Haruyuki Kanehiro

Analysis of mesh breaking loads in cotton gill nets: Possible solution to ghost fishing

A small number of fishers in Chiba Prefecture of eastern Japan use cotton gill nets to catch Japanese spiny lobster Panulirus japonicus. To examine the advantages of cotton gill nets, we analyzed changes in mesh breaking load of a new cotton gill net used in a fishing operation. A new cotton gill net was also soaked in a seawater tank to simulate ghost fishing conditions. The average mesh breaking load of new cotton mesh was 50.3 N. This value decreased to 19.0 N after 38 days (∼912 h), and after 82 days (∼1968 h) the mesh could be easily torn (breaking load 0.07 N). Under fishing conditions, the cumulative soak time was only 744.4 h over 19 months. The average breaking load at the end of this period was 43.1 N, a strength 86% that of the presoaked mesh. The mesh breaking load of a cotton gill net continuously soaked for 744.4 h was 26.1 N, as estimated from tank experiment data. Thus, a cotton gill net maintains reasonable strength under typical use conditions, but will degrade if lost at sea.

Disposal and recycling of fisheries plastic wastes:Fishing net and expanded polystyrene

Abstract Fishing net wastes generating from fisheries amounts to about 2,000 tons/year. Most fishing net wastes are made with only a single type of plastic and they are suited to recycling. As for disposal of fishing net wastes, “material recycling” and “chemical recycling” are now positioned as a rational and efficient method. Of the wasted netting materials, polyethylene and nylon are mostly conductive to material and chemical recycling. At present, the recycling of fishing net wastes is still limited in quantity, but has been increasing inch by inch in these years. Expanded polystyrene is largely utilized for containers and packaging materials in fishery, agriculture and electrical appliances, and its output amounts to 209,000 tons in 2000. Of this, 183,000 tons are discharged after use and disposed. About 45% of total wastes were disposed by incineration and landfill. The remaining 55% (100,000 tons) are being used to recover resources from wastes. Of the remaining portion, 33% (60,400 tons) is used in material recycling and 23% (39,600 tons) in thermal recycling. Recycled products have frequently been used again as packaging material for household and electrical appliances, and gardening pots, etc. A part of the recycled products is exported to Asian countries as raw materials to manufacturing other products.

Change in Diving Behavior by Lip Shape and Tied-eye Position of Crank Bait Lure.

クランクベイトルアーのタイドアイの位置と潜行姿勢およびライン張力との関係をリップ形状別に把握するために回流水槽実験を行った。リップの形状, タイドアイの位置を可変設定できるルアーを試作し, 流速40,60,80cm/sにおいてルアーの迎角, ラインにかかる張力およびルアーの振動周波数を測定した。その結果, ルアーの平面図上におけるタイドアイより前方の面積(SF)と全平面投影面積(S)との比CS=SF/Sを指標としてタイドアイの位置を表すと, リップの形状が異なる場合でもCSとルアーの迎角およびライン張力とはそれぞれ一定の関係を示した。また, 各リップ形状においてCS=0.2a0.3で安定した潜行運動が得られた。

Fatigue Life of Nylon Filaments as Fishing Gear Material for Tuna Long-line.

The fatigue phenomena of nylon filaments which have recently been used as fishing gear material for tuna long-line fishing have been investigated. Fatigue tests were carried out with two nylon filaments (No. 150 monofilament and No. 50×3-ply filament). Results show that there is a great difference in fatigue behavior between the two nylons. Monofilament is found to have high resistance to tension cycling fatigue. In addition the measurement of the residual strength after 104 cycles of fatigue testing at a higher loading level (S/T0=75%) shows no strength loss but rather an increase (5 to 10%). In 3-ply filament, however, a decrease in fatigue life and a marked loss of residual strength after fatigue testing (12 to 27%) are observed. As a result, it can be concluded that the fatigue behavior of nylon filaments depends heavily on the construction of the filaments (number of filaments and twist). In 3-ply filament, fiber-fiber interactions and the twist of filament have a greater influence on the decrease of its fatigue life and the residual strength loss.

On the weight of knotted (English knot) and knotless netting.

An estimation method for the weight of netting was proposed and weight properties of several knotted and knotless netting were examined. Netting weight (W) could be expressed as a function of number of yarns (n) and mesh size (2L) by the following linear relation, W/(n•de)=A+B•(√n•de/p/2L) where A and B are constants respectively expressed as follows; A=23.8q(kD)2 and B=A(b-2a)kD, where q and kD are the degree of packing of netting twine and a constant relating to the twine diameter, and a and b are constants depending on the type of the knot. The values of A and B calculated from the netting weight table (W(n, 2L) data) for knotted and knotless netting were found to be constant regardless of the kind of fiber. However, A was different depending on the kind of netting (knot type and twine construction). The value of B was found to be remarkedly different between knotted and knotless netting; B=0.00134 for knotted netting while B=0.00003-0.00009 for knotless netting. It was shown that the weight ratio of both netting changed in the range of 0.7 to 3 depending on the size of 2L and n.

Mechanism of photodegradation of nylon netting twine.

Photodegradation of nylon netting twine exposed in the carbon arc Fade meter was investigated from the relation between mechanical and chemical properties. The progress of photodegradation of the samples after u.v. irradiation was evaluated by measuring breaking strength, molecular weight (viscometeric method) and density. With increasing of irradiation, a continuous decrease in molecular weight and also an increase in density were observed in proportion to a significant loss of breaking strength. Form these results, the mechanism of the photodegradation of nylon was estimated as follows. The loss of molecular weight during irradiation suggested the occurrence of photochemical degradation causing the scission of chemical bonds in polymer chain. And also the occurence of reorientation of polymer chain and/or the formation of crosslinking were suggested by the results of the increase in density of nylon fiber with progressive degradation. The results on breaking strength, molecular weight and density of nylon samples treated with additive (702) showed that this additive acted as a good retardant to photodegradiation of nylon.

Water tank experiments of fish behavior to model of trap nets.

In order to compare the performance of trap nets, water tank experiments of fish behavior to the model nets constructed by PVC plate were conducted, using bitterling Rhodeus ocellatus ocellatus as an experimental fish and a video behavior analyzer system for observing and analysing the fish behavior. Six different types of trap nets as the model net were examined; two mouth trap net with two bags, two mouth trap net with a left bag, two mouth trap net with a right bag, single mouth trap net with two bags, single mouth trap net with a left bag and single mouth trap net with a right bag. For the fish school comming from the left hand side of the leader net, the rate of right turn behavior just after entering the net was extremely high; 89% for the two mouth trap nets and 76% for the single mouth trap nets. The rate of remaining, which was employed as an index of the catching performance, showed the maximum in the right bag net and the minimum in the left bag net for both the two mouth trap nets and the single mouth trap net. The results seem to support an empirical rule that the bag net should be set to the same side as the leader net in the single mouth trap net.

An analysis of the fish behavior to model trap net.

Water tank experiments of fish behavior to model net were made by using Rhodues ocellatus ocellatus. Trap net with a leader net, play-ground and two bag nets was used as the model net and its function was discussed in conjunction with fish behavior. Also the entering and leaving behavior of fish to the model trap net was observed and analyzed by using a video position analyzer system.Results of analysis of fish behavior showed that the average swimming speed and separation distance between fishes were different in each part of the net in the following decreasing order; outside of the net>inside of the net (play-ground)>inside of the bag net.It was suggested that the difference of the fish behavior before and after entering the net was closely connected with the different functions (swimming space and the construction) of each part of the net.