Skip Navigation


ICES Journal of Marine Science: Journal du Conseil Advance Access originally published online on July 27, 2007
ICES Journal of Marine Science: Journal du Conseil 2007 64(8):1573-1578; doi:10.1093/icesjms/fsm113
This Article
Right arrow Abstract Freely available
Right arrow FREE Full Text (PDF) Freely available
Right arrow All Versions of this Article:
64/8/1573    most recent
fsm113v1
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Alert me to new issues of the journal
Right arrow Add to My Personal Archive
Right arrow Download to citation manager
Right arrowRequest Permissions
Google Scholar
Right arrow Articles by Sala, A.
Right arrow Articles by Fryer, R. J.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Sala, A.
Right arrow Articles by Fryer, R. J.
Social Bookmarking
Add to CiteULike   Add to Connotea   Add to Del.icio.us  
What's this?

© 2007 International Council for the Exploration of the Sea. Published by Oxford Journals. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

Experimental method for quantifying resistance to the opening of netting panels

A. Sala1,, F. G. O'Neill2, G. Buglioni1, A. Lucchetti1, V. Palumbo1 and R. J. Fryer2

1 Istituto di Scienze Marine—Consiglio Nazionale delle Ricerche (ISMAR-CNR), Sede di Ancona, Largo Fiera della Pesca 1, 60125 Ancona, Italy
2 Fisheries Research Services, Marine Laboratory, 375 Victoria Road, Aberdeen AB11 9DB, UK

Correspondence to A. Sala: tel: +39 071 2078841; fax: +39 071 55313; e-mail: a.sala{at}ismar.cnr.it

Sala, A., O'Neill, F. G., Buglioni, G., Lucchetti, A., Palumbo, V., and Fryer, R. J. 2007. Experimental method for quantifying resistance to the opening of netting panels. – ICES Journal of Marine Science, 64: 1573–1578.

In recent years, the tendency in some sectors of the fishing industry has been to use thicker and stiffer twines in the manufacture of netting material. This increases the mesh's resistance to opening and, consequently, reduces the selective performance of fishing gears. The main characteristic of netting twine contributing to mesh resistance to opening is flexural rigidity (EI). We present a methodology for quantifying mesh resistance to opening through the use of a prototype called the resistance to opening and deflection meter (ROD-m). It incorporates four tension load cells and four stepping motors that are driven by four ministep bipolar chopper drives. Small panels of netting (3 x 3 meshes) can be mounted on ROD-m, which can measure the forces acting on the netting and the position of the knots. Estimates of the flexural rigidity of the netting twine can be made from these data. We present the results of the analysis of Mediterranean polyamide netting, and standard and Brezline polyethylene (PE) netting from the North Sea. In all netting, regardless of the material used, EI increases with increasing linear density. Furthermore, Brezline is much stiffer than standard green PE, although they are equally thick.

Keywords: flexural rigidity, mesh resistance to opening, netting materials, netting twine

Received 9 September 2006; accepted 11 June 2007; advance access publication 27 July 2007.


   Introduction
Top
 Introduction
Material and Methods
 Results
 Discussion
 References
 
In recent years, some sectors of the fishing industry have begun to use stronger and thicker twines in the manufacture of netting materials. Lowry and Robertson (1996) and Sala et al. (2007) demonstrated that the use of such netting material reduces the selective performance of fishing gears. They suggest that a number of mechanisms, such as the increased bending stiffness and the reaction of fish to more highly contrasting barriers, may contribute to the reduction in selectivity that they observed. These explanations are supported by the studies of O'Neill (2004), who demonstrates how an increase in twine bending stiffness will increase the resistance of codend meshes to opening; Glass et al. (1993), who reveal that haddock (Melanogrammus aeglefinus) are less likely to penetrate netting that presents a greater visual contrast; and Herrmann and O'Neill (2006), who carry out theoretical simulations using the PRESEMO model of codend selectivity.

Since 2000, EU legislation, aimed at improving codend selectivity, has prohibited the use of codends made from twine with a diameter greater than 5 mm (Anon., 2000). Although restrictions of this sort may help with mechanisms related to fish escape behaviour, they will not necessarily limit the use of twine with greater bending stiffness, which increases mesh resistance to opening. This is particularly the case with new twine types developed by the netting and twine industries that offer increased tensile and bending strength using the same or smaller diameter.

In this paper, we develop a method of quantifying the resistance of meshes to opening laterally. We present a prototype instrument that measures (i) the forces that act on small netting panels, and (ii) their deformation. These data were analysed using a number of assumptions about the netting construction and the mechanical properties of the twine. The analysis, in effect, estimates the mechanical and geometric parameters of an idealized netting panel that best approximates the behaviour of the netting panel tested.


   Material and Methods
Top
 Introduction
 Material and Methods
Results
 Discussion
 References
 
Resistance to opening and deflection meter
A prototype instrument, named resistance to opening and deflection meter (ROD-m), was designed at ISMAR-CNR, Ancona, Italy, and developed in collaboration with Asystel S.r.l., Trevi, Italy. It incorporates four tension load cells and four stepping motors that are driven by four ministep bipolar chopper drives. The ministep operation is connected to an electronic resonance damping facility that ensures excellent operating smoothness and low noise. The careful design for this specific purpose led to a highly reliable instrument (Figures 1 and 2).


Figure 1
View larger version (18K):
[in this window]
[in a new window]
[Download PowerPoint slide]
  		Figure 1. Detailed design of the ROD-m.

 


Figure 2
View larger version (113K):
[in this window]
[in a new window]
[Download PowerPoint slide]
  		Figure 2. General view of the ROD-m equipment and a North Sea PE double-twine netting panel specimen during the test.

 
The ROD-m is symmetric along the vertical and horizontal axes; a tension load cell and a stepping motor are mounted in each quadrant. Each stepping motor drives an endless screw, which, when rotating, positions a load cell. Three-by-three mesh netting panels (Figure 2) can be mounted between the four load cells, using the three steel hooks on each of the four linear guideways. The two outer hooks on each guideway are free to move along its length in response to the deformation of the netting.

The tension load cells and stepping motors are connected to a PC through signal conditioners. The operation of the stepping motors is governed by a Delphi program, which simultaneously records the resulting position and measurements of the four load cells. To ensure that the netting panel is positioned centrally on the instrument, the stepping motors on opposite sides (SM1 and SM4 and/or SM2 and SM3; Figure 1) are operated simultaneously. Thus, the forces in the normal and transverse directions (the average of the LC2 and LC3, and the LC1 and LC4 values, respectively) that act on the panel and the netting panel deformation can be measured. The load cells have a nominal load capacity of 60 kg with a combined error declared of 0.02%. The stepping motors have a basic step angle of 1.8° with a tolerance of 0.09°.

Experimental methodology
The netting material specifications (i.e. fibre type, twine construction, mesh type, etc.) were selected according to the netting commonly used in commercial Mediterranean and North Sea trawl codends. The main characteristics of the 13 different types of netting materials chosen are given in Table 1. Four samples of 3 x 3 mesh netting panels of each of the material types were tested. Each 3 x 3 mesh netting panel was subject to a series of pre-tension cycles, similar to those carried out during stress/strain experiments on netting twine, to remove the irreversible part of the elongation and to safeguard against knot slippage (Klust, 1983; Sala et al., 2004). The pre-tension cycle, which was repeated ten times for each panel, applying a load of 10 kg on each mesh bar, entailed loading the panel from 0 to 60 kg for the single twine, and from 0 to 120 kg for the double twine.


View this table:
[in this window]
[in a new window]

  		Table 1. Main characteristics of the netting tested.

 
The complete set of measurements on a single netting sample were performed in these steps:
  1. a netting panel is mounted on the ROD-m in a predetermined initial position;
  2. the normal load cells (LC2 and LC3) are moved to stretch the netting panel until the load force reaches 3 kg (and subsequently 6, 9, 12, and 15 kg) for single-twine mesh, and 6 kg (and subsequently 12, 18, 24, and 30 kg) for double-twine mesh;
  3. to account for relaxation of the netting twines, the panel is held stationary for 5 min and measurements of positions, and loads were taken every minute;
  4. steps (ii) and (iii) are repeated until the maximum value (15 or 30 kg) of the normal load is reached;
  5. the normal load cells are moved back to the initial position, and the position of the transverse load cells (LC1 and LC4) is increased by a predetermined increment;
  6. steps (ii)–(v) are repeated four times, at which point the positions of the transverse load cells (by setting the increment appropriately) are such that the mesh opening is approximately square.

Data analysis
To study mesh resistance to opening, it is necessary to consider the forces required to deform each of the component mesh bars. O'Neill (2002) investigated the system governing the bending of mesh bars under tension. He found an asymptotic solution of the governing differential equations in the bending stiffness of the twine, the mesh bar length, the tensile forces acting on a mesh bar, and the angle of slope where the mesh bars emerge from the knots, by assuming that:

  1. the bending moment of the twine is proportional to the curvature;
  2. there is no twine extension;
  3. the slope angle, where the mesh bars emerge from the knots, is fixed.
To analyse the experimental data, it is also necessary to assume that:
  1. the mesh structure is symmetric across the vertical and horizontal planes;
  2. the knots can be represented by rectangles;
  3. the mesh bars emerge from the knots at the corners of the rectangles.
These assumptions define an idealized panel of netting as portrayed in Figure 3. The mechanical and geometric parameters of this idealized netting panel are estimated by fitting solutions of the deformation of this panel to the experimental data. The parameters estimated are the mesh bar length, the knot dimensions, the angle at which a mesh bar emerges from a knot, and the twine bending stiffness.


Figure 3
View larger version (48K):
[in this window]
[in a new window]
[Download PowerPoint slide]
  		Figure 3. Netting material and the definition of the parameters of the idealized netting. In this diagram, {theta}, the slope angle at either end of the bars of the idealized mesh, is shown to be zero.

 
O'Neill (2002) demonstrated that the deformation of a mesh bar under tension can be expressed as:


Formula

and


Formula

where x and y define the bar deformation, {theta} is the slope angle (which is assumed to be the same) at either end of the mesh bar, m the mesh bar length, EI the twine bending stiffness, and f = (fx2 + fy2)0.5, where fx and fy are the respective tensile components that act at the end of each mesh bar and {theta} = tan–1fy/fx. Although these solutions are approximate, they are very accurate when EI/m2f < 0.04 (which can be proven retrospectively to be the case here; O'Neill, 2002).

The distances between successive knots, 2x' and 2y', were measured, rather than the deformations themselves, and these are given by:


Formula

and


Formula

where a and b are the dimensions of the knot (Figure 3). For each set of measurements, the parameters a, b, m, EI, and {theta} were estimated by non-linear regression, assuming that the x' and y' are distributed with constant variance and, for simplicity, uncorrelated.


   Results
Top
 Introduction
 Material and Methods
 Results
Discussion
 References
 
Figure 4 shows an example of the preliminarily data collected for one sample of North Sea PE single-twine netting, where the positions of the transverse load cells were kept constant and the panel was enlarged to a predetermined position in the normal direction. The netting panel was then left to relax and the loads continuously recorded. This figure plots the reduction of the tensile forces over a 5 min period and demonstrates the extent to which the netting panel can relax.


Figure 4
View larger version (8K):
[in this window]
[in a new window]
[Download PowerPoint slide]
  		Figure 4. Preliminarily ROD-m test on North Sea PE single-twine netting showing the decrease in magnitude of the transverse and normal loads with time, as the netting twine relaxes and settles.

 
Preliminary analysis demonstrated that the estimates of a, b, m, EI, and {theta} were all highly correlated, particularly the twine bending stiffness and the angle at which a mesh bar emerges from a knot.

Furthermore, {theta} was often negative, which is physically unrealistic. Consequently, it was very difficult to compare EI across netting types. To circumvent this, the data for each netting panel were re-analysed assuming {theta} = 0, the smallest physically acceptable value. The results of the subsequent analysis are presented in Tables 2Go4 and are summarized in Figures 5 and 6. It can be seen in Table 2 that the minute-by-minute variation for a given panel is negligible. This was generally the case, and Tables 3 and 4 contain only the average results for each panel. For a given netting type, the variation between panels is also usually small. The one exception is 4478 Rtex PA netting, which has a large between-panel variation. It is not clear whether these are representative readings of the panel mechanics or whether there were measurement problems for this set of trials.


Figure 5
View larger version (7K):
[in this window]
[in a new window]
[Download PowerPoint slide]
  		Figure 5. The average EI estimate of PE twines vs. linear density. The empty circles represent the Brezline, and the empty squares represent the standard green braid. The linear regression lines have been included for both PE twines: solid line, Brezline; dashed line, standard green braid.

 


Figure 6
View larger version (6K):
[in this window]
[in a new window]
[Download PowerPoint slide]
  		Figure 6. The average EI estimate of PA twines vs. linear density. The solid triangles represent the 4478 Rtex twine, which appears to be an outlier and is not included in the linear regression. A single linear regression line has been included for all PA twines.

 


View this table:
[in this window]
[in a new window]

  		Table 2. Result of analysis: a and b, size of the knot (mm); m, mesh bar length (mm); EI (N mm2), where the {theta} slope angle at the knot is assumed to be zero.

 


View this table:
[in this window]
[in a new window]

  		Table 3. A summary of the results for all other North Sea netting materials.

 


View this table:
[in this window]
[in a new window]

  		Table 4. A summary of the results for all Mediterranean-netting materials.

 
In Figures 5 and 6, we have plotted the mean EI estimates for the different netting types against Rtex, with linear fits to the data. The hollow circles in Figure 5 represent netting made from Brezline PE, and the hollow squares represent netting made from a traditional green-braid PE. The solid and dashed lines show the respective linear fits. For both materials, the estimates of EI increase with Rtex. These results also suggest that the Brezline material is stiffer than the traditional green braid.

In Figure 6, all the netting was made from braided nylon fibres. The solid triangles refer to the results for the 4478 Rtex netting. This netting is an outlier and is not included in the linear regression. Here, the relationship between EI and Rtex is not so obvious, and there is very little change over the range of materials tested.


   Discussion
Top
 Introduction
 Material and Methods
 Results
 Discussion
References
 
Attempts to obtain simultaneously plausible estimates of the five parameters a, b, m, EI, and {theta} were thwarted, because the parameter estimates were highly correlated. Further investigation revealed a more basic problem with parameter estimation that needs to be addressed. When only the transverse data x' were used to estimate a, m, EI, and {theta}, the estimates of EI and {theta} were usually plausible. However, when only the normal data y' were used to estimate b, m, EI, and {theta}, the estimates of {theta} were often large and negative. Therefore, there appear to be some inconsistencies between the two sets of data. Further, residual plots from both sets of analyses revealed a small but systematic lack of fit.

This lack of fit probably results from the many assumptions made concerning the twine mechanics and the netting construction. Other studies have demonstrated that the assumption that bending moment is proportional to curvature is unlikely to hold for the twines used in the manufacture of fishing gears, where there will be friction between the component fibres, which are likely to bend non-linearly and move over each other (Leaf, 1995). Therefore, the regression equations above may not represent the physical situation accurately enough to obtain good estimates of all five parameters. In particular, the estimate of {theta} might be too sensitive to small model departures to be reliable. Such model departures would also explain the occasional negative estimates of the knot dimensions a and b (Tables 3 and 4).

Another concern is that, strictly speaking, the transverse data x' should not be used as the response variable in a regression model. These values are set as part of the experimental design [in steps (i) and (v) of the experimental procedure], and it is the transverse force fx that varies accordingly and is measured. However, inverting the equations so that fx can be expressed in terms of x' is not possible analytically. In future experiments, it might be possible to adjust the experimental procedure so that both forces are set and both resulting displacements are measured.

Clearly then, great care must be exercised in interpreting the estimates of EI, a, b, and m, because they may be biased. We must not assume that we are obtaining an absolute estimate of EI for the netting twine and that a, b, and m are actual dimensions of the knot/mesh. Rather, we should view them as estimates of the mechanical and geometric parameters of idealized netting panels that best approximate the behaviour of those tested.

In this sense, the method we have presented is very useful. It is robust, in so far as the parameter estimates for the netting panels tested are not influenced by twine relaxation, and they are consistent between different panels of the same netting material. Therefore, the estimates of EI, a, b, and m provide a quantitative means of comparing the mesh resistance to opening of different netting panels and the bending stiffness of netting twines, which perhaps could be used as an explanatory variable in a selectivity analysis. This analysis also permits the introduction of mesh resistance to opening in netting deformation models (O'Neill, 1997, 1999; Priour, 1999). These models have demonstrated how twine bending stiffness can affect net and codend geometry (O'Neill, 2004) and have been used by Herrmann and O'Neill (2006) to simulate the influence of twine bending stiffness on codend selectivity.


   Acknowledgements
 
This paper was made possible with financial support from the Commission of the European Communities, specifically the RTD programme Quality of Life and Management of Living Resources, "PREMECS-II: development of predictive model of cod-end selectivity". It does not necessarily reflect the Commission's view and in no way anticipates its future policy in this area.


   References
Top
 Introduction
 Material and Methods
 Results
 Discussion
 References
 

    Anon. Commission Regulation (EC) No 2549/2000 of 17 November 2000 establishing additional technical measures for the recovery of the stock of cod in the Irish Sea (ICES Area VIIa). Official Journal of the European Union (2000) L292:5–6.

    Glass C. W., Wardle C. S., Gosden S. J. Behavioural studies of the principles underlying mesh penetration by fish. ICES Marine Science Symposia (1993) 196:92–97.

    Herrmann B., O'Neill F. G. Theoretical study of the influence of twine thickness on haddock selectivity in diamond mesh cod-ends. Fisheries Research (2006) 80:221–229.[CrossRef][Web of Science]

    Klust G. Fibre Ropes for Fishing Gear. In: FAO Fishing Manuals (1983) Farnham: Fishing New Books Ltd. 200.

    Leaf G. A. V. The friction couple in yarn bending. Journal of the Textile Institute (1995) 86:45–54.[Web of Science]

    Lowry N., Robertson J. H. B. The effect of twine thickness on cod-end selectivity of trawls for haddock in the North Sea. Fisheries Research (1996) 26:353–363.[CrossRef][Web of Science]

    O'Neill F. G. Differential equations governing the geometry of a diamond mesh cod-end of a trawl net. Journal of Applied Mechanics (1997) 64:1631–1648.

    O'Neill F. G. Axisymmetrical trawl cod-ends made from netting of generalized mesh shape. IMA Journal of Applied Mathematics (1999) 62:245–262.[Abstract/Free Full Text]

    O'Neill F. G. The bending of twines and fibres under tension. Journal of the Textile Institute (2002) 93:1–10.

    O'Neill F. G. The influence of bending stiffness on the deformation of axisymmetric networks. (2004) Twenty-third International Conference on Offshore Mechanics and Arctic Engineering. Vancouver, Canada.

    Priour D. Calculation of net shapes by the finite element method with triangular elements. Communications in Numerical Methods in Engineering (1999) 15:755–763.[CrossRef][Web of Science]

    Sala A., Lucchetti A., Buglioni G. The change in physical properties of some nylon (PA) netting samples before and after use. Fisheries Research (2004) 69:181–188.[CrossRef][Web of Science]

    Sala A., Lucchetti A., Buglioni G. The influence of twine thickness on the size selectivity of polyamide codends in a Mediterranean bottom trawl. Fisheries Research (2007) 83:192–203.[CrossRef][Web of Science]


Add to CiteULike CiteULike   Add to Connotea Connotea   Add to Del.icio.us Del.icio.us    What's this?



This Article
Right arrow Abstract Freely available
Right arrow FREE Full Text (PDF) Freely available
Right arrow All Versions of this Article:
64/8/1573    most recent
fsm113v1
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Alert me to new issues of the journal
Right arrow Add to My Personal Archive
Right arrow Download to citation manager
Right arrowRequest Permissions
Google Scholar
Right arrow Articles by Sala, A.
Right arrow Articles by Fryer, R. J.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Sala, A.
Right arrow Articles by Fryer, R. J.
Social Bookmarking
Add to CiteULike   Add to Connotea   Add to Del.icio.us  
What's this?