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Lee, J. H., Karlsen, L., and Lee, C. W. 2008. A method for improving the dynamic simulation efficiency of underwater flexible structures by implementing non-active points in modelling. – ICES Journal of Marine Science, 65: 1552–1558. The implementation of non-active points into a conventional mass spring network for improving the simulation efficiency of fishing gear systems is presented. A conventional mass spring network generally consists of just active mass points involved in physical calculations, and non-active points located between neighbouring active mass points are not involved in the calculations. For each time-step, new positions for non-active points are calculated based on the active mass point positions already calculated for that time-step. By replacing some active mass points with non-active ones, computing time can be reduced while maintaining accuracy in the ground-interference simulation. As this method is more efficient in demonstrating the behaviour of fishing gear such as bottom longlines and bottom gillnets, an adaptive non-active point model represents the shape of a system more accurately than one consisting of just active mass points.

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Sinking speed is one of the main factors affecting the performance of longlines. Demersal and pelagic longlines need to sink quickly, to prevent bycatch of marine animals and bait loss to seabirds. The incidental catch or bycatch of seabirds by longlining is a problem from the environmental conservation point of view. Thus, it is necessary to develop techniques to reduce the incidental catch or bycatch by longlining. This study focuses on the sinking speed of longlines. To determine the significant factors that may influence the sinking speed, the study considers the various parameters for longlining such as bait properties, rope materials, rope thickness, anchor weights, shooting speeds, shooting methods, and the ratio between the depth and mainline length. The results of the numerical analysis were verified by comparing them with observations from field experiments. The results suggest that the sinking speed can be increased and the bycatch during the sinking process can be reduced when baits have elliptical shapes with a small projected area relative to the sinking orientation, as well as when high‐density and thicker materials are used. Heavier anchors also help to increase the sinking speed of the anchor part, excluding other parts such as the middle part of a mainline. If the water is shallow compared with the line's length, the anchor's weight has a smaller influence on the speed of sinking in the middle of the line. In this case, the factors that influence the sinking speed of a longline 10, 30, and 50 m from the stern are rope material, rope thickness, and shooting speed. These results may be used to design techniques for bycatch prevention. Based on these results, accounting for these important factors when designing a longline is expected to increase the fishing efficiency. Copyright © 2013 John Wiley & Sons, Ltd.

The paper presents investigations on to which degree the sinking speed of longlines is influenced by type of bait, bait sinking orientations and anchor weights. The main aim of this study is to obtain further insight in the ocean current displacement phenomena in demersal longlining. The sinking speed is one of the main factors deciding the current displacement. In an ongoing project, sinking speed experiments with longlines with 6 kg and 10 kg anchor weights have been carried out in the Trondheim fjord. The longlines used in the first experiments were rigged without bait and hook. The results of these experiments with two different anchor weights have revealed only a slight difference in the sinking speed, except for the part near to the anchors, even though the sinking speed of longlines in general is supposed to be much influenced by the anchor weights. The reason for the obtained result is supposed to be that the experiments have been carried out at relative shallow waters. Further studies have included bait sinking experiments in the flume tank. The experiments showed that the drag coefficient of fillet type (flat) bait varied from 0.763 to 1.735, while it for elliptic type bait varied from 0.62 to 1.483. Other activities have included calculation of the sinking speed of longlines as a function of the established resistance coefficients of bait of various shape and size for commercial longlining. The calculated sinking speed of a longline with the fillet type bait was found to be 12.4 to 16.5% lower than for a longline without bait.