In the Mediterranean Sea artisanal fisheries are important and complex, considering the mixed-species catches, the diversity and seasonality of gears used. Passive nets are considered as the most important gears used by this fishing sector. Passive nets include the stationary or fixed nets and the driftnets, for which the catch operation does not require an active movement of the gear. Four main types of passive nets are commonly used by the fishermen in the Mediterranean Sea: gillnet, trammel net, combined net and driftnet. Gillnet consists of a single netting wall generally made of mono-filament nylon. A trammel net consists of two/three layers of netting with a slack small mesh inner netting between two layers of large mesh netting within which fish will entangle. The fish entangle themselves in a pocket of small mesh webbing between the two layers and large meshed walls. Combined net is a bottom-set gillnet combined with a trammel net which constitutes the lower part. Small driftnet is composed by a single netting panel. This kind of net is left free to drift with the current, usually near the surface or not far below it. The catch efficiency of the passive nets, that is the ability of the net to catch marine organisms, is strongly correlated with the net dimensions (net drop and net length). Therefore in the Mediterranean Sea the EC Regulation 1967/2006 has defined the allowed maximum drop and maximum length for each type of net in order to guarantee the responsible use of natural resources. However the enforcement of the net length can be a difficult task for fisheries inspectors considering that a passive net can measure several kilometres. Considering that the passive nest are usually stored in net containers/boxes or amassed on the vessel's deck or on the dock of a port, the main goal of the Archimedes project was the estimate the net length on the basis of the net volume. Secondly the project aimed at providing the EU Commission with updated information on the passive net technical properties in the Mediterranean. In order to collect and classify information on the technology, description and use of the passive net fishing gears in the Mediterranean, a detailed review of the information available in scientific and technical papers has been conducted. Technical information on 313 gillnets, 324 trammel nets, 54 combined nets and 40 driftnets were gathered. The main goal of this review was to support the European Commission with updated information on the passive net characteristics (mesh opening, twine thickness, net length, net drop, hanging ratio etc.) in order to provide fisheries managers with a complete scenario of the Mediterranean situation. All this information can represent a useful tool for fisheries managers for the adoption of reasonable measures on the basis of the real situation. The main goal of the project was to develop a simple and reliable mathematical model for the estimation of the net length from the net volume based on technical data of passive nets collected through direct measurements and/or interviews at on deck harbour, factories, fishing boat etc. Only the parameters which are easy to be directly measured have been measured (net drop, net length, mesh opening etc.). Information on 55 gillnets, 88 trammel nets, 13 combined nets and 19 driftnets were gathered through direct measurements and interviews. The data collected in this phase were used for the development of a reliable, effective and practical formula which allowed the correlation between the net length and net volume. Practically the formula should be considered as a "conversion factor" which can be used for the estimation of the net length starting from few parameters. The fisheries inspector shall be able to measure only the net volume, the mesh opening and the vertical number of meshes; by inserting these variables into the formula it will be possible to estimate the net length. In parallel, a simple device for the assessment of passive net length and volume which can be used during fishery inspections was also developed. The objective was to provide the fisheries inspectors with suitable advices for enforcement and control purposes. To achieve this goal five different actions have been performed: development of a net length gauge, development of a simple tool for the assessment of the net volume, development of a model for the estimation of the net length from the net volume, production of a book of pictures with the different types of net and types of storage, production of a manual for the measurement of the mesh opening, net drop, net volume and the application of the model for the estimation of the net length. Three different gauges (REV-Counter, Bull Machine and NETER) for the direct measurement of the net length were independently developed and tested. The three prototypes were composed of a net hauler were the net was scrolled for the measurement. The net was then recovered on the hauler manually or automatically and the gauge was able to measure the net length. The three net length gauges allowed a precise and simple estimation of the net length. Moreover the new gauges reduced the time for the direct net length measurement of about 37-50%. When the net is simply amassed, for example on a deck vessel, the estimation of the net volume can be problematic and the error in the measurement can be really high. Therefore a simple, feasible and practical method for the measurement of the net volume was also developed and tested. In particular the use of a calibrated net container consistently improved the quality and precision of the net volume estimation as a cheap, easy to use and broadly applicable solution. The net was transferred into a graduated box and covered with a plastic lid. In order to standardize the procedures a weight of 10 kg was placed over the lid to press the net. The level of the lid in the box allowed a quite precise estimation of the net volume. In addition a series a series of pictures of the nets and net containers have been taken according with the standard protocol established. Hence it was possible to implement a book of pictures of the different types of net and types of storage (boxes, net amassed etc.) by different areas in order to give a complete overview of the situation. Moreover it should be considered that the net containers and the storage of nets generally vary from a port to another. For fisheries inspectors it is essential to know the characteristics of the nets, the type and volume of the net container to estimate the amount of net: in fact it can be feasible to guess the net used by a fishermen by simply counting the number of boxes on board. Hence the Archimedes project provides useful information for the improvement of management measures in the Mediterranean Sea. Moreover it also produces tools and advices that can be used for a practical enforcement of the management measures.
Estimation of maximum net length of trammel nets, gillnets and combined bottom set nets by using the volume or the mass of the net (acronim. ARCHIMEDES)
Alessandro Lucchetti;Gabriele Buglioni
2012
Abstract
In the Mediterranean Sea artisanal fisheries are important and complex, considering the mixed-species catches, the diversity and seasonality of gears used. Passive nets are considered as the most important gears used by this fishing sector. Passive nets include the stationary or fixed nets and the driftnets, for which the catch operation does not require an active movement of the gear. Four main types of passive nets are commonly used by the fishermen in the Mediterranean Sea: gillnet, trammel net, combined net and driftnet. Gillnet consists of a single netting wall generally made of mono-filament nylon. A trammel net consists of two/three layers of netting with a slack small mesh inner netting between two layers of large mesh netting within which fish will entangle. The fish entangle themselves in a pocket of small mesh webbing between the two layers and large meshed walls. Combined net is a bottom-set gillnet combined with a trammel net which constitutes the lower part. Small driftnet is composed by a single netting panel. This kind of net is left free to drift with the current, usually near the surface or not far below it. The catch efficiency of the passive nets, that is the ability of the net to catch marine organisms, is strongly correlated with the net dimensions (net drop and net length). Therefore in the Mediterranean Sea the EC Regulation 1967/2006 has defined the allowed maximum drop and maximum length for each type of net in order to guarantee the responsible use of natural resources. However the enforcement of the net length can be a difficult task for fisheries inspectors considering that a passive net can measure several kilometres. Considering that the passive nest are usually stored in net containers/boxes or amassed on the vessel's deck or on the dock of a port, the main goal of the Archimedes project was the estimate the net length on the basis of the net volume. Secondly the project aimed at providing the EU Commission with updated information on the passive net technical properties in the Mediterranean. In order to collect and classify information on the technology, description and use of the passive net fishing gears in the Mediterranean, a detailed review of the information available in scientific and technical papers has been conducted. Technical information on 313 gillnets, 324 trammel nets, 54 combined nets and 40 driftnets were gathered. The main goal of this review was to support the European Commission with updated information on the passive net characteristics (mesh opening, twine thickness, net length, net drop, hanging ratio etc.) in order to provide fisheries managers with a complete scenario of the Mediterranean situation. All this information can represent a useful tool for fisheries managers for the adoption of reasonable measures on the basis of the real situation. The main goal of the project was to develop a simple and reliable mathematical model for the estimation of the net length from the net volume based on technical data of passive nets collected through direct measurements and/or interviews at on deck harbour, factories, fishing boat etc. Only the parameters which are easy to be directly measured have been measured (net drop, net length, mesh opening etc.). Information on 55 gillnets, 88 trammel nets, 13 combined nets and 19 driftnets were gathered through direct measurements and interviews. The data collected in this phase were used for the development of a reliable, effective and practical formula which allowed the correlation between the net length and net volume. Practically the formula should be considered as a "conversion factor" which can be used for the estimation of the net length starting from few parameters. The fisheries inspector shall be able to measure only the net volume, the mesh opening and the vertical number of meshes; by inserting these variables into the formula it will be possible to estimate the net length. In parallel, a simple device for the assessment of passive net length and volume which can be used during fishery inspections was also developed. The objective was to provide the fisheries inspectors with suitable advices for enforcement and control purposes. To achieve this goal five different actions have been performed: development of a net length gauge, development of a simple tool for the assessment of the net volume, development of a model for the estimation of the net length from the net volume, production of a book of pictures with the different types of net and types of storage, production of a manual for the measurement of the mesh opening, net drop, net volume and the application of the model for the estimation of the net length. Three different gauges (REV-Counter, Bull Machine and NETER) for the direct measurement of the net length were independently developed and tested. The three prototypes were composed of a net hauler were the net was scrolled for the measurement. The net was then recovered on the hauler manually or automatically and the gauge was able to measure the net length. The three net length gauges allowed a precise and simple estimation of the net length. Moreover the new gauges reduced the time for the direct net length measurement of about 37-50%. When the net is simply amassed, for example on a deck vessel, the estimation of the net volume can be problematic and the error in the measurement can be really high. Therefore a simple, feasible and practical method for the measurement of the net volume was also developed and tested. In particular the use of a calibrated net container consistently improved the quality and precision of the net volume estimation as a cheap, easy to use and broadly applicable solution. The net was transferred into a graduated box and covered with a plastic lid. In order to standardize the procedures a weight of 10 kg was placed over the lid to press the net. The level of the lid in the box allowed a quite precise estimation of the net volume. In addition a series a series of pictures of the nets and net containers have been taken according with the standard protocol established. Hence it was possible to implement a book of pictures of the different types of net and types of storage (boxes, net amassed etc.) by different areas in order to give a complete overview of the situation. Moreover it should be considered that the net containers and the storage of nets generally vary from a port to another. For fisheries inspectors it is essential to know the characteristics of the nets, the type and volume of the net container to estimate the amount of net: in fact it can be feasible to guess the net used by a fishermen by simply counting the number of boxes on board. Hence the Archimedes project provides useful information for the improvement of management measures in the Mediterranean Sea. Moreover it also produces tools and advices that can be used for a practical enforcement of the management measures.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
