Fishing by Nets

Most of the catch of inland and marine fisheries landed by small scale fishermen is contributed by the gill net fisheries. In marine sector, about one third of the total catch is taken by gill net fisheries particularly drift net. This fishery has been undergoing changes as a result of introduction of mechanized fishing boats, improvement of fishing gears and method of capture with better performance than those previously used by the fishermen.

Gill net, fishing in the marine sector:

There was no comprehensive survey on the status of gill net fisheries. The major problems in conducting survey are associated with scattered beach landing and carrier boats which collect and transport the high valued fish form the capture site at sea or intercepting incoming vessels in the mouth of the river gill net fisheries is summarized below:

1. Drift Net Fisheries:

Two main types of drift net are operating in the coastal water of Bangladesh: The small mesh net ( 2.5 to 5.5 inches/65 to 140 mm mesh) and large mesh net ( 7 to 9 inches/180 to 230 mm mesh) The nets are made of nylon twine variable trade number and thickness. The main catch of the small mesh drift nets are hilsa, pomfret, jew fish, mackerel, catfish etc. of which 70-80% is composed of hilsa.

Large mesh drift nets are operated by the limited number of boats in Cox’s Bazar and Chittagong area. Major catches of the gear are larger species of fish such as Indian salmon, red snapper, tuna, black jew fish, sharks etc:

The length and width of the drift net greatly varies depending on the type and place. In Cox’s Bazar area both small and large mesh size drift nets consist of mostly 15 to 20 pieces of unit having 100 m length and 25 to 40 m width per unit and each unit piece of net is joined together with others to form 1500-2000 m of one fishing net operating form a single fishing boat. 

In other areas such as Chittagong, Barisal, Hatiya, Noakhali and Chadpur use mainly small mesh size nets consisting of 10 to 12 pieces of unit having 100 m length and 20 m length and 20 m depth/width per unit to form 1000 to 1200 m length of one fishing net.

2. Fixed Gill Net:

Fixed gill nets are mainly operated in the coastal area of Khepupara in Patuakhali and Kumira in Chittagong districts. The net consists of mostly 10 to 12 pieces of unit having 12 to 20 m length and 2 m width per unit. Each unit joins together one after another to form one long net and operates the net form a single fishing boat.

The soaking time per haul is appprox. 8 to 12 hours and there is one to two hauls per day depending on the abundance of fish. Normally fishing takes place day and night at 10 to 20 m depth. The catch obtained by daily fishing are transported to the landing places by boats. The major species of the catches are Hilsa ilisha, C. dussumari, S. taty, S. domina, all species of Scianneida, Aridae etc.

3. Bottom Gill Net:

Two types of bottom gill nets, small mesh size ( 33-52 mm mesh ) and large mesh ( 180-190 mm) locally called “Dova jal” and “ Patore jal” respectively, are used in St. Martin island and Teknaf area. The length of the “ Dova jal” varies form 1200-25 m joining several pieces of unit and width/height varies from 1.9 to 2.3 m. The boats propelled with 5-18 hp engine are used to operate this gear around 20-30 m depth. The major species caught with this pomfret, shark, mackerel, tuna, shad, shrimp and lobster on season. Four to six peoples usually operate the gear.

“ Patore jal” is mainly used in the rocky bottom area of St. Martin island, Teknaf and near Elephant point. The length of the net varies form 850-1500 m joining 10-12 pieces of unit to form one single net. The net is operated by the boat ( 9-13 m length propelled by 6-15 hp with maximum 12 hp. 4-5 peoples are required to operate this gear. The catch composition includes shrimp and lobster, red snapper, grunter, catfish, sharks etc. The catch rate varies from October to December. Nylon twist of different trade names and numbers are used for making net.

4. Mullet Gill Net:

 Mullet gill net is made of monofilament nylon material. The mesh size of the gear varies from 30-40 mm. The pieces of unit of the net having 80-100 m length and 1.8-2.0 width each are joined together to form a long net. Two to three people can operate the gear in shallow water. The net may operated by drifting in the water  or fixing in the bamboo pole two free end. Sufficient weight in the lead line and floats in headline are given to maintain the vertical stretch. The main species caught by this gear are mullet species and other small fishes of shallow water.

Combined gillnets-trammel nets

This bottom-set gear has two parts

• the upper part is a standard gillnet where semi-demersal or pelagic fish can be gilled

• the lower part is a trammel net where bottom fish can entangle.

The combined nets are maintained more or less vertically in the usual way by floats on the floatline and weights on the groundline. They are set on the bottom. After a time depending on the target species, they are hauled on board. (Wikipedia)

Combined gill net – trammel nets:

This bottom set gear is made with a gill net, the lower part of which is replaced by trammel net. It may catch bottom fish in the lower trammel net part, together with semi-Demersal or pelagic fish in the upper gill net part.

Several types of nets may be combined in one gear (for example, trammel net combined with gill net). According to their design, ballasting and buoyancy, these nets may be used to fish on the surface, in mid-water or on the bottom

TRAWLING

Trawling is a types of fishing by using trawl net from a mechanized vessel called trawler. Trawler is a mechanized fishing vessel with trawl net and other navigator instrument, fish finding equipment saving equipment, some important facilities for crew’s.

Bottom trawl net

It is a beg net of synthetic twine, which is drawn along the seabed to harvest the fish on or near the bottom. Bottom trawling is a method of fishing in which the trawl net is drawn along seabed to scoop up fish on or near the bottom.

Types:

1.      Bottom otter trawl-trawl net connected with otter board/door.
2.       Bottom beam trawl- trawl net with a beam connected to the head rope.
3.           Bottom pair trawl-trawl net with two boats each towing one rope.

The trammel net

The trammel net is actually three nets in one. There are the two outside nets which have a larger opening that fish can freely swim through, but in between the two there is a finer meshed net.

The way this net works is that a fish will swim through the first net without a problem, but than will encounter the fine meshed net and continue to swim through the outer net, thus getting tangled in the different layers of the net.

The way this net would be used in fishing is that it would be loaded on a boat and transported to the desired location to fish. Slowly this net would be let out of the boat parallel to the shore, making a wall along the bottom. In deeper water different shapes could be used to set this net.

The fishermen begin to make noise and turbulence by splashing with their oars and stamping on the bottom of the boat to terrify the fish.  The frightened fish dive to the bottom, and in their flight toward deep water, find themselves facing the net. Now the fish passed easily through the large mesh of the first layer, but immediately comes to the narrow meshing of the middle layer. Pushing against it, he takes it through the third wall. Trying to retreat, he finds himself hopelessly entangled in a kind of net bag. Now the net is hauled out and the fish disentangled by hand, one by one.

Selectivity

Gill nets are basically a series of panels of meshes with a weighted "foot rope" along the bottom, and a "headline", to which floats are attached. They can therefore be set to fish at any height in the water column. The meshes of a gill net are uniform in size and shape, hence highly selective for a particular size of fish. 

Conclusion
Fish which are smaller than the mesh of the net are able to pass through unhindered, while those which are too large to push their heads through the meshes as far as their gills are not retained. This gives a selectivity ogive (Wikipedia) which is skewed towards medium sized fishes, unlike active gears such as trawling, in which the proportion of fish entering the net which are retained increases with length.