曳網水深図表の応用

Previously, the author prepared a chart for estimation of the working depth of mid-water trawl net upon computation on the basis of supposition that the configuration of towing warp in operation takes a part of circular arc. This time, the chart was applied for estimation of trolling depth of plankton net. It was concluded that the chart is helpful fer simple estimation of working depth within the range of practical towing speed

アイザックキッド中層網の曳網水深について

The author has been studying on the simple estimation of working depth of mid-water trawl and had drawn a chart considering that the warp in operation takes of a cir-cular arc. The chart was applied to the field experiments of mid-water trawl and plankton net, and considerably good results were obtained. This time, the author applied the chart on Isaacs-Kidd mid-water trawling. The net was towed by a fine towing warp which differs from the ones that were employed in previous experiments. It was clarified that the diameter of the towing warp in operation has some influence upon the shape of the warp within a relatively low towing speed range, and that the chart is helpful for simple estimation of working depth of the gear within the practical towing speed range

六枚仕立底曳網の研究―1 : 六枚仕立底曳網の力学的特性

In order to improve the design of fishing gear and its performance, the knowledge of the characteristics of the gear in operation is of a major importance. Meanwhile, the behaviour of a trawl gear is much affected by its geometrical performance accompanied by hydrodynamic forces acting on. Considering that the hydrodynamic forces by the current on an operating trawl net finally act on the harness lines, and the geometric planform of the harness lines could be approximated to be a part of circular arc of various curvatures or a straight line, a set of twenty equations from the theoretical analysis of the geometrical configuration and the mechanical characteristics of a six-seam constructed trawl net is presented in this paper. To verify the validity of the theoretical analysis, a series of experiments on the six-seam trawl model was conducted in a ferro circulating water tank. Accordingly, the agreement between theoretical analysis and experiments of model was not close enough to be accepted uncritically, especially for its performance, however the analysis of the total resistances was in favorable agreement to the experimental results

旋回曳網中の網の軌跡

The horizontal position of the towed net in relation to the towing vessel in a circular towing is discussed. For the sake of simplicity, letting a locus of a towing vessel and a locus of a net be concentric, mechanical equations were obtained. A chart which is drawn by numerical calculation of equations was also obtained. A series of experiments were carried out in order to verify these equations, and a simple estimation of the horizontal position of the net in relation to the towing vessel in a circular towing is possible with these relationships when the angle made by the towing warp and center line of the towing vessel is below 25°

水中音に関する研究-5 : 南米産河イルカの水中鳴音について

The investigation on the underwater calls of three kinds of river dolphins, i.e., Amazon Dolphin (Inia geoffrensis), Buffeo negro (Sotalia fluviatilis) and La Plata river dolphin (Pontopolia blainvillei) that live in waters in South America was carried out. It has been considered that these dolphins made better use of the underwater calls than any other sea dolphins on account of their circumstances. However, conversely it was made clear that the underwater calls were lacking of the variety and frequencies, and moreover they were different in character from the underwater calls of the Ganges river dolphin (Platanista gangetica). For example, Buffeo negro did not emit the stratiformed sound, and both whistle and clicks of Amazon dolphin were limited in the range of low frequency. And also the frequency in their use of calls was exceedingly low. The underwater calls of La Plata river dolphin could not be recorded though the opportunity of observation was little.!?1. The underwater calls of the river dolphins in South America (Buffeo negro and Amazon dolphin) were recorded in natural environment and analys ed in comparison with the sea dolphin and the Ganges river dolphin. 2. Clear difference between the Buffeo negro and the Amazon river dolphin was obtained and further the underwater calls of these dolphins were different from that of the sea dolphins and the Ganges river dolphin. 3. For Buffeo negro, the stratiformed sound was not emitted. The frequency of whistle was higher than that of Amazon dolphin and clicks also showed the same tendency. 4. The frequency in use of calls was lower compared with any other kinds of dolphins

六枚仕立底曳網の研究―2 : 六枚仕立と四枚仕立底曳網の力学的比較

After analyzing theoretically the mechanical characteristics of a sixseam trawl and a four-seam trawl, a series of experiments of both models was conducted in a ferro circulating water tank to compare their relative merits. The comparison of their important performance such as the spread of wings, elevation of headline and total resistance of nets was carried out theoretically as well as experimentally. Since no remarkable difference of the spread of wings of both trawls could be noted even though the wing of four-seam trawl was longer than that of the six-seam trawl, the conclusion could be drawn that the greater total resistance of the six-seam trawl nets than that of the four-seam trawl nets is much affected by higher elevation of its headline

流れの中にある物体の抗力測定の一方法

In order to make an accurate measurement of the drag force of the immersed body against the stream, a simple device was devised by attaching the strain gauge on a steel plate. It has become clear that direct measurement of the drag force of the immersed body by use of the device is feasible, and also the value obtained by indirect measurement which has been employed in this laboratory must be revised by approximately 10%

西日本小型底曳網の力学的研究

The mechanical characteristics of a four-seam trawl net being used in the west Japan district was studied on the basis of its construction. The authors, in this report, watched especially on the wing length of the net. It was found that when the transversal distance between the danlenos exceeds the length of 13.6 cm in the model net and 408 cm in the practical net, the force acting on the net becomes smaller as the wing becomes longer at the towing speeds that are given in the practical fishing

Amazon river dolphins (Inia geoffrensis) use a high-frequency short-range biosonar

Author Posting. © The Company of Biologists, 2015. This article is posted here by permission of The Company of Biologists for personal use, not for redistribution. The definitive version was published in Journal of Experimental Biology 218 (2015): 3091-3101, doi:10.1242/jeb.120501.Toothed whales produce echolocation clicks with source parameters related to body size; however, it may be equally important to consider the influence of habitat, as suggested by studies on echolocating bats. A few toothed whale species have fully adapted to river systems, where sonar operation is likely to result in higher clutter and reverberation levels than those experienced by most toothed whales at sea because of the shallow water and dense vegetation. To test the hypothesis that habitat shapes the evolution of toothed whale biosonar parameters by promoting simpler auditory scenes to interpret in acoustically complex habitats, echolocation clicks of wild Amazon river dolphins were recorded using a vertical seven-hydrophone array. We identified 404 on-axis biosonar clicks having a mean SLpp of 190.3±6.1 dB re. 1 µPa, mean SLEFD of 132.1±6.0 dB re. 1 µPa2s, mean Fc of 101.2±10.5 kHz, mean BWRMS of 29.3±4.3 kHz and mean ICI of 35.1±17.9 ms. Piston fit modelling resulted in an estimated half-power beamwidth of 10.2 deg (95% CI: 9.6–10.5 deg) and directivity index of 25.2 dB (95% CI: 24.9–25.7 dB). These results support the hypothesis that river-dwelling toothed whales operate their biosonars at lower amplitude and higher sampling rates than similar-sized marine species without sacrificing high directivity, in order to provide high update rates in acoustically complex habitats and simplify auditory scenes through reduced clutter and reverberation levels. We conclude that habitat, along with body size, is an important evolutionary driver of source parameters in toothed whale biosonars.Field work was funded by Danish National Research Council grants to P.T.M., Associação Amigos do Peixe Boi da Amazônia (AMPA) and Petrobras Ambiental grants to V.M.F.d.S., Augustinus Fonden grants to M.L. and a travelling fellowship awarded to M.d.F. by Journal of Experimental Biology. M.L. was funded by a PhD stipend from the Faculty of Science and Technology, Aarhus University, and National Research Council grants to P.T.M. F.H.J. was funded by a Carlsberg Foundation travel grant.2016-10-0