The experts from the World Wildlife Fund (WWF) are convinced that in order to reduce the negative im-pact of bottom trawling on the marine ecosystem (both within and outside the EEZ) it is necessary to minimize the impact of trawls on bottom ecosystems
(a comprehensive assessment of the impact on bottom communities, arranging the areas closed to bottom trawling, reducing the number of trawlers); develop and implement new systems of management and regulation of marine ecosystems . If the bottom trawling process is carried out taking into account the ecosystem approach and meets the criteria of independent certification systems, the WWF will support such fishing. The general fishery policy of the world community is that fisheries are provided in terms of the ecosystem perspective and considering the environmental impact of bottom trawling. Therefore, for the rational and effective management of trawl fisheries it is necessary to quantitatively measure the effect of bottom trawls on the benthos of the reservoir [2-8]. To date, most studies and methodologies for assessing the impact of bottom trawls on benthos differ in the type and trawling equipment. In order to correctly design and operate bottom trawl systems with reduced impact on benthos, it is necessary to take into account their influence at the level of individual elements of the trawl system (trawl doors, trawl doors and cables) and at the level of individual components and types of sediments on the bottom of the reservoir (depth of movement
of trawl doors, trawl and cables) shown in Fig. 1.
Fig. 1. Bottom trawl system
Bottom trawl system includes bottom trawl, trawl doors, ground rope, rigging, cable line, warps and con-trol sensors.
The movement of trawl doors on the ground, crashing into it, as well as the ground rope and cables, increases the total resistance of bottom trawls and the wear of these parts. Consequently, there may be envi-ronmental implications as well as impacts on the effi-ciency and safety of the bottom trawl system and its fishing operations, which increases emissions of nitro-gen oxides, sulfur oxides and greenhouse gases.
Formulation of the problem
An important component of the Strategy for the development of the fishery complex of the Russian Federation is the definition of a policy of sustainable fishing and its implementation through the promotion of competitive, environmentally sustainable and eco-nomically profitable fisheries. Indicators relating to the physical impact of bottom trawling on the benthos
of the reservoir and to the influence on the habitat and integrity of the seabed will make a particular contribu-tion to the development of sustainable fisheries. Bot-tom trawl fishing proposals are expected helpful to make decisions about permitted fishing activities. This will allow the fishing industry in the Russian Federa-tion to prepare for and respond to future management measures, modify their bottom trawl systems, develop elements of bottom trawl systems with less impact on the benthos of the water body, and select the appropri-ate trawl fishing methods. Thus, constant access to fishing areas is provided and environmentally sustain-able and economically profitable exploitation of bot-tom and near-bottom accumulations of aquatic organisms is ensured .
When considering any physical and biological sys-tems (bottom trawl systems and fishing objects) it is advisable to distribute all variables that characterize the system or are related to it into three sets as objects of study (Fig. 2):
– input variables U1, U2, ..., Um – characterizing ex-ternal influences on system inputs;
– state variables X1, X2, ..., Xn – internal (intermedi-ate) variables, the totality of which completely charac-terizes the properties of the system;
– output variables Y1, Y2, ..., Yz, representing those reactions to external influences and those states of the system that are of interest to the researcher.
Fig. 2. Input, variable and output parameters of the system
Let us consider abiotic, biotic and anthropogenic factors of influence on the control systems of the trawl complex.
Abiotic factors include all the parameters of the surrounding water-air environment and the bottom
of the reservoir that affect the bottom trawl system: depth of the fishing area and the associated water pressure; flow of water masses; wind power; excitement; air and water temperature; presence of ice salinity; transparency; nature and topography of the soil; etc. Abiotic factors are the input parameters of the U system (Fig. 2). Input effects U depend on many factors of the surrounding water-air environment and the bottom of the reservoir.
Biotic factors include all possible influences that aquatic organisms experience from the surrounding aquatic organisms. In other words, the effect of popu-lations of fish or other organisms on the fishery stock. Restrictions can be imposed on bottom one-time fish-ing due to the presence of sea animals and birds in the area, a large number of valuable fish species juveniles in the distributed fish schools at the bottom of the res-ervoir, as well as the distribution of food objects. These factors include the behavior of fish – conditions that determine certain actions in order to survive and protect the offspring: movement speed (throwing, swimming); cluster density; distribution; reaction to stimuli; visibility range; migrations. Fig. 3 shows rep-resentatives of the bottom layers of the reservoirs.
Fig. 3. Hydrobionts: a – fever; b – haddock; c – sea flounder
Biotic factors are the variable parameters of the
X system (Fig. 2). Anthropogenic factors are factors caused by human impact on the benthos of a reservoir. Anthropogenic factors are input and variable parameters U and X (Fig. 2). The contact of bottom trawls with the seabed can lead to the penetration of elements of the bottom trawl system (trawl doors and ground rope) into the soil, lateral displacement of the benthos and a pressure field transmitted through the benthos (Fig. 4).
Fig. 5 presents a diagram of a bottom trawl system without sensors.
Fig. 4. Details of rigging the bottom trawl system that touch and cut into the bottom of a reservoir:
a – trawl door; b – ground rope; c – cable
Fig. 5. Scheme of the bottom trawl system (without sensors)
When designing bottom trawl systems, there is al-ways a specific list of performance requirements that bottom trawls must meet. In the general case, the list
of requirements includes functional requirements, i. e.
a list of quantitative indicators of the object of fishing, to which the action of the bottom trawl is directed, indicators of special fishing conditions and restrictions under which fishing is performed, environmental friendliness of fishing, energy costs, information costs, the value of forces etc. As part of the study, it is necessary to develop models of the operation processes of the bottom trawl complex, taking into account the complex influence of abiotic, biotic and anthropogenic factors, as well as the impact of the human factor on the control systems of the trawl complex. In particular, they include:
– determination of the depth to which the elements of the trawl system (trawl doors, trawl and cables) penetrate the seabed;
– development of predictive models of the physical impact of the elements of the trawl system (trawl doors, trawl and cables);
– determine which elements of the elements of the trawl system have the greatest impact on benthos;
– to determine the influence of the bottom trawl speed v on the soil-dynamic resistance of the elements of the trawl system (trawl boards, trawl and cables);
– prepare proposals for fishers on how to modify their bottom trawls to reduce the impact on the benthos of the reservoir.
It is possible to control the variable states X1,
X2, …, Xn of the bottom trawl system by using the autotrawl system  to reduce the negative impact on the benthos of the reservoir, but in the absence of sensors for monitoring the bottom trawl system, control is only possible by means of regulating the speed
of trawling and length of etched warps, taking into account the experience of the trawlmaster or a captain, trawl probe and sonar readings.
Biotic factors will be considered in the context
of the research . The swimming speed of a fish in water is proportional to the frequency and amplitude
of its body and tail vibrations and fits into the frame-work of certain mathematical equations.
To describe the input anthropogenic factors affect-ing the control systems of the trawl complex there are demonstrated the main parameters of the mechanical properties of soils . It is the mechanical properties of soils that are the basis for determining the output parameters of the bottom trawl system, designing bot-tom trawls and their elements. Soil characteristics are its features that depend on the composition and relationships between the components. The mechanical characteristics of soils are properties that manifest themselves when loads are applied to the soil. Characteristics serve as initial information and are of great importance for the study and prediction of trawling processes.
To calculate the deformations, the load that the soil can withstand and evaluate the indentation of the elements of the bottom trawl system, as well as the strength of the elements, it is necessary to have data on the mechanical properties of the exploited soils. The mechanical characteristics of soils are influenced by their composition, parameters of the physical state, as well as the features of their structure. The parameters of the physical state include:
– natural humidity (W) is the amount of water that is contained in the pores of the soil in its natural occur-rence. This value is the most important natural charac-teristic of the physical and mechanical state of the soil, which determines the strength;
– soil density (ρгр) is directly proportional to soil cohesion, which increases with increasing density and affects water permeability;
– modulus of deformation (E) – the value of the forecast of subsidence of the soil. Reflects the re-sponse of soils to external load impacts.
For rocky soils, strength is evaluated by the ulti-mate value of compressive strength in one axis, and for non-rocky soils by their mechanical parameters. The following strength levels are distinguished; super strong – the value is more than 120 MPa; strong – more than 50 MPa, but less than 120 MPa; the average strength index is less than 50 MPa, but more than 15 MPa; low strength – less than 15 MPa, but more than
5 MPa; reduced strength – less than 5 MPa, but more than 3 MPa; reduced strength – less than 3 MPa, but more than 5 MPa; very low strength – less than 1.
Consider the schematization of the movement of the bottom trawl door on the ground (Fig. 5). The first studies in this direction were carried out by P. G. Grewe , who found that the soil resistance forces acting on the trawl door significantly exceed the forces of sliding friction and can reach values of the same order as hydrodynamic forces. The most complete data on the characteristics of trawl doors moving along the ground were obtained by V. L. Vedeneev [14, 15]. He also developed a method for calculating the forces of soil resistance acting on the board. This does not take into account the angle of attack, roll and trim.
We introduce the condition of not cutting the trawl door into the soil (minimum impact on the soil of the reservoir):
where ω – angle between the force vectors F and Gd in the yOx plane,
where T0 – tension at the bottom of the warp; F – re-sultant forces; Gd – weight in water of the trawl door. According to the actions of forces (parallelogram rule),
where φ – angle between the force vectors T0 and Gd in the yOx plane. The horizontal projection of the tension at the lowest point of the warp F is determined by the expression under ω = 90°:
where Rxa – aggregate resistance of the trawl system; Rxв – warp resistance.
Fig. 6 shows a schematic movement of the bottom trawl door on the ground.
Fig. 6. Schematization of the movement of the bottom trawl door on the ground:
b – trawl door chord; l – sweep of the trawl door; td – trawl board thickness;
lk – sediment of the soil (cutting the trawl door into the soil);
bk – chord of soil settlement (cutting the trawl door into the soil); To – tension at the bottom of the warp;
Gd – is the weight in water of the trawl door; F – resultant forces; φ – is the angle between
the force vectors To and Gd in the yOx plane; ω – is the angle between the force vectors F and Gd in the yOx plane
Areas where the trawl door is cut into the ground along the yOx and yOz axes
Let us write down the basic formulas for strength 
where σ – soil strength; E – modulus of soil deforma-tion (in the planes yOx and yOz E = const);
v – trawling speed; cгр – coefficient taking into account the mechanical properties of the soil; ε – relative com-pression of the soil.
Mechanical characteristics of the soil in the yOx plane
where εy – is the relative compression of the soil along the Oy axis; Ey – soil deformation modulus along the
Oy axis; Ex – soil deformation modulus along the Ox axis; h = lk – is the depth to which the element of the trawl system (trawl doors, trawl and cables) penetrates into the seabed; H is the thickness of the soil (benthos).
We write (4) as
where vy is the rate of subsidence of the trawl door or other element of the trawl system into the ground; vx – trawling speed; εx – relative soil compression along the Ox axis.
We write (5) as
since Ey = Ex, then from (8) and (9) we get
From the equations (6) and (7) we obtain, taking into account (3),
From (11) we find h taking into account (10)
Based on (11), we obtain
Relations (13) characterize the strength properties of the reservoir soil. Based on the equations (1) and (2) we obtain the value of the angle φ:
Poisson's ratio μ for the soil of the reservoir along the axes Oy and Ox
where μy – is Poisson's ratio along the Oy axis; μx – Poisson's ratio along the Ox axis.
In the absence of experimental data, the values
of the Poisson's ratio can be taken according to 184.108.40.206 of GOST 12248-96: for coarse clastic soils it is 0.27; for sand is from 0.30 to 0.35 depending on the density; for sandy loam is from 0.30 to 0.35 depending on the density; for loams is from 0.35 to 0.37, depending on the density; for hard clay is from 0.20 to 0.30 depending on the density for semi-hard clay is from 0.30 to 0.38 depending on the density; for hard plastic clay is from 0.38 to 0.45 depending on the density; for soft plastic clay is from 0.38 to 0.45 depending on the density; for flowable clay is from 0.38 to 0.45, depending on the density. In accordance with experimental studies , a plot of Poisson's ratio for model loams and sandy loams depending on soil moisture was obtained (see Fig. 7).
Fig. 7. Relationship between the Poisson ratio and the degree of moisture content of loams and sandy loams
The value of εy directly depends on h, whose value is the main factor affecting the benthos of the reser-voir. This means that if
where hд is the allowable depth to which an element
of the trawl system (trawl doors, trawl and cables) can penetrate the seabed, provided that the precautionary principle is observed.
The value hд is determined experimentally for each area where bottom trawling is carried out. Thus, a de-viation from the fulfillment of condition (14) will af-fect the threat of causing serious or irreversible dam-age to the benthos of the reservoir where bottom trawl systems are operated. For rocky rocks, condition (14) is satisfied. With a known value of Poisson's ratio μн for soils, it is not difficult to determine the value
of relative deformation εy; also, with a known value
of benthos thickness H, it is possible to determine the value of h.
Let us represent the aggregate resistance of the trawl system in the following form:
where Rxd – is the drag of the trawl door (Rxd = Rxdг +
+ Rxdгр, where Rxdг is the hydrodynamic drag of the trawl door; Rxdгр – is the soil dynamic resistance of the trawl door); Rxk – cable resistance (Rxk = Rxkг + Rxrгр, where Rxrг – cable hydrodynamic resistance, Rxk = 0; Rxkгр – cable soil-dynamic resistance); Rxo – is the tool-ing resistance (Rxo = Rxoг + Rxoгр, where Rxoг – is the hydrodynamic resistance of the tooling; Rxoгр – is the soil-dynamic resistance of the tooling) .
Movement on the ground of a cable, cable, reel, reel, trawl board or other element of the trawl system is associated with the influence of both the characteris-tics and characteristics of the soil of the reservoir (soil connectivity, soil density, etc.). An important value that affects the trawling process is the towing speed
of the bottom trawl system v.
The value of soil-dynamic resistance of the cable is determined by the expression
where fk – soil-dynamic friction coefficient of the ca-ble; Gk – cable weight in water; qk – weight in water
of one meter of cable; lk – cable length.
The value of the soil-dynamic friction coefficient of the cable at fk ≤ 1 indicates that it does not cut into the benthos, only sliding along the ground occurs . Let us consider the process of movement and cutting into the benthos of a cable and a section of soft ground rope in the form of a cable. The process can be considered if the angle of attack of the cable is 0° ≤ α ≤ 90°, the ratio is 25.0 ≤ qk / dk ≤ 300.0 N/m2, where dk is
a cable diameter. The value of the soil-dynamic coefficient of friction of the cable fk at qk / dk = 300 N/m2 and α = 90° reaches the value fk = 6-8, this indicates that at high pressure on the ground and the value of the angle of attack of the cable (cable section) α = 90° the cable is immersed in ground and is towed in the ground, and not on its surface.
The ratio qk / dk can be represented as
then expressions (12) and (13) are valid for the cables.
In general, the formula for calculating the soil-dynamic coefficient of cable friction fk for the condi-tions: 25.0 ≤ qk / dk ≤ 300.0 N/m2 and 0° ≤ α ≤ 90° turns into the expression:
where v is the towing speed in m/s.
In accordance with the results of mathematical modeling of the process of cable movement and cut-ting into benthos, it is proposed to use a cable of dif-ferent ratios along the length qk / dk, taking into ac-count its angle of attack α in the process of bottom trawling. It is important to note that in this case it is advisable to use the parameters of the chain line, the shape
of which is the lower line of the bottom trawl . The bottom trawl speed v has a positive effect on the soil-dynamic resistance of the cable, that is, the value of the soil-dynamic coefficient decreases with an in-crease in the speed of trawling v and cutting into the ground does not occur, but the cable parameters qk / dk and α must be taken into account in combination. So, to reduce the cutting of the bottom seine edges, it is advisable to reduce the length of the sagging part, in order thereby to reduce the area of the seabed lost for fishing. The length of the sagging part is the smaller, the heavier the edge. But heavy ropes cut into the ground more strongly, increasing the tension of the edges during hauling. To avoid this, in practice, the cuts are completed from several pieces of ropes of different weights, and the ropes at the net bag should have the smallest weight, and as they approach the ship, the weight of the ropes increases. In this case, the sagging part of the edge of the bottom seine may con-sist of ropes having different weights in the water. We shall proceed accordingly assembling the central part of the lower headline from the cable with its pressure on the ground qk / dk → min, the middle part of the half of the ground rope qk / dk → medium and in the approach to the ends of the lower headline (at the junction of the bare ends) qk / dk → max, taking into account qk / dk value of the bare ends. Thus, the mini-mum impact of the cables will be on the benthos, pro-vided that the pressure on the soil is minimal, while its deepening characteristic remains.
Fig. 8 shows a diagram of a composite ground rope in the form of a cable (steel cable, Hercules cable, etc.).
Fig. 8. Scheme of a composite ground rope in the form of a cable
The movement of trawl reels on the ground was studied by G. E. Bidenko [12, 20, 21]. From the results obtained by him, it follows that almost all the spools
of the ground trawl do not roll over the ground, but slide along its surface. This nature of the movement is explained by the fact that when the bobbin axis deviates from the normal to the direction of movement by only 2-3°, its rotation stops. In addition, in the process
of movement, significant friction forces arise in the bushings of the bobbins and between the bobbins, which also prevent their rotation. By analogy with the movement of a cable, let us imagine the movement of bobbins under the condition of 25.0 ≤ Gг / Dг ≤ 300.0 N/m2, where Gг is the weight of the reel in water; Dг – bobbin diameter. The compression of the soil by the bobbin occurs at significant pressures Gг / Dг ≤ 300.0 N/m2.
The ratio Gг /Dг can be represented as
then, expressions (12) and (13) are valid for the bob-bins.
In the course of the study, theoretical and experi-mental data of the Russian and foreign scientists on the effect of cutting into the ground of trawl doors and bottom trawls were analyzed. In the course of the study there have been obtained:
– models of the operation processes of the bottom trawl complex, taking into account the complex influ-ence of abiotic, biotic and anthropogenic factors, and the impact of the human factor on the control systems of the trawl complex;
– theoretical calculations relating the mechanical properties of soils, the characteristics of the elements of the trawl system (trawl doors, trawl and cables), as well as the hydrodynamics of the movement of the trawl system;
– dependence of the depth to which the elements
of the trawl system (trawl doors, trawl and cables) penetrate the seabed;
– predictive models of the physical impact of the elements of the trawl system (trawl doors, trawl and cables);
– influence of bottom trawl trawl speed v on the mechanical properties of soils and soil-dynamic resis-tance of trawl system elements (trawl doors, trawl and cables);
– the influence of the mechanical properties of the reservoir soils on the soil-dynamic resistance of the elements of the trawl system (trawl boards, trawl and cables).
The research revealed:
– soil-dynamic resistance of trawl doors increases as its weight increases;
– the angle of attack, roll and trim of the trawl door affects the soil dynamic resistance, since the coefficient that takes into account the energy for discarding the soil layer depends on the shape of the blade and the properties of the soil;
– the ground-dynamic resistance of the ground-rope parts increases as their weight increases;
– soil-dynamic resistance of the fixed parts of the ground rope is greater than the rolling resistance;
– the dependence of the soil-dynamic resistance coefficient of their cables on elongation, cutting into the ground and the angle of attack and details of the ground rope was obtained;
– the fixed rigging of bottom trawl systems may penetrate the seabed to a lesser depth when they are towed at higher speeds, but when they are rolling there is no such dependence.
Based on mathematical models (12) and (13), it is possible to minimize the impact of the anthropogenic factor on the benthos of the reservoir.