| Tuna Longline is made up of flexible components mainly, such as mainlines, branch lines and the float lines, and a lot of hooks and swivels etc. While the external forces acting on the longline appeared any changes, the shape of longline would change coordinately. However, the shape and positions of longline and its components varied, it would cause the changes of the magnitude of loads and their distribution on the longline, and also affected the depths of hooks in water. The longline is a kind of the structure system which effected by all forces, loads and shapes and positions of components which effecting each other strongly.The depths of hooks reached will directly be decided according to the habitat water lays of target species. So the properly control the depth of hooks is the key of fishing operation success and selectivity of fishing. However, the depths of hooks reached were affected by fishing gear configuration and materials used, operating parameters and sea conditions etc. Among them, the physical parameters of fishing gear, such as the flow and hydrodynamics around the gear system and its components, materials properties and their gravities are the important factors affecting the shape of fishing gear and hooks'depth. With the rapid development of computer technology, the ability to handle a large number of data has been greatly developed and it became possible for numerical calculation on a complex mechanical system. By established a fishing gear dynamic model, we could make numerical simulation on the shape and load distribution under varied fishing conditions, and then it could be possible to forecast the longline fishing operating conditions. By controlling the operating conditions, the hooks will reach the habitat of the water layers as expected; it will greatly improve the catch efficiency on target species and reducing bycatch.However, based on the assumption of the catch efficiency of fishing gear higher, which indicates the fishing gear is suitable to the fish behaviour and reaching their habitats properly, so that the degree of catch efficiency will be the important indicators of fish distribution and the parameters of their habitats. By means of the fishing gear math-dynamics model the values of parameters can be obtained from the simulate results, which can be refered for the improving of fishing gear performance and operation. So the dynamics models of longline (DML) and simulation will create valuable contribution on the theory and practices.The tuna longline was made up of float lines, mainline, branch lines and hooks etc. This system could be regarded as the flexible line system, which could bear tension only and not withstand any torques and extrusion pressures. In this study, the longline fishing gear system was discrete as the large number of tension bar using the finite element theory. The bar elements were linked by frictionless hinge. The hydrodynamic force, gravity and buoyancy of bar would be distributed to the two nodes of bar element. The loads were set on the appropriate positions according to the characteristics of forces. The movement of each node was analyzed by using the dynamics method, and a set of dynamic equations of nodes. The three-dimensional dynamic model of tuna longline (DMTL) was established. The implicit Euler method and software on language R were used to solve the differential equations.In order to verify and improve the longline dynamic model, the data were measured at sea, including the gear parameters, operating conditions, hook depths (116 hooks) and ocean current data in three-dimensions, which collected and measured on board of fishing vessel"Shenliancheng 719"at Kiribati Sea (1 o 01'S-5 o 01'N, 169 o 52'E-176 o 42 ' E) in the Western Pacific from October 2009 to January 2010. SPSS software is used to analyze the variance and t-test between the depths calculated by two models and the measured data. In addition, the movements of special nodes and the tension distribution were analyzed and compared with the measured data. The dynamics model of longline fishing gear could be used to calculate the velocity around each node, the tension of node and the shape of longline in a variety of ocean current in 3D, i.e. X, Y and Z currents was non-constant flow.The value of the node tension was larger compared to its dynamical force and gravity, so the tension was the subjects in this study. And the three-dimensional ocean current was an important factor affecting the shape of longline. In this study, the method used is only one single factor varied in a reasonable range and the other parameters keeping unchanged, while calcute the movement, tension and spatial location of the nodes.The results showed that: (1) the value of CN90 selected is the key factor representing the flow state around the longline. When CN90 value selected was 1.12, the results from the mechanical model obtained the highest accuracy and best matched with the measured hook depths in 2009. So the value of CN90 was recommended to be 1.12 for whole longline system in the dynamic model; (2) this dynamic model could be used to analyze and predicate the shape of longline fishing gear, the location and depth of hook, and the stress of critical component in different sea conditions. (3) by means of the paired sample t-test between the numeric hook depths of standard catenary formula (SCF) and Satio's catenary equation (SCE), the results of test showed that there were no significant difference between them (P value = 0.90> 0.05). So these two methods could be used for estimate the hook depth of longline under no water current circumstance. In this paper, the SCE was used to provide the initial location of each node in DMTL; (4) By the paired sample t-test between the measured depths and the numeric hook depths from DMTL, the results of test showed that there were no significant difference between them (P value = 0.17> 0.05). And the numeric hook depths from DMTL were less than the hook depths from SCF and SCE, which means the water current push the hooks into depth less; (5) based on the single factor changed analysis, it was found that X and Y currents had greater impact on the main node tension and the longline shape changing, other than in Z direction. |
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