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PREMECS-II: Development of predictive model of cod-end selectivity

  • January 2006
Authors:
Daniel Priour at Institut Français de Recherche pour l'Exploitation de la Mer
Daniel Priour
Finbarr Gerard O'Neill at Technical University of Denmark
Finbarr Gerard O'Neill
Antonello Sala at Italian National Research Council
Antonello Sala
P.Chevaillier
P.Chevaillier
P.Chevaillier
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... This wake vortex shedding can cause a deviation of fish trajectories thus, facilitating capture of juveniles. This is the reason why researchers such as Priour (2009), Priour et al. (2006), O'Neill et al. (2003, Arkley (2008), and Broadhurst and Millar (2009) recommended the use of square mesh or hexagonal mesh on the codend to facilitate flow passage, as this reduces the impact of turbulent flow on the fish response, thus minimizing undesirable bycatch. ...
Unsteady turbulent flow developing inside and around different parts of fluttering trawl net in flume tank
Article
Fluttering motions of a trawl are affected by its geometrical shape, natural underwater flows, and unsteady turbulent flow inside and around it. Thus, knowledge of the interaction between fluttering motions and the turbulent flow developing inside and around the different parts of the trawl is essential for understanding the hydrodynamic performance and selectivity of fishing gear. This paper assesses the coupled dynamics of the fluttering motions of different parts of a trawl and their hydrodynamics behavior. The analysis is based on three-dimensional electromagnetic current velocity meter measurements carried out inside and around different parts of a 1/18 scaled bottom trawl model. Instantaneous flow velocity fields and turbulent flow parameters were analyzed from obtained flow data. Moreover, fast Fourier transform and wavelet transform analysis were conducted to analyze the time–frequency contents of instantaneous flow velocity fields and fluctuating parts. The motions of different parts of the trawl net were oscillatory quasi-periodic and related to low frequency activity. The mean velocity ratios inside and around trawl wing, square, trawl body, and codend were 0.92, 0.89, 0.86, and 0.84, respectively. These results demonstrate the existence of unsteady turbulent flow inside and around different parts of the trawl. This unsteady turbulent flow corresponds to turbulent boundary layer flow, turbulent flow because of trawl wake, and vortex shedding. The dominant frequencies of these unsteady turbulent flows showed low frequency activities and wavelet energy was transferred from small-scale to large-scale motions under unsteady shear turbulence. In addition, knowledge of unsteady turbulent flow is greatly important for better understanding the hydrodynamic forces acting on a trawl and its geometrical shape as well as for implementing various selective devices and mesh shapes to reduce juvenile by-catch.
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... Therefore, prior to implementing such a stimulating device, it seems important to determine the codend hydrodynamics and its associated oscillations frequency content. Moreover, a great disparity is obtained in the measurements when dealing with different flow conditions not only during real fishing operations [16] but also in a laboratory [20]. In this study, we propose a preliminary specific flow configuration which has been scaled to approach the real flow field around full scale codend. ...
Analysis of hydrodynamics of a moving trawl codend and its fluttering motions in flume tank
Article
The coupled dynamics of the codend fluttering motion and its hydrodynamics behavior are investigated based on PIV measurements carried out around a 1/10 scaled model codend. The mean wake flow characteristics are firstly analyzed demonstrating the great effect of the codend motion. The differences in flow contour between fluttering motion and no-fluttering codend are highlighted by analyzing the classical mean characteristics of a body wake flow. In presence of fluttering motion, the classical results are recovered only if one considers the vertical extent of the codends oscillations as a reference length scale. Secondly, the frequency content of codend oscillations is analyzed and it is linked to the physical mechanisms. It is observed that the motion of the codend is mainly of a low-frequency activity and of another component related to the vortex shedding street. It is then demonstrated a complex fluid-structure interaction where the near wake hydrodynamics of the moving codend structure are an intricate interplay between the codend fluttering motion and the shear layer instabilities and large scale vortex shedding. The knowledge of such flow instabilities is of great importance to improve the understanding of the force acting on a trawl and for the implementation of some selected device to drive fish near the codend and then to reduce juvenile by-catch.
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... Though low selectivity for whiting has been reported in several studies (Zuur et al., 2001, Madsen et al., 1999, it is not always the case , and the lack of lengthbased selection observed here for whiting in the standard net was unexpected. Cod-end selectivity is known to depend on other factors as well as net design, such as water temperature and physiological condition of the fish (Özbilgin and Wardle, 2001, Breen et al., 2004, Ozbilgin et al., 2006 and on girth which, for a particular length, varies with season and maturity stage (Anon., 2004b). Such factors, studied in haddock, could explain these whiting results. ...
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... Flume tank tests carried out in Danemark (Anon. 2000) have shown that the mesh opening in the cod-end is largely affected by the catch but also by the mesh opening stiffness. In figure 1 it is quite clear that the catch has a large effect on the cod-end shape: on the top of the figure there are 40kg of catch when on the bottom the catch is 300kg. When the catch increases, the length and th ...
Investigation of methods for the assessment of the flexural stiffness of netting panels
Conference Paper
Full-text available
  • Oct 2011
There is a tendency to use stiff nettings in the fishing industry. This could affect the selectivity of fishing gears: a stiff netting in flexion has a large mesh opening stiffness and therefore the mesh opening during the fishing operations could be small, which could affect the escapement of small fish. We have developed and compared methods for the measurement of the flexural stiffness of nettings (EI). These methods are based on two phases. The first phase consists in pretty simple tests of the rectangular pieces of netting: one end is fixed horizontally while the other end is free to bend under its own weight. For each length of free netting the vertical deflection is recorded. The second phase consists in simulating this deformation with a Finite Element Method model which takes into account the flexural stiffness of nettings (EI). The EI is assessed by adjusting its value in the model in order to tune the simulated vertical deflection with the recorded one. Three nettings (PA, PE, Breizline) have been evaluated and the EI values have been compared with values measured by Sala & al (2007).
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... Selectivity of trawl occurs mostly in the cod-end, just in front of the catch. The mesh opening largely affects the ability of fish to escape. It is quite clear that a mesh well opened facilitates the fish escapement. Flume tank tests carried out in Danemark (Anon. 2000) have shown that the mesh opening in the cod-end is largely affected by the catch but also by the mesh opening stiffness. In figure 1 it is quite clear that the catch has a large effect on the cod-end shape: on the top of the figure there are 40kg of catch when on the bottom the catch is 300kg. When the catch increases, the length and th ...
Contributions on the Theory of Fishing Gears and Related Marine Systems (Vol. 7)
Book
Full-text available
  • Oct 2011
1. Methods and development in fishery science 2. Underwater robotics 3. Investigations on fluid-netting interactions 4. Fishing gear-seabed interactions 5. Simulations and model tests 6.Methods and concepts for energy reduction
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... Druault). net structure with different size of catches (Pashen et al., 2002;Priour et al., 2006) may be possible, the determination of the drag contribution associated with each part of the trawl (net, catch, hydrodynamic forces. . .) remains today inconceivable. ...
Time-Resolved PIV investigations France of the flow field around cod-end net structures
Article
Flow field measurements past a fixed rigid cod-end structure and past a porous fishing net structure are conducted using Time-Resolved PIV method. The rigid cod-end is first used to characterize finely the wake flow. Proper Orthogonal Decomposition (POD) is then applied in order to extract the large scale energetic vortices of the flow from the measured velocity field. It is then observed that the first POD modes are associated to the Karman's type flow structure of vortex shedding. It is shown that the characteristics of the wake flow behind the rigid cod-end flow configuration compare quite well with previous ones obtained from bluff cylinder or sphere wake analyzes. Second, PIV measurements are performed around a non-rigid bottom trawl which is free to move. Preliminary analyses show that the frequencies associated with the oscillatory motion of the porous structure are the same as the ones detected in the near wake flow demonstrating the lock-in regime. It is then expected that these preliminary results provide some interesting informations about future investigations on the drag force acting on fishing net structure.
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Contributions on the Theory of Fishing Gears and Related Marine Systems, Vol. 2
Book
Full-text available
  • Feb 2002
Specialists from 14 countries in Europe, Asia and Latin America met in Rostock in order to present and discuss their Research results which have been gathered since the last work shop two years ago. The scientific emphasis of this Workshop concentrated on the following Topics: 1. Theoretical and experimental methods for the design of fishing gears 2. Investigation on cod-end selectivity 3. interaction between maritime Systems and the sea bed.
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Numerical method for energy optimisation of bottom trawl
Conference Paper
Full-text available
  • Jan 2010
Bottom trawl energy efficiency is greatly affected by the drag, as well as by the area covered during fishing operations. Generally, the drag results in an overall increase of the energy consumption; moreover the value of the sweeping width affects directly the amount of fish caught. Many types of optimisation techniques have been developed to tackle bottom trawl design in order to reduce the volume of fuel per kg of fish caught and consequently the drag per swept width of the bottom trawl. Based on a finite element method model adapted to fishing net structures, our constrained optimisation tool modifies a reference design and selects the best according to the drag to swept width ratio. Previously, our strategy was built on a fixed percentage of the panel dimensions, adapted to pelagic trawls optimizing the drag to swept surface ratio. In contrast, the present method is adapted to bottom trawls and based on a set of percentages among which a selection is made with respect to the best drag to swept width ratio. For each step the best-modified design, in terms of drag per swept width is kept. This technique while adapted to bottom trawl design might lead to a decrease of the drag or an increase of the swept area. In the second case, that might lead to a substantial increase in fishing effort. This tool shows potential saving in fuel cost since it reduces drag and increases moderately catch volume leading nonetheless to a sizeable decrease in the fishing effort.
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Effect of Cross-sectional Shape of Fish Body on Mesh Selectivity of Trawl Codend.
Article
Full-text available
  • May 1999
The conclusion given by Tokai (1998) using the relative girth, the ratio of body girth to mesh perimeter (G/P), instead of the common body length of fish was verified by the result of 20 species' mesh selectivity in Isei pair-trawl and stern-trawl net of Japan. And, the influence of the relation between the cross-sectional shape of the fish body and shape of mesh opening on mesh selectivity was examined. The Tokai (1998)'s conclusion which says that the selectivity starts roughly at 0.5 in G/P and attains 100% at 1.0 G/P was well applicable to the case of the most fishes except a few fish species in this study. The relation between the relative girth and the cross sectional coefficient give the domeshaped curve with a mode at the aspect ratio of mesh. However the selection range seems related to other characteristics such as the hardness of the fish body part with the maximum body girth rather than the cross sectional coefficient. For the relation between the type of mesh and the fish body shape, it was suggested that the use of diamond and square mesh would be effective for fish with flat bodies and round bodies, respectively, to obtain a highly selective catch.
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Variation of Square Mesh Codend Selectivity for Walleye Pollock Theragra chalcogramma with Respect to Difference in Body Shape.
Article
  • Jan 1997
Square mesh codend selectivity experiments for walleye pollock Theragra chalcogramma were carried out along the Pacific coast of the Kuril Islands and off Miyagi, Tohoku. Selection curves varied by differences in body shape. Catches taken off Miyagi tended to retain smaller individuals when compared with catches using the same codend mesh size off the Kuril Islands. A detailed investigation of the total length-body girth relationship showed that there were significant differences in the maximum body girth of walleye pollock for fish of the same length. It was also observed that when the catch of fish in the selection range was relatively small, the selection curve tended to be sharp. The selection master curve was expressed in terms of the retention ratio of the codend to the body girth (G) and mesh perimeter (P) ratio (G/P). From these findings, it is suggested that selectivity research should include investigations of changes in the target body shape in the fishing grounds.
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Fish Behaviour and Fishing Gear
Chapter
  • Jan 1986
Throughout history, human hunters for fish have made use of their knowledge of fish behaviour in order to make catches. There are more than 22000 different species of teleost fishes, each with its own characteristic world of reaction and behaviour, so that numerous appropriate fish capture systems have been invented. Outlines of our knowledge of the sensory ability and behaviour of fishes have been summarised in the earlier chapters of this book. Fish behaviour is involved in catching fish, both on the oceanic scale, where the annual cycles of maturity cause migrations so that fish are found in different locations that become known to the fisherman by observation of their availability, and on a smaller scale, where the reactions of a fish to each part of an approaching trawl can cause the fish to swim into the codend. In order to be successful, the fisherman must have local knowledge of the day-to-day movements of the fish and of their likely distribution. In all fisheries, one of the most important of the fisherman’s skills is to use the appropriate gear at the right time in the right place.
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