Net Twine Research Articles

Review of Gillnet Selectivity

This paper reviews studies on gillnet selectivity: the relevant characteristics of fish and nets, graphical and mathematical representation of selectivity curves, and methods of estimation. F. I. Baranov started these studies 60 yr ago, and most subsequent work has been consistent with his axioms that a) gillnets catch fish whose head girths are smaller but maximum girths larger than the mesh perimeter, and b) selectivity curves for all mesh sizes have the same shapes and heights. Unfortunately these axioms are inaccurate. The heights of selectivity curves increase with mesh sizeSelectivity depends mainly on fish size and shape and mesh size, but is also affected by the thickness, material, and color of net twine, hanging of net, and method of fishing. The left slopes of selectivity curves represent smaller fish wedged in the meshes; the right slopes, larger fish mainly tangled by head parts. The curves may be very skewed or multimodal for fish that are easily tangled.The most reliable, though expensive, estimates of gillnet selectivity are by "direct" methods of fishing a population of known size-frequency distribution. The more popular but biased "indirect" estimates compare catches by two or more mesh sizes. Other methods used are prediction from girth measurements and the DeLury method.

水中視程に影響をおよぼす付着物質の反射率について

Colour and size of an object change as a result of adhesive organisms after it is set under-water for a long time and results in a great difference in underwater visibility. Samples of articles hung in the sea and removed at different time intervals for examination in the laboratory of its luminous reflectance. Underwater visibility of net twines (blue, grey and natural twine) increases with the adhesion of organisms on the twines and it was found that in about 40 days after being set it increases about two fold. In the present estimation, the apparent contrasts increase rectilinearlly with the decrease of visual angles and the luminous reflectances of objects decrease with the increasing of the underwater visibility. Apparently, it shows that the optical properties of the adhesive organisms greatly influence on the underwater visibility.

網糸の繰返し衝撃試験

The experiments were carried out on a trawl net twine using a newly devised impact repeating apparatus.The results obtained are as follows;A definite direct tendency relative to the increase in the number of repeated impacts was not shown for such measured values as breaking strength, flexibility and breaking energy for 100 and 800 repeated impacts.The elongation caused by repeated impact seems to increase with the increase of the number of repeated times and impact load.Uneven values were observed more in netting twine composed of spun yarn than in multi-filament in the present experiments.

人間の目による網糸の水中視程の研究-III

人間の目による網糸の水中視程の研究-III

LABORATORY INVESTIGATION INTO THE SINKING OF OIL SPILLS WITH PARTICULATE SOLIDS

ABSTRACT This paper describes experimental laboratory work relevant to the sinking of floating oil spills. The results show that aqueous sand slurries treated with an amine acetate salt have satisfactory sinking and retention properties. In general, aqueous sand slurries containing cationic wetting agents of widely different water solubilities can effect sinking provided that the concentration of the wetting agent is within the range necessary to give oleophilic sand surfaces. Fine powders have better retention properties than have sands, but are unsatisfactory sinkers unless mechanical stirring is provided. To sink stable water-in-oil emulsions effectively a sinker application rate is required which allows for the cohesive structure of the emulsion as well as for the wetting of the sinker by the oil phase. Work relevant to fears that bottom trawling fishing gear may be fouled in areas where oil slicks had been sunk shows that under very severe experimental conditions hydrophilic and oleophilic fishing net twines can be fouled, but the significance of this in relation to practical considerations is unknown. The feasibility of preventing carpet formation by sunken oil/solid masses by reducing the size of the sinking oil/solid droplets and delaying or preventing their coalescence by spraying a dispersant onto the oil before sinker application is discussed. Standard laboratory procedures in direct support of the Shell Sand Sink method are detailed.

サケ・マスの流網罹網時における網目の伸びと魚体のくびれについて

At the moment of capture of salmon with gill-net, twines of the net extended to some extent, and at the same time, body of salmon contracted at netted portion. The extensibility of twines is concerned with the strength of the twines and also with the elasticity of fish body which varies with fish species and duration of time after death. At every fishing operation authors measured the girth length of fishes on netting condition and also after taken off from the net. Mean extension of mesh size of net on netting condition in desirable portion was 3 percent in comparison with original girth length. Load-contraction curves of net twines and load-extension curve of fish body were obtained from the experiments made by using a simple device of meter (Fig. 2). Combining the curve of load-extension of net twine and the curve of load-contraction of fish body, it is presumed that the crossing point of these two curves is the netting point; that is, the point at which contractibility of fish body and extensibility of twine is in an equilibrium keeping the fish body firmly.

長繊維漁網糸の摩擦に対する強さ-I

Among various physical properties of fishing net twines, wear resistance seems to be affected by the number of twists given to yarn, strand and twine. In the present study the sample twines used were the same in quality (vinyle chloride synthetics, Teviron, 300 deniers ZN1×SN2×ZN3) and in number of yarns of which they consisted. But they differed one another in number of twists given to yarn, strand and cord as shown in Table 1. When the breaking strength of a material rubbed n times with an apparatus (Fig. 1) is represented by Tn, and that of a non-rubbed material by To, one may denote the wear resistance as Tn/To for comparing it among the differently twisted samples. The results obtained are summarized as follows. 1) There is much difference in the wear resistance (Tn/To) even in the same quality of the synthetic twines (Fig. 2 and Table 1). 2) It has been recongnized that the difference in the wear resistance among these twines tested is concerned with the number of the second twists (N2) of the twines (Table 2). 3) Neither the weight nor the diameter of the twines seems to affect the wear resistance. 4) There are marked differences in rigidity closely related the 2 nd twist (N2) but hardly with the 1 st and the 3 rd twists (N1, N3) (Fig. 3). See the foot note under Table 1 concerning for N1, N2 and N3. 5) Relationship between coefficient of the friction and either the rigidity or the wear resistance had an inclination inverse from each other: the smaller the coefficient of friction, the greater is the rigidity as well as the wear resistance (Figs. 5 and 6). 6) On the other hand, a high correlation has been noticed between the wear resistance and rigidity of the tested twines (Fig. 4). 7) It is inferable that the differences in the wear resistance are primarily attributed to the number of twists, and in the present case, 2 nd twists, as the twists govern the rigidity which in turn affects coefficient of friction. 8) It is doubtful, however, whether or not the number of 2 nd twists is the only factor that always increases the rigidity and decreases the coefficient.

漁網の水中視程に関する研究-I

Visual range of net twines with different colors and diameters (Table 1) was observed succesively in an experimental tank (Fig. 1) containing water of serially different turbidity (Fig. 2). Variations of visual range with the wave length of incident light was also observed using a series of color filters (Table 2 and Fig. 3). Visibility or visual range γ of net twine in water of turbidity k seen from the light source of parallel illumination is expressed by the following equation; e=e-2kr(8R/f180-1), where e is the contrast threshold of observer, R is the luminous albedo of the net twine and f180 is the mean relative backward scattering function of surrounding water. The value of e of an observer is generally a function of the brightness of background and the visual angle θ of target. In this experiment the value of e was calculated by observing long and narrow strips of grayish paper of different width with known albedo, backgrounded by a constant brightness, and the following experimental formula was obtained (Fig. 4 and Fig. 5); e=e0/1-e-θθ where e0 and α are parametric constants depending probably on the background brightness and the shape of target, respectively. The backward scattering is generally very difficult to obtain, therefore the values of e/e-2kr=8R/f180-1≅B were calculated to test their constancy through a series of experiments with a fixed twine (sample No. 1) and any one fixed color filter, the results being shown in Table 4. The variation of this value with the turbidity of water is not negligibly small except in the case of green light.