otolith dimensions in the common sole ( Solea solea (Linnaeus, 1758)) captured in the Northeastern Mediterranean

Fish specimens were captured by a commercial bottom trawler at a depth of 50–80 m from Iskenderun Bay (Hatay, Turkey) between December 2017 and May 2018. The bottom trawl gear used was equipped with a 44 mm stretched mesh size net at the cod-end. Blind side and eyed side otolith lengths (OL), otolith breadths (OB) and otolith weights (OW) were measured from each specimen to the nearest 0.001 mm and 0.0001g respectively. A total of 110 fish (43 females and 67 males) were collected. Total length ranged from 20.8 to 28.2 cm and 68.0 to 166.1 g (males) and 21.1 to −28.5 cm and 74.5 to 201.4 g (females). The coefficients of determination between fish weight and otolith weight, and total length and otolith weight (sexes combined) were found as R2 = .7694 (0.77) and R2 = .6274 (0.63), respectively. A moderate positive relationship between the total length-otolith dimensions, and fish weight-otolith dimensions, was also demonstrated.

Growth characteristics of Uranoscopus scaber Linnaeus 1758, inhabiting the Iskenderun Bay (Northeastern Mediterranean)

This study aimed to determine the age and some growth characteristics of Atlantic stargazer (Uranoscopus scaber) from Iskenderun Bay (Northeastern Mediterranean). For this purpose, a total of 150 Atlantic stargazer ranging in size from 9.1 to 28.0 cm in total length (weight: 11.7–345.7 g) were collected as by-catch from a commercial trawl fishing boat at a depth of 80–100 m between May 2015 and January 2016. The bottom trawl gear used was equipped with a 44 mm stretched mesh size net at the cod-end. The percentage of females and males were 46.7% and 53.3% respectively. The total length–weight relationships equation with coefficient of determination (R2) were found as W = 0.011*TL3.131, R2 = .9728, for all individuals, W = 0.015*TL3.021, R2 = .9512 for females and W = 0.0102*TL3.136, R2 = .9553 for males. By using the von Bertalanffy equation, the growth parameters of Atlantic stargazer were estimated to be L∞ = 42.35 cm, k = 0.098, t0 = −1.8474 for all individuals; L∞ = 36.92 cm, k = 0.138, t0 = −1.2693 for females and L∞ = 38.77 cm, k = 0.1, t0 = −2.334 for males. In this study, age reading was done by two independent readers and index of average percentage error (IAPE) was calculated as 6.1%. The highest condition factor value calculated as 1.81 in the age group 6 and the lowest condition factor value was calculated as 1.48 in the age group 1.

Quantifying the performance of selective devices by combining analysis of catch data and fish behaviour observations: methodology and case study on a flatfish excluder

AbstractThis article describes a method for the quantitative analysis of fish behaviour relative to selection devices in trawl gears. Based on video observations, the method estimates probabilities for a given event to happen and establishes behavioural tree diagrams representing and quantifying behavioural patterns in relation to the selection device under assessment. Double bootstrapping is used to account for the uncertainty originating from a limited number of fish observations and the natural variation in fish behaviour. The method is used here to supplement standard analysis of catch data for the performance assessment of a flatfish excluder (FLEX). The Baltic Sea trawl fishery targeting cod (Gadus morhua) provides the pilot case. Results obtained by comparing catches with and without FLEX installed revealed that &amp;gt;75% of bycaught flatfish individuals escaped through the device, while no evidence was found that catches of cod in the targeted sizes were reduced. The behavioural analysis produced values of escape efficiency comparable to those obtained in the catch analysis. Furthermore, it revealed that ∼80% of the flatfish went calmly into the excluder, while most of the roundfish displayed avoidance swimming reactions. The method provides quantitative information of fish behaviour that can be relevant for developing and optimizing selection devices.

THE EFFECT OF TRAWL BOARD PULL-OVER LOAD ON SUBSEA PIPELINE OF OFFSHORE SABAH

Trawling is a method of catching fish in a large volume where fish net is pulled through water using one or two boats. Bottom trawling is where the nets are pulled over or close to seabed and can affect the subsea pipeline if found along the route. Therefore, the objective of this study was to determine the impact of pull-over to selected subsea pipelines in Sabah and Labuan waters. This study involved four oil and gas pipelines in Sabah and Labuan waters from the oil fields to shore terminals. The research started with obtaining data of the pipelines and specification of trawl gear in Sabah. Fishing trawler traffic data along the pipelines route was determined by AIS system and site observation to determine the density of the trawlers. Trawl gear pull-over load was calculated using DNV algorithm and the inputs were trawl gear specification ^and fishing trawl speed. The severity was based on pull-over load calculated and pipeline yield stress. Then frequency was based on AIS data and density of fishing trawl per area. Based on the comparison between trawl pull-over load and yield strength/stress, the effect of trawl board pull-over is considered as minor, which is the lowest in the severity index.

Growth Characteristics Layur Fish Lepturacanthus savala in Juata Waters, Tarakan, Indonesia

The city of Tarakan, Indonesia, benefits economically from abundant marine biological resources, one of which is the layur fish, known commercially as ribbonfish. The purpose of this study was to analyze the growth character of the layur fish (Lepturacanthus savala) obtained from the juata waters of Tarakan City. The method of this quantitative and descriptive study was to analyze the growth of layur fish Lepturacanthus savala in the Juata waters off the coast from the city of Tarakan. Body shape, length, weight, and ratio sex data were collected from sites selected by the purposive sampling method based on trawl fishing catch. Secondary data, collected from interviews of fishing personnel, included the number of catches using trawling gear and trawl fishing location. The length ranges of male from 20,4 to 54,0 cm with an average length of 37,2±16.8 cm (n = 255) and the length ranges of female from 20,5 to 68 cm with an average length of 44,25±23.75 cm (n = 275). The results showed that maximum length of male layur fish was smaller than the maximum length of female fish, which were 59.352 cm at 267 d and 72.638 cm at 315 d, respectively. The result shows a negative allometric growth pattern male and female layur fish was found to be the same and sex ratio of male to female has a ratio of 1,0: 1,8 with a percentage of males at 48.11% and females at 51.89%. The majority of male and female fish exhibited a thin body shape (51,4% of male and 52% of female).