Trawl Doors Research Articles

Mathematical Modeling of the Hydrodynamic Forces on a Trawl Door

Precise control of trawl systems is assumed to be beneficial from both economic and environmental reasons. Using the trawl doors as actuators could increase the amount of available control forces. Adequate mathematical models of the hydrodynamic fores on the trawl doors are needed for control system design and verification. This paper presents a method for mathematical modeling of the hydrodynamic forces on the trawl doors. These forces are divided into steady-state forces and transient effects. The six degrees of freedom (six dof) steady-state hydrodynamic coefficients of a trawl door have been found as a function of its angles of attack and slip, based on wind-tunnel experiments. The coefficients are parameterized for smoothing and computational performance, and methods for extending the validity of the model in terms of orientation and trawl door shape are presented. The transient effects are described as functions of relative accelerations between the trawl door and the ambient water, angular velocities of the trawl door and circulation buildup. These effects are manifestations of variations in the flow around the trawl door and its wake, and a numerical method based on potential theory is employed to investigate them. A computational efficient, nonlinear, state-space model of the hydrodynamic forces is finally proposed. It accounts for steady-state and unsteady hydrodynamic forces and moments in six dof, suitable for trawl control system design and analysis

When fish meet a trawling vessel: examining the behaviour of gadoids using a free-floating buoy and acoustic split-beam tracking

The reaction of individual gadoids to a bottom-trawling vessel has been observed in situ in the Barents Sea using a free-floating buoy and acoustic target-tracking methods. More than 20 000 tracks were analysed in terms of velocity changes in vertical, athwarthship, and alongship direction relative to the vessel, the warps, and the trawl, respectively. The fish starts diving about 15 min before vessel passing. This coincides with the time the trawl is running and not with the gradual increase in vessel noise caused by the approaching vessel. The change in horizontal movement is more gradual and is directed away from the vessel in the alongship direction, but towards the vessel in the athwarthship direction. The strongest and sharpest response is related to the trawl warps. A strong herding in front of the warps is seen. Closer to the bottom, an athwarthship herding reaction is seen away from the trawl doors or possibly the lower parts of the warps. There were only minor differences when grouping the tracks according to light level, fish size, and fish density.

The influence of warp length on trawl dimension and catch of walleye pollock Theragra chalcogramma in a bottom trawl survey

Variations in trawl dimension, bottom contact, and catch with differing warp lengths during walleye pollock surveys conducted by the Fisheries Research Agency were examined. The ratio of warp length to fishing depth (scope ratio) was set at 2.5, 3.0 and 3.5 at depths of 110 m. At tact the bottom. Footgear contact was complete when the scope ratio was 3.0 or more. Walleye pollock, sculpins and flatfishes were the main catch in all tows, and the catch increased with scope ratio. There was no difference in the length frequency of walleye pollock (n=300) captured at 2.5 and 3.0 scope ratio. However, the length frequency at 3.5 was significantly different from that of other scope ratios. These results suggest that at a scope ratio of 3.0 or more, the trawl door will not leave the bottom at any depths. To complimentecho surveys for walleye pollock, a 3.0 scope ratio would be optimal, since the catch data for a 3.5 scope ratio was dissimilar from that of scope ratios.

Hydrodynamic properties important for control of trawl doors

The paper presents a method for mathematical modelling of threedimensional foils applied totrawl doors. Experimental data are parametrized to give the steady-state hydrodynamic coefficients. Transient effects are investigated by the use of a numerical vortex lattice method. A method for approximating these effects is suggested, along with the appropriate coefficients. The proposed model accounts for steady-state and transient hydrodynamic forces and moments in 6 degrees of freedom. It will be used for the purpose of trawl control system design and analysis.

RoxAnn bottom classification system, sidescan sonar and video-sledge: spatial resolution and their use in assessing trawling impacts

Abstract Three complementary seabed characterization tools with different spatial resolution were used to locate a research site and to assess physical effects of experimental otter trawling in the Barents Sea: an acoustic seabed classification system (RoxAnn), sidescan sonar and a video-sledge. The marine protected area (MPA) around Bear Island was chosen as it offered unfished reference sites. The area was topographically complex which resulted in certain challenges for choice of the experimental site due to the requirements of representativity and homogeneity and suitable sampling substrate. Systematic waylines with RoxAnn gave broad-scale patterns of bottom conditions, the more informative sidescan revealed topographic reliefs, whilst detailed information on sediment composition and small-scale seabed features was provided by the video-sledge. Accurate positioning of towed gears (trawl, sidescan and video-sledge) ensured unbiased data acquisition. Trawl doors and rockhopper gear created furrows that were visible by sidescan sonar and video. Intensive trawling also caused changes in the acoustic properties by increasing roughness and decreasing hardness. Results are consistent with a possible resuspension of the sediment and a homogenizing effect from the trawl doors and ground gear ploughing the area. The suitability and advantages of using spatially overlapping tools in trawl impact studies are discussed.

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

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

By‐catch begone: changes in the philosophy of fishing technology

AbstractSince humans began fishing (at least 90 000 years ago), fishing technology has developed with the objective of trying to catch the greatest quantities of fish possible, of an ever‐increasing variety. Fishing technology has evolved from simple harpoons and hooks to the industrial factory trawlers of the 20th century. After millennia of assuming that seafood resources are inexhaustible, and centuries of somewhat muted concerns that advanced fishing technologies may have detrimental impacts on stocks and ecosystems, the last century has seen advances in fishing technology blamed as a major cause of the current over‐exploitation of fish stocks. It has mainly been during the last few decades that fishing technologists have begun to focus on more conservation‐orientated goals. This occurred initially in response to concerns over the by‐catch of charismatic species (like dolphins in tuna purse‐seines), but quickly broadened to address concerns over the discarding of not‐so‐charismatic species (like juvenile fish killed by shrimp trawling). To ameliorate these issues, technologists and commercial fishers successfully developed various innovative gear‐based and operational solutions. The steps involved in successfully reducing by‐catches have tended to follow a certain incremental framework involving identification of problems using observer programmes, developing technological solution to these problems, experimentally testing these solutions, implementing these solutions throughout industry and finally gaining acceptance of the solutions from concerned interest groups. Most recently public concern has broadened once again from by‐catch issues to encompass a much wider context involving the impacts of fishing on entire ecosystems, i.e. the impacts of fishing on all species affected – not just those species caught, retained or discarded. As a consequence, there have been many calls for ecosystem‐based fisheries management to ensure that fisheries operate under the principles of ecologically sustainable development. Scientists are gradually filling the gaps in our knowledge about how fishing affects whole ecosystems but, because of the scales and complexities involved, such studies are usually difficult, expensive and of long duration. While this descriptive work is difficult, finding solutions to any identified problem is an even greater challenge, particularly for fishing technologists. The easiest solutions to such problems involve rather draconian management strategies like closures. A less extreme alternative involves the development of new technologies that reduce the impacts of fishing on ecosystems – in a similar way as that done to reduce by‐catch problems. Innovations like altering ground‐chains, footropes, sweeps and trawl doors have been suggested as possible ways to ameliorate the environmental damage done by trawling, but such research is still very much in its infancy. Nevertheless, the recent history of fishing technology is chequered with successfully meeting such challenges, giving one confidence that solutions to such issues may eventually be developed. Integral to the success of any solutions that strive towards the goal of perfect selectivity, however, is a corresponding improvement in the adoption of these methods by fishers. As our framework shows, this is best achieved by involving fishers in all aspects of the work.

Bridle efficiency of a survey trawl for flatfish: measuring the length of the bridles in contact with the bottom

A bottom trawl catch of flatfish is composed of fish that were initially in the path of the net and fish that were initially in the path of the bridles and were subsequently herded into the net path. Bridle efficiency (i.e. the proportion of fish in the bridle path herded into the net path) can be estimated by fitting a model to catch and trawl measurement data collected in a field experiment in which the bridle path is varied in width by varying bridle length. Since flatfish are herded by the direct contact or close approach of the bridles with the bottom, part of the necessary field work is estimating the length of the bridles in contact with the bottom. Previously, bottom contact of the bridles has been determined using video cameras, but this is not possible when the bridles are entirely obscured by the mud clouds generated by the trawl doors. As an alternative approach, a bridle-mounted bottom contact sensor (BCS) was developed to allow measurement of the off-bottom distance of a bridle and the percentage of time it is in contact with the bottom. For the survey trawl studied, approximately 43% of the lower bridle was sufficiently close to the bottom to presumably elicit a herding response; however, the middle of the three bridles sagged to such an extent that it touched the bottom ahead of the lower bridle and extended the length to 51%.

The deep-water coral Lophelia pertusa in Norwegian waters: distribution and fishery impacts

The paper presents documentation on the distribution of, and damages to, deep-water reefs of the coral Lophelia pertusa in Norwegian waters. The reef areas have traditionally been rich fishing grounds for long-line and gillnet fisheries, and the coral habitat is known to support a high diversity of benthic species. Anecdotal reports claim that trawlers often use the gear, wires, chains and trawl doors to crush the corals and clear the area before fishing starts. To get an overview of the situation, information about the distribution and damage were collected from the literature, fishermen, and our own investigations. The results show that the corals are abundant particularly on the mid Norwegian continental shelf between 200 and 400 m depth. In general it seems that the largest densities are distributed along the continental break and at ridges of morainic origin. The reports from fishermen suggested severe damage to the corals and in situ observations using ROV confirmed the presence of mechanically damaged corals located on trawling grounds. A first estimate of the fishery impact indicates that between 30 and 50% of the reef areas are damaged or impacted. Fishermen claim that catches are significantly lowered in areas where the reefs are damaged. Potential ecological consequences of the destruction are discussed.

Modelling and Control of a Trawl System in the Transversal Direction

Today trawling is probably the most important fishing tackle for the fisheries. Still, this is a tool that. is not. friendly to the environment. A lot of undesirable small fish are caught, sea floor and coral reefs are often damaged, and the efficiency in proportion to energy consumption could be increased. Conventional trawl systems are not equipped with actuators for feedback control. In this paper a mathematical model for automatic control of the trawl system in the horizontal plane is developed. Minor modifications on the trawl doors making them suitable for automatic control are suggested, and local PID controllers for motion control in sway are developed. Sway control may reduce the average towing drag. This will contribute to higher efficiency, and may also reduce the risk for collisions with obstacles on the sea bed. Numerical simulations will demonstrate the performance of the proposed control system.

Impacts of trawl door scouring on infaunal bivalves: results of a physical trawl door model/dense sand interaction

The physical interaction of otter trawl doors with the seabed and the associated damage to infaunal bivalves were simulated in a laboratory test tank using a full-scale otter trawl door model. A scour test was performed in a sand testbed constructed to simulate a seabed on the northeastern Grand Banks of Newfoundland. As it scoured the testbed, the trawl door model created a 2 cm deep furrow, the pre-determined scouring depth of the trawl door shoe, and an adjacent berm of displaced frontal spoil along the trailing edge of the trawl door. Bivalves in the scour path at the sediment-water interface in two replicate experimental blocks were displaced to the berm, and 58% and 70% of displaced specimens which were originally buried were completely or partially exposed at the testbed surface. Out of a total of 42 specimens which had been placed in the scouring zone, two showed major damage. We propose a mechanism to explain the apparent anomaly of bivalve displacement with little associated damage based on sediment mechanics, and size and life position of infaunal bivalve species living on this bottom type.

Impacts of Flounder Trawls on the Intertidal Habitat and Community of the Minas Basin, Bay of Fundy

Four experimental trawls were made at highwater over the intertidal zone of the Minas Basin and the effects assessed when the tide was out to determine the physical and biological impacts of groundfish trawling on the benthos. The trawl doors made furrows 30–85 cm wide and up to 5 cm deep. The rollers compressed surficial sediments but did not scour a depression. The bridle caused no obvious disturbance. Door furrows and roller marks remained visible for 2–7 mo. No significant impacts were observed on either benthic diatoms or macrobenthos. The macrobenthos was dominated by polychaetes, some of which may have the ability to take evasive action as a trawl approaches. There were few molluscs, crustaceans, or echinoderms present; these taxa have been shown to be more susceptible to trawling damage in studies done elsewhere. Nematode numbers were initially depressed in the door furrows but did recover with time. It is not known whether nematodes were killed or displaced but the latter is thought more likely. Overall, the impacts in this particular environment are judged to be minor, especially since the intertidal sediments of the Minas Basin are already exposed to similar natural stresses imposed by storms and winter ice.

The effect of commercial trawling on sediment resuspension and transport over the Middle Atlantic Bight continental shelf

Numerous field observations have revealed that turbulence created in the wake of trawl doors can generate large and highly turbid clouds of suspended sediment. Time-averaged concentrations of sediment resuspended by trawls from various areas of the Middle Atlantic Bight continental shelf have been estimated using a simple mathematical model and National Marine Fisheries Service records of commercial trawling activity. Mean concentrations of sediment put into suspension by currents have also been computed using a modified form of the Glenn and Grant model. The results indicate that sediment resuspension by trawling can be a primary source of suspended sediment over the outer shelf, where storm-related bottom stresses are generally weak. The concentration estimates further suggest that sediment resuspended by trawls makes a sizeable contribution to the total suspended sediment load over the heavily trawled central shelf area of Nantucket Shoals during all times except winter and early spring. The level of trawling activity declines dramatically going seaward across the outer shelf. This decline coupled with cross-shore water motions in the area appears to result in a net offshore transport of sediment across the shelf edge. However, the estimated magnitude of this transport indicates that trawling does not produce significant short-term erosion of outer shelf sediments.

Estimation of Trawl Door Spread from Wing Spread

Estimation of Trawl Door Spread from Wing Spread

Teaching unit for blood pressure screening and control for secondary schools: Jim Glick, Health Education Division, State of Washington Department of Social and Health Services, Olympia, Washington

To address the relative importance of shrimp trawling on seabed resuspension and bottom characteristics in shallow estuaries, a series of disturbance and monitoring experiments were conducted at a bay bottom mud site (2.5 m depth) in Galveston Bay, Texas in July 1998 and May 1999. Based on pre- and post-trawl sediment profiles of 7Be; pore water dissolved oxygen and sulfide concentration; and bulk sediment properties, it was estimated that the trawl rig, including the net, trawl doors, and “tickler chain,” excavate the seabed to a maximum depth of approximately 1.5 cm, with most areas displaying considerably less disturbance. Water column profile data in the turbid plume left by the trawl in these underconsolidated muds (85–90% porosity; <0.25 kPa undrained shear strength) demonstrate that suspended sediment inventories of up to 85–90 mg/cm2 are produced immediately behind the trawl net; an order of magnitude higher than pre-trawl inventories and comparable to those observed during a 9–10 m/s wind event at the study site. Plume settling and dispersion caused suspended sediment inventories to return to pre-trawl values about 14 min after trawl passage in two separate experiments, indicating particles re-settle primarily as flocs before they can be widely dispersed by local currents. As a result of the passage of the trawl rig across the seabed, shear strength of the sediment surface showed no significant increase, suggesting that bed armoring is not taking place and the trawled areas will not show an increase in critical shear stress.

The missing link in the compliance chain: Jan Gilmore, Sharon Horn Holguin, High Blood Pressure Council of Los Angeles, California

To address the relative importance of shrimp trawling on seabed resuspension and bottom characteristics in shallow estuaries, a series of disturbance and monitoring experiments were conducted at a bay bottom mud site (2.5 m depth) in Galveston Bay, Texas in July 1998 and May 1999. Based on pre- and post-trawl sediment profiles of 7Be; pore water dissolved oxygen and sulfide concentration; and bulk sediment properties, it was estimated that the trawl rig, including the net, trawl doors, and “tickler chain,” excavate the seabed to a maximum depth of approximately 1.5 cm, with most areas displaying considerably less disturbance. Water column profile data in the turbid plume left by the trawl in these underconsolidated muds (85–90% porosity; <0.25 kPa undrained shear strength) demonstrate that suspended sediment inventories of up to 85–90 mg/cm2 are produced immediately behind the trawl net; an order of magnitude higher than pre-trawl inventories and comparable to those observed during a 9–10 m/s wind event at the study site. Plume settling and dispersion caused suspended sediment inventories to return to pre-trawl values about 14 min after trawl passage in two separate experiments, indicating particles re-settle primarily as flocs before they can be widely dispersed by local currents. As a result of the passage of the trawl rig across the seabed, shear strength of the sediment surface showed no significant increase, suggesting that bed armoring is not taking place and the trawled areas will not show an increase in critical shear stress.