The Experts below are selected from a list of 201 Experts worldwide ranked by ideXlab platform
Aida Campos - One of the best experts on this subject based on the ideXlab platform.
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Selectivity of diamond and square Mesh cod ends for horse mackerel (Trachurus trachurus), European hake (Merluccius merluccius) and axillary seabream (Pagellus acarne) in the shallow groundfish assemblage off the south-west coast of Portugal*
Scientia Marina, 2003Co-Authors: Aida Campos, Paulo FonsecaAbstract:The effects of an increase in cod end Mesh size from 65 to 70 and 80 mm and a change of Mesh Configuration from 65 mm diamond to 65 mm square Mesh on the size selectivity of horse mackerel (Trachurus trachurus), hake (Merluccius merluccius) and axillary seabream (Pagellus acarne) of the shallow groundfish assemblage off the Portuguese southwest coast were evaluated. The increase in Mesh size had a small but significant effect on size selectivity for the three species, while the change in Mesh Configuration led to a much more pronounced increase in the selectivity parameters. For horse mackerel, the L50 estimates ranged from 14.4 to 16.0 cm in the diamond Mesh cod ends-values that are close to the minimum landing size of 15 cm. For hake, L50 of 17.0 and 18.3 cm were estimated for the 70 and 80 mm diamond cod ends respectively, while for the axillary seabream the L50 estimated was 13.9 cm for the 80 mm diamond Mesh cod end. These values are well below the minimum landing sizes of 27 and 18 cm for these species. The corresponding estimates in the square Mesh cod end were 21.9, 32.4 and 19.6 cm, with the loss of a high percentage (76%) of horse mackerel above the minimum landing size. For all the cod ends tested, the observed retention was presented as a function of maximum girth/Mesh perimeter, which allowed a better understanding of the selection process for the species in study. Selectivity estimates for horse mackerel and hake were also compared to those obtained by Campos et al. (2003) for the same species in 1993 off the south coast, where they are captured as a by-catch in the crustacean fishery.
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Size selectivity for four fish species of the deep groundfish assemblage off the Portuguese southwest coast: evidence of Mesh size, Mesh Configuration and cod end catch effects
Fisheries Research, 2003Co-Authors: Aida Campos, Paulo Fonseca, Victor HenriquesAbstract:Abstract The effects were evaluated of an increase in cod end Mesh size from 65 to 70 and 80 mm and change of Mesh Configuration from 65 mm diamond to 65 mm square Mesh on the size selectivity of hake ( Merluccius merluccius ), horse mackerel ( Trachurus trachurus ), four-spot megrim ( Lepidorhombus boscii ) and blue whiting ( Micromesistius poutassou ) captured on the upper continental slope off the Portuguese southwest coast, at depths from approximately 200–400 m. A number of individual hauls were analysed in the diamond Mesh cod ends, for hake and blue whiting, and in the square Mesh cod end, for horse mackerel, and mean selection curves were estimated for these cod ends taking into account between-haul variation. For the four-spot megrim, the selectivity estimates were based on pooled data for all the cod ends tested. Selectivity models are proposed for hake and blue whiting in which positive effects of both the increase in Mesh size and in cod end catch were estimated for L 50 , while SR was only affected by Mesh size. For horse mackerel a positive effect of cod end catch was estimated for SR in the square Mesh cod end. The selectivity was greatly affected by the change in Mesh Configuration for all species with the exception of the four-spot megrim, for which only Mesh size was found to affect selectivity.
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Size selectivity of diamond and square Mesh cod ends for four by-catch species in the crustacean fishery off the Portuguese south coast
Fisheries Research, 2003Co-Authors: Aida Campos, Paulo Fonseca, Karim ErziniAbstract:Abstract The effects of an increase in cod end Mesh size from 55 to 60 and 70 mm and a change of Mesh Configuration from 55 mm diamond to 55 mm square Mesh on the size selectivity of four by-catch species (the red shrimp Aristeus antennatus , the European hake Merluccius merluccius , the horse mackerel Trachurus trachurus and the blue whiting Micromesistius poutassou ) commonly captured in the crustacean fishery off the Portuguese south coast, were evaluated. Selectivity parameters for blue whiting, the most abundant species in the catches, were estimated taking into account between-haul variation, while for the remaining species, captured in much lower quantities, the selectivity estimates were based on pooled data by length class for all hauls within the same cod end. Length at 50% retention, L 50 , was found to increase with Mesh size and with the change in Mesh Configuration for all the studied species. For blue whiting trawling depth and cod end catch were found to play a role in between-haul variation by increasing L 50 as well. The results suggest that an increase in the current minimum Mesh size of 55–70 mm would be advisable to be compatible with the minimum landing sizes (MLSs) of 29 mm carapace length and 27 cm total length for red shrimp and hake, respectively, while it would greatly reduce the amount of discards, particularly those for blue whiting, that accounted for approximately 50% of the total catch weight. Horse mackerel was the only species for which the use of a larger Mesh size would result in a significant escapement of individuals above the MLS of 15 cm.
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size selectivity of diamond and square Mesh cod ends for rose shrimp parapenaeus longirostris and norway lobster nephrops norvegicus off the portuguese south coast
Fisheries Research, 2002Co-Authors: Aida Campos, Paulo Fonseca, Karim ErziniAbstract:Abstract The effects of an increase in cod end Mesh size from 55 to 60 and 70 mm and a change of Mesh Configuration from diamond to square Mesh on the size selectivity for rose shrimp Parapenaeus longirostris and Norway lobster Nephrops norvegicus captured off the Portuguese south coast were evaluated. The results were analysed taking into account between-haul variation in selectivity, and indicate a significant increase in L 50 for rose shrimp with an increase in Mesh size or with the use of a square Mesh cod end, while for Norway lobster only Mesh Configuration was found to affect this parameter. Two other important external variables were identified; the trawling depth and the cod end catch, which influence between-haul variation, by increasing the selection range for rose shrimp and Norway lobster, respectively. The results obtained suggest that an increase in the current minimum Mesh size of 55 mm would be advisable for rose shrimp in order to respect the minimum landing size of 24 mm carapace length presently established for this species. Moreover, trawling for rose shrimp should be avoided at depths above 200 m, in order to avoid catches consisting almost exclusively of juveniles. Such an increase in Mesh size would have a minor impact in terms of losses of individuals above the minimum landing size for Norway lobster and would contribute to reducing the amount of discards in this fishery.
Karim Erzini - One of the best experts on this subject based on the ideXlab platform.
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Size selectivity of diamond and square Mesh cod ends for four by-catch species in the crustacean fishery off the Portuguese south coast
Fisheries Research, 2003Co-Authors: Aida Campos, Paulo Fonseca, Karim ErziniAbstract:Abstract The effects of an increase in cod end Mesh size from 55 to 60 and 70 mm and a change of Mesh Configuration from 55 mm diamond to 55 mm square Mesh on the size selectivity of four by-catch species (the red shrimp Aristeus antennatus , the European hake Merluccius merluccius , the horse mackerel Trachurus trachurus and the blue whiting Micromesistius poutassou ) commonly captured in the crustacean fishery off the Portuguese south coast, were evaluated. Selectivity parameters for blue whiting, the most abundant species in the catches, were estimated taking into account between-haul variation, while for the remaining species, captured in much lower quantities, the selectivity estimates were based on pooled data by length class for all hauls within the same cod end. Length at 50% retention, L 50 , was found to increase with Mesh size and with the change in Mesh Configuration for all the studied species. For blue whiting trawling depth and cod end catch were found to play a role in between-haul variation by increasing L 50 as well. The results suggest that an increase in the current minimum Mesh size of 55–70 mm would be advisable to be compatible with the minimum landing sizes (MLSs) of 29 mm carapace length and 27 cm total length for red shrimp and hake, respectively, while it would greatly reduce the amount of discards, particularly those for blue whiting, that accounted for approximately 50% of the total catch weight. Horse mackerel was the only species for which the use of a larger Mesh size would result in a significant escapement of individuals above the MLS of 15 cm.
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size selectivity of diamond and square Mesh cod ends for rose shrimp parapenaeus longirostris and norway lobster nephrops norvegicus off the portuguese south coast
Fisheries Research, 2002Co-Authors: Aida Campos, Paulo Fonseca, Karim ErziniAbstract:Abstract The effects of an increase in cod end Mesh size from 55 to 60 and 70 mm and a change of Mesh Configuration from diamond to square Mesh on the size selectivity for rose shrimp Parapenaeus longirostris and Norway lobster Nephrops norvegicus captured off the Portuguese south coast were evaluated. The results were analysed taking into account between-haul variation in selectivity, and indicate a significant increase in L 50 for rose shrimp with an increase in Mesh size or with the use of a square Mesh cod end, while for Norway lobster only Mesh Configuration was found to affect this parameter. Two other important external variables were identified; the trawling depth and the cod end catch, which influence between-haul variation, by increasing the selection range for rose shrimp and Norway lobster, respectively. The results obtained suggest that an increase in the current minimum Mesh size of 55 mm would be advisable for rose shrimp in order to respect the minimum landing size of 24 mm carapace length presently established for this species. Moreover, trawling for rose shrimp should be avoided at depths above 200 m, in order to avoid catches consisting almost exclusively of juveniles. Such an increase in Mesh size would have a minor impact in terms of losses of individuals above the minimum landing size for Norway lobster and would contribute to reducing the amount of discards in this fishery.
Paulo Fonseca - One of the best experts on this subject based on the ideXlab platform.
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Selectivity of diamond and square Mesh cod ends for horse mackerel (Trachurus trachurus), European hake (Merluccius merluccius) and axillary seabream (Pagellus acarne) in the shallow groundfish assemblage off the south-west coast of Portugal*
Scientia Marina, 2003Co-Authors: Aida Campos, Paulo FonsecaAbstract:The effects of an increase in cod end Mesh size from 65 to 70 and 80 mm and a change of Mesh Configuration from 65 mm diamond to 65 mm square Mesh on the size selectivity of horse mackerel (Trachurus trachurus), hake (Merluccius merluccius) and axillary seabream (Pagellus acarne) of the shallow groundfish assemblage off the Portuguese southwest coast were evaluated. The increase in Mesh size had a small but significant effect on size selectivity for the three species, while the change in Mesh Configuration led to a much more pronounced increase in the selectivity parameters. For horse mackerel, the L50 estimates ranged from 14.4 to 16.0 cm in the diamond Mesh cod ends-values that are close to the minimum landing size of 15 cm. For hake, L50 of 17.0 and 18.3 cm were estimated for the 70 and 80 mm diamond cod ends respectively, while for the axillary seabream the L50 estimated was 13.9 cm for the 80 mm diamond Mesh cod end. These values are well below the minimum landing sizes of 27 and 18 cm for these species. The corresponding estimates in the square Mesh cod end were 21.9, 32.4 and 19.6 cm, with the loss of a high percentage (76%) of horse mackerel above the minimum landing size. For all the cod ends tested, the observed retention was presented as a function of maximum girth/Mesh perimeter, which allowed a better understanding of the selection process for the species in study. Selectivity estimates for horse mackerel and hake were also compared to those obtained by Campos et al. (2003) for the same species in 1993 off the south coast, where they are captured as a by-catch in the crustacean fishery.
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Size selectivity for four fish species of the deep groundfish assemblage off the Portuguese southwest coast: evidence of Mesh size, Mesh Configuration and cod end catch effects
Fisheries Research, 2003Co-Authors: Aida Campos, Paulo Fonseca, Victor HenriquesAbstract:Abstract The effects were evaluated of an increase in cod end Mesh size from 65 to 70 and 80 mm and change of Mesh Configuration from 65 mm diamond to 65 mm square Mesh on the size selectivity of hake ( Merluccius merluccius ), horse mackerel ( Trachurus trachurus ), four-spot megrim ( Lepidorhombus boscii ) and blue whiting ( Micromesistius poutassou ) captured on the upper continental slope off the Portuguese southwest coast, at depths from approximately 200–400 m. A number of individual hauls were analysed in the diamond Mesh cod ends, for hake and blue whiting, and in the square Mesh cod end, for horse mackerel, and mean selection curves were estimated for these cod ends taking into account between-haul variation. For the four-spot megrim, the selectivity estimates were based on pooled data for all the cod ends tested. Selectivity models are proposed for hake and blue whiting in which positive effects of both the increase in Mesh size and in cod end catch were estimated for L 50 , while SR was only affected by Mesh size. For horse mackerel a positive effect of cod end catch was estimated for SR in the square Mesh cod end. The selectivity was greatly affected by the change in Mesh Configuration for all species with the exception of the four-spot megrim, for which only Mesh size was found to affect selectivity.
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Size selectivity of diamond and square Mesh cod ends for four by-catch species in the crustacean fishery off the Portuguese south coast
Fisheries Research, 2003Co-Authors: Aida Campos, Paulo Fonseca, Karim ErziniAbstract:Abstract The effects of an increase in cod end Mesh size from 55 to 60 and 70 mm and a change of Mesh Configuration from 55 mm diamond to 55 mm square Mesh on the size selectivity of four by-catch species (the red shrimp Aristeus antennatus , the European hake Merluccius merluccius , the horse mackerel Trachurus trachurus and the blue whiting Micromesistius poutassou ) commonly captured in the crustacean fishery off the Portuguese south coast, were evaluated. Selectivity parameters for blue whiting, the most abundant species in the catches, were estimated taking into account between-haul variation, while for the remaining species, captured in much lower quantities, the selectivity estimates were based on pooled data by length class for all hauls within the same cod end. Length at 50% retention, L 50 , was found to increase with Mesh size and with the change in Mesh Configuration for all the studied species. For blue whiting trawling depth and cod end catch were found to play a role in between-haul variation by increasing L 50 as well. The results suggest that an increase in the current minimum Mesh size of 55–70 mm would be advisable to be compatible with the minimum landing sizes (MLSs) of 29 mm carapace length and 27 cm total length for red shrimp and hake, respectively, while it would greatly reduce the amount of discards, particularly those for blue whiting, that accounted for approximately 50% of the total catch weight. Horse mackerel was the only species for which the use of a larger Mesh size would result in a significant escapement of individuals above the MLS of 15 cm.
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size selectivity of diamond and square Mesh cod ends for rose shrimp parapenaeus longirostris and norway lobster nephrops norvegicus off the portuguese south coast
Fisheries Research, 2002Co-Authors: Aida Campos, Paulo Fonseca, Karim ErziniAbstract:Abstract The effects of an increase in cod end Mesh size from 55 to 60 and 70 mm and a change of Mesh Configuration from diamond to square Mesh on the size selectivity for rose shrimp Parapenaeus longirostris and Norway lobster Nephrops norvegicus captured off the Portuguese south coast were evaluated. The results were analysed taking into account between-haul variation in selectivity, and indicate a significant increase in L 50 for rose shrimp with an increase in Mesh size or with the use of a square Mesh cod end, while for Norway lobster only Mesh Configuration was found to affect this parameter. Two other important external variables were identified; the trawling depth and the cod end catch, which influence between-haul variation, by increasing the selection range for rose shrimp and Norway lobster, respectively. The results obtained suggest that an increase in the current minimum Mesh size of 55 mm would be advisable for rose shrimp in order to respect the minimum landing size of 24 mm carapace length presently established for this species. Moreover, trawling for rose shrimp should be avoided at depths above 200 m, in order to avoid catches consisting almost exclusively of juveniles. Such an increase in Mesh size would have a minor impact in terms of losses of individuals above the minimum landing size for Norway lobster and would contribute to reducing the amount of discards in this fishery.
Walter Boscheri - One of the best experts on this subject based on the ideXlab platform.
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high order direct arbitrary lagrangian eulerian ale finite volume schemes for hyperbolic systems on unstructured Meshes
Archives of Computational Methods in Engineering, 2017Co-Authors: Walter BoscheriAbstract:In this work we develop a new class of high order accurate Arbitrary-Lagrangian–Eulerian (ALE) one-step finite volume schemes for the solution of nonlinear systems of conservative and non-conservative hyperbolic partial differential equations. The numerical algorithm is designed for two and three space dimensions, considering moving unstructured triangular and tetrahedral Meshes, respectively. As usual for finite volume schemes, data are represented within each control volume by piecewise constant values that evolve in time, hence implying the use of some strategies to improve the order of accuracy of the algorithm. In our approach high order of accuracy in space is obtained by adopting a WENO reconstruction technique, which produces piecewise polynomials of higher degree starting from the known cell averages. Such spatial high order accurate reconstruction is then employed to achieve high order of accuracy also in time using an element-local space–time finite element predictor, which performs a one-step time discretization. Specifically, we adopt a discontinuous Galerkin predictor which can handle stiff source terms that might produce jumps in the local space–time solution. Since we are dealing with moving Meshes the elements deform while the solution is evolving in time, hence making the use of a reference system very convenient. Therefore, within the space–time predictor, the physical element is mapped onto a reference element using a high order isoparametric approach, where the space–time basis and test functions are given by the Lagrange interpolation polynomials passing through a predefined set of space–time nodes. The computational Mesh continuously changes its Configuration in time, following as closely as possible the flow motion. The entire Mesh motion procedure is composed by three main steps, namely the Lagrangian step, the rezoning step and the relaxation step. In order to obtain a continuous Mesh Configuration at any time level, the Mesh motion is evaluated by assigning each node of the computational Mesh with a unique velocity vector at each timestep. The nodal solver algorithm preforms the Lagrangian stage, while we rely on a rezoning algorithm to improve the Mesh quality when the flow motion becomes very complex, hence producing highly deformed computational elements. A so-called relaxation algorithm is finally employed to partially recover the optimal Lagrangian accuracy where the computational elements are not distorted too much. We underline that our scheme is supposed to be an ALE algorithm, where the local Mesh velocity can be chosen independently from the local fluid velocity. Once the vertex velocity and thus the new node location has been determined, the old element Configuration at time \(t^n\) is connected with the new one at time \(t^{n+1}\) with straight edges to represent the local Mesh motion, in order to maintain algorithmic simplicity. The final ALE finite volume scheme is based directly on a space–time conservation formulation of the governing system of hyperbolic balance laws. The nonlinear system is reformulated more compactly using a space–time divergence operator and is then integrated on a moving space–time control volume. We adopt a linear parametrization of the space–time element boundaries and Gaussian quadrature rules of suitable order of accuracy to compute the integrals. We apply the new high order direct ALE finite volume schemes to several hyperbolic systems, namely the multidimensional Euler equations of compressible gas dynamics, the ideal classical magneto-hydrodynamics equations and the non-conservative seven-equation Baer–Nunziato model of compressible multi-phase flows with stiff relaxation source terms. Numerical convergence studies as well as several classical test problems will be shown to assess the accuracy and the robustness of our schemes. Finally we briefly present some variants of the algorithm that aim at improving the overall computational efficiency.
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arbitrary lagrangian eulerian discontinuous galerkin schemes with a posteriori subcell finite volume limiting on moving unstructured Meshes
Journal of Computational Physics, 2017Co-Authors: Walter Boscheri, Michael DumbserAbstract:Abstract We present a new family of high order accurate fully discrete one-step Discontinuous Galerkin (DG) finite element schemes on moving unstructured Meshes for the solution of nonlinear hyperbolic PDE in multiple space dimensions, which may also include parabolic terms in order to model dissipative transport processes, like molecular viscosity or heat conduction. High order piecewise polynomials of degree N are adopted to represent the discrete solution at each time level and within each spatial control volume of the computational grid, while high order of accuracy in time is achieved by the ADER approach, making use of an element-local space–time Galerkin finite element predictor. A novel nodal solver algorithm based on the HLL flux is derived to compute the velocity for each nodal degree of freedom that describes the current Mesh geometry. In our algorithm the spatial Mesh Configuration can be defined in two different ways: either by an isoparametric approach that generates curved control volumes, or by a piecewise linear decomposition of each spatial control volume into simplex sub-elements. Each technique generates a corresponding number of geometrical degrees of freedom needed to describe the current Mesh Configuration and which must be considered by the nodal solver for determining the grid velocity. The connection of the old Mesh Configuration at time t n with the new one at time t n + 1 provides the space–time control volumes on which the governing equations have to be integrated in order to obtain the time evolution of the discrete solution. Our numerical method belongs to the category of so-called direct Arbitrary-Lagrangian–Eulerian (ALE) schemes, where a space–time conservation formulation of the governing PDE system is considered and which already takes into account the new grid geometry (including a possible rezoning step) directly during the computation of the numerical fluxes. We emphasize that our method is a moving Mesh method, as opposed to total Lagrangian formulations that are based on a fixed computational grid and which instead evolve the mapping of the reference Configuration to the current one. Our new Lagrangian-type DG scheme adopts the novel a posteriori sub-cell finite volume limiter method recently developed in [62] for fixed unstructured grids. In this approach, the validity of the candidate solution produced in each cell by an unlimited ADER-DG scheme is verified against a set of physical and numerical detection criteria, such as the positivity of pressure and density, the absence of floating point errors (NaN) and the satisfaction of a relaxed discrete maximum principle (DMP) in the sense of polynomials. Those cells which do not satisfy all of the above criteria are flagged as troubled cells and are recomputed at the aid of a more robust second order TVD finite volume scheme. To preserve the subcell resolution capability of the original DG scheme, the FV limiter is run on a sub-grid that is 2 N + 1 times finer compared to the Mesh of the original unlimited DG scheme. The new subcell averages are then gathered back into a high order DG polynomial by a usual conservative finite volume reconstruction operator. The numerical convergence rates of the new ALE ADER-DG schemes are studied up to fourth order in space and time and several test problems are simulated in order to check the accuracy and the robustness of the proposed numerical method in the context of the Euler and Navier–Stokes equations for compressible gas dynamics, considering both inviscid and viscous fluids. Finally, an application inspired by Inertial Confinement Fusion (ICF) type flows is considered by solving the Euler equations and the PDE of viscous and resistive magnetohydrodynamics (VRMHD).
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arbitrary lagrangian eulerian discontinuous galerkin schemes with a posteriori subcell finite volume limiting on moving unstructured Meshes
arXiv: Numerical Analysis, 2016Co-Authors: Walter Boscheri, Michael DumbserAbstract:We present a new family of high order accurate fully discrete one-step Discontinuous Galerkin (DG) finite element schemes on moving unstructured Meshes for the solution of nonlinear hyperbolic PDE in multiple space dimensions, which may also include parabolic terms in order to model dissipative transport processes. High order piecewise polynomials are adopted to represent the discrete solution at each time level and within each spatial control volume of the computational grid, while high order of accuracy in time is achieved by the ADER approach. In our algorithm the spatial Mesh Configuration can be defined in two different ways: either by an isoparametric approach that generates curved control volumes, or by a piecewise linear decomposition of each spatial control volume into simplex sub-elements. Our numerical method belongs to the category of direct Arbitrary-Lagrangian-Eulerian (ALE) schemes, where a space-time conservation formulation of the governing PDE system is considered and which already takes into account the new grid geometry directly during the computation of the numerical fluxes. Our new Lagrangian-type DG scheme adopts the novel a posteriori sub-cell finite volume limiter method, in which the validity of the candidate solution produced in each cell by an unlimited ADER-DG scheme is verified against a set of physical and numerical detection criteria. Those cells which do not satisfy all of the above criteria are flagged as troubled cells and are recomputed with a second order TVD finite volume scheme. The numerical convergence rates of the new ALE ADER-DG schemes are studied up to fourth order in space and time and several test problems are simulated. Finally, an application inspired by Inertial Confinement Fusion (ICF) type flows is considered by solving the Euler equations and the PDE of viscous and resistive magnetohydrodynamics (VRMHD).
Alessandro Lucchetti - One of the best experts on this subject based on the ideXlab platform.
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the effect of Mesh Configuration and codend circumference on selectivity in the mediterranean trawl nephrops fishery
Fisheries Research, 2010Co-Authors: Antonello Sala, Alessandro LucchettiAbstract:Abstract The Pomo pit area is both the main Adriatic nursery area for European hake (Merluccius merluccius) and Norway lobster (Nephrops norvegicus) and an important trawling ground for the Adriatic trawl fleet. The current paper is not only restricted to the escapement of Norway lobster but also takes note of the associated white fish community (European hake, blue whiting Micromesistius poutassou and poor-cod Trisopterus minutus capelanus) which makes an important contribution to the commercial catch of trawlers. This study was undertaken to analyse the effect on trawl selectivity of inserting first a 40 mm square-Mesh codend and second of increasing the circumference in the diamond-Mesh codend. A traditional 40 mm diamond-Mesh codend, typical of the commercial trawl fisheries, was selected as a reference codend for the comparative analysis. The square-Mesh codend was always more selective than the diamond-Mesh codends, while the influence of codend circumference on diamond-Mesh selectivity was less evident. In addition to Mesh Configuration and the number of Meshes around the codend, the selectivity data were modelled by estimating the individual contribution of catch size and season, but there was no evidence of a coherent effect on the selectivity of these two parameters. This study confirms that all the four investigated species make the best use of the square-Mesh opening, either because of their body shape or because the form of the Mesh facilitates forcing of the net. Results seem to suggest that for the main target species, Norway lobster, square-Mesh codend would protect specimens under the 20 mm of MLS (carapace length). Square-Mesh also improves the L50 of European hake, but it seems not to be sufficient to avoid the catch of specimens under MLS of 20 cm as fixed by the EC Reg. Nr. 1967/2006. In conclusion, enforcement of installation of square-Mesh codends in Mediterranean trawl Nephrops fishing could contribute to decreasing the capture of individuals from the particular nursery area of Pomo pit.
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Size selection by diamond- and square-Mesh codends in multi-species Mediterranean demersal trawl fisheries
Fisheries Research, 2008Co-Authors: Antonello Sala, Alessandro Lucchetti, Corrado Piccinetti, Mario FerrettiAbstract:Abstract Mediterranean demersal trawl fisheries traditionally operate using small diamond-shape Meshes in the codend, which tend to retain almost all animals. We investigated the effect of Mesh Configuration on the size selectivity of nine species (the scaldfish Arnoglossus laterna , the broad-tail shortfin squid Illex coindettii , the European hake Merluccius merluccius , the red mullet Mullus barbatus , the Norway lobster Nephrops norvegicus , the common pandora Pagellus erythrinus , the deepwater rose shrimp Parapenaeus longirostris , the Mediterranean horse mackerel Trachurus mediterraneus and the poor-cod Trisopterus minutus capelanus ) commonly captured in the Mediterranean demersal trawls. Two codends having Meshes with the same opening (ca. 38 mm) but different Mesh Configuration (DM38: diamond-Mesh and SM38: square-Mesh Configuration) were fished daily and alternately on the same trawl. They demonstrated a substantial improvement in selectivity with square-Mesh. Selectivity was measured using the covered codend technique, the cover being supported by circular hoops. The results were analysed taking into account the between-haul variation in selectivity. Two other important external variables were identified: the trawling depth and the codend catch, which for some species influenced between-haul variation, but there was no evidence of a coherent effect on selectivity parameters. The selectivity of 40 mm diamond-Mesh codend has been reported to be rather poor because a large proportion of the codend catch is immature and smaller than the minimum landing size (MLS) or first maturity size. With the exception of the flatfish ( A. laterna ), the effect of a change of Mesh Configuration from diamond- to square-Mesh on size selectivity positively affected the retention length at 50% ( L 50 ). However, square-Meshes were found to be unsuitable for flat and/or deep-bodied fish as these escape more readily from diamond-Meshes. The Council Regulation (EC) No. 1967/2006, concerning management measures for the sustainable exploitation of fishery resources in the Mediterranean, establishes the MLS of marine organisms. However, the increase in L 50 with square-Mesh would not avoid some of the existing contradictions in allowing the use of codend Mesh which leads to lower L 50 than the MLS.
