Choice of trawl netting materials
Selection of the netting material depends on type of fishing gear and fishing conditions in relation to geographic, economic, sociological aspects. In general the choice depends on the type and size of fishing gear to be made, the species of fish to be caught , the fishing ground and the the fishing conditions under which the gear is to be operated, the type, size and engine power of the vessel. Synthetic fibers are considered the suitable raw material for fishing gears.
Choice of netting material for bottom trawl
While selecting the netting materials following properties have to be taken into consideration.
High wet knot and breaking strength
Netting material for bottom trawl should have high wet knot strength, high extensibility, small diameter and high abrasion resistance. For trawl nets lower towing resistance allows either a reduction of towing power with consequent saving in fuel, or increased towing speed or the use of a larger net by which the catching efficiency may be increased. For bottom trawl PA continuous filament netting are yarn is suitable as it has highest wet knot breaking strength, followed by PP, PE, PES whereas PVD is too poor to be eligible for bottom trawl as per as wet knot breaking strength is concerned.
High extensibility and toughness
High elongation combined with high elasticity and high wet knot strength result in a high toughness which enables the net to withstand rough treatment during fishing and to safely haul in large catch in general the elongation of PA staple fiber netting yarn is considered too high and that of medium twisted PP and PES netting yarns too low for bottom trawl.
Wet knot strength and fiber density determine the diameter of the netting yarns. High density and high breaking strength both favour the thin netting yarns. With the same wet knot strength, PES continuous filaments netting yarns have the smallest diameter followed by those made of PA continuous filament.
High abrasion resistance
This property is of particular importance for bottom trawl because of all fishing gears they suffer the most wear and tear on the bottom during towing as well as on board when being dragged over the rail or ramp of the trawler. PA is superior to all other fiber materials. Netting yarns made of monofilaments are superior to those made of continuous filaments netting yarns made of staple fiber have the poorest abrasion resistance.
Size of netting yarn
The different part of a trawl is naturally subjected different stress during fishing and has different mesh size. A perfect adjustment of netting yarn size would result in the lightest net possible. PP should be used in the form of split fibers or folded monofilaments.
Choice of netting material for midwater trawl
Midwater trawl is important fishing gear for bulk fishing pelagic species in the large free water area between the near bottom range of the bottom trawl and the near surface range of gill nets and encircling nets. Most of the properties required for bottom trawl are also needed for midwater trawl but midwater trawl have additional requirement which are basically due to the reaction of fish to be caught. In midwater trawl the chance of fish escapement are more compare to bottom trawl. To overcome this problem net size and filtering efficiency can be increased.
Kind and size of netting yarns
After taking all above factors into consideration the best material for midwater trawl is PA continuous filaments. Compared to the material for bottom trawl the netting yarns for midwater trawl, in spite of their larger dimension and partly larger size, have to be finer. Most of the strengthening ropes must have similar elongation characteristic as the netting material and should therefore also be made of PA.
Choice of netting material for purse seine nets
Purse seine is efficient for bulk fishing of schooling pelagic fish. The fishing range extends from the surface down to about 150 m depth depending on size and construction of the net. Requirement regarding the netting material are determined by the operational characteristic of the gear as a surrounding net and by its comparatively enormous size. The main demands are great sinking speed, high breaking strength, low resistance to water flow and, finally, low price. The catching efficiency largely depends on the speed by which the fish school can be encircled, the long line net wall sinks down and the net can be closed by pursing. The sinking speed is influenced by the kind of netting materials as well as by the construction of the gear, including mesh size, hanging ratio of netting, weight of lead line and also net impregnation. With regard to netting material, high density of fiber, smooth surface and small diameter of netting yarn accelerate the sinking speed of the net and latter two reduce resistance to water.
Kind and size of netting yarn
It is impossible to find an optimum combination of all characteristic required for purse seine in one kind of netting material PVD has the highest specific gravity and therefore the highest sinking speed, but the lowest breaking strength and is therefore not suitable for larger purse seine. Cheap material like PE monofilament or PP split fiber have sufficient breaking strength but low specific gravity so that they float in water and cannot contribute to the sinking speed at all. Considering the whole, the most suitable materials for purse seine are probably netting yarns made of PA continuous filament and of PES continuous filaments. PEs has a reasonably high specific gravity and accordingly a favourable sinking speed. Pa has a lower specific gravity and sinking speed but a higher breaking strength and therefore the smallest netting yarn diameter.
Choice of netting material for gill net
As it is passive fishing gear, the catching efficiency requires low perceptibility of the gear by sight or the lateral organ of the fish before being gilled or entangled. Gill net must be adjusted to the behavior of the fish even better than trawls and purse seines. Material of gill nets should have lowest possible visibility in water, particularly for day time fishing in clear water. The netting yarn should also be as fine and soft as possible to reduce the perceptibility by the side line organ. The netting material should be strong enough to with stand the forces of the caught fish in its fight for freedom. As the size of the fish to be caught by gilling depends on the mesh size, the netting should have good knot stability and the mesh size should not affected by water.
Kind and size of netting yarns
PA continuous filament is the softest of all synthetic material in wet condition, but its natural shiny white colour is far too visible in clear water. It should be dyed just other kinds of white material made of continuous filaments or staple fibers. Since there is no uniformity of option and since the efficiency depends on the fishing conditions, the fish species sought and eventually even on the time of the day and year, no recommendations on the selection of colour can be offered. Green, blue, grey and brownish colours appear to be most common in present commercial fisheries. Netting yarn made of the very strong PA continuous filaments should have soft twist. Hard and even medium twisted PA netting yarns are not recommended because of their high elongation at low loads which is undesirable for the function of gill netting. Because of the large variety of gillnets with regards to size, mesh size fish sought, operational patterns, fishing conditions etc., it is not possible to offer comprehensive recommendations for the selection of net materials.
At present there is a concern regarding biodiversity conservation and fish species sustainability. Practically optimization of these two things is rather difficult but maximum extent to which it is possible that can be done. For this purpose mesh size regulation, selectivity of the fishing gears, most important is to select suitable netting material for major commercial fishing gears. In this article different suitable netting material for commercially important fishing gears, are briefed which will provide a useful information to the fishermen community. These information will reduce cost of manufacture the gears, will extend their shelf life by manifolds in aquatic environment, enhances catching efficiency of the gear which turns in maximizing the fish yield.