Active gears
Active fishing methods are especially suitable for sampling large proportions of the whole fish stock or large numbers of fish. The term “active” means that the fishing gear is dragged through the water by human, animal or engine power. In most cases the efficiency of active gear is considerably higher than that of “passive” gear, such as gillnets and traps, which rely for their efficiency on the movement of the fish themselves.
Beam trawl
In this type of trawl, the mouth or opening of the net is kept open by a beam, which is mounted at each end on guides or skids, which travel along the seabed. The trawls are adapted and made more effective by attaching tickler chains (for sand or mud) or heavy chain matting (for rough, rocky ground) depending on the type of ground being fished.
These drags along the seabed in front of the net, disturbing the fish in the path of the trawl, cause them to rise from the seabed into the oncoming net. Modern beam trawls range in size from 4 to 12 m (weighing up to 7.5 tonnes in air) beam length, depending on the size and power of the operating vessel. |
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Demersal otter trawl
Bottom trawl
The demersal or bottom trawl is a large, usually cone-shaped net, which is towed across the seabed. The forward part of the net – the ‘wings' – is kept open laterally by otter boards or doors. Fish are herded between the boards and along the spreader wires or sweeps, into the mouth of the trawl where they swim until exhausted. They then drift back through the funnel of the net, along the extension or lengthening piece and into the cod-end, where they are retained.
The selectivity of trawl fisheries may be increased by the use of devices known as separator trawls. Separator trawls exploit behavioural differences between fish species and can be used, for example, to segregate cod and plaice into the lower compartment of the net, whilst haddock are taken in the upper part. The mesh size for the two compartments can be altered according to the size of the adult fish being targeted. Insertion of square mesh panels also improves selectivity of the net because square meshes, unlike the traditional diamond shape meshes, do not close when the net is towed. Sorting grids are compulsorily fitted in nets in some prawn and shrimp fisheries to reduce bycatch of unwanted or non-target species, including small prawns and shrimp. |
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Depending on the depth of water fished and the way in which the gear is constructed and rigged, trawling may be used to catch different species. Trawls can be towed by one vessel using otter boards, as in bottom-trawling, or by two vessels, each towing one warp, as in pair-trawling or more than one trawl can be towed simultaneously as in multi-rig trawling.
Pelagic trawl
When trawling takes place in the water column or in mid-water between the seabed and the surface, it is referred to as mid-water or pelagic trawling. Pelagic trawls target fish swimming, usually in shoals, in the water column i.e. pelagic species. These include seabass, mackerel, Alaska pollack, redfish, herring and pilchards for example. Their effectiveness relies on traversing a considerable volume of water, and consequently nets are larger than bottom trawls and require a large vertical and horizontal mouth opening to provide net stability and capture large shoals of fish.
The length of time the net is towed through the water is shorter than in bottom trawling in order to capture the shoals of fish the net passes through. To handle the large amounts of fish, pumps are used to transfer the catch from the cod-end to the boat.
In mid-water pair trawling the otter boards are replaced, and the mouth of the net kept open, by a pair of trawlers. This enables vast nets, often ¼ mile wide and ½ mile long, to be towed through the water column to capture the fish. |
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Seine netting
This is a bottom fishing method and is of particular importance in the harvesting of demersal or ground fish including cod, haddock and hake and flat-fish species such as plaice and flounder. The fish are surrounded by warps (rope) laid out on the seabed with a trawl shaped net at mid-length. As the warps are hauled in, the fish are herded into the path of the net and caught.
Effectiveness is increased on soft sediment by the sand or mud cloud resulting from the warps' movement across the seabed. This method of fishing is less fuel-intensive than trawling and produces a high quality catch, as the fish are not bumped along the bottom as with trawling.
Purse seining
This is the general name given to the method of encircling a school of fish with a large wall of net. The net is then drawn together underneath the fish (pursed) so that they are completely surrounded.
It is one of the most aggressive methods of fishing and aims to capture large, dense shoals of mobile fish such as tuna, mackerel and herring.
Trolling
Trolling involves towing baited hooks or lures through the water. The method is particularly suited to the capture of pelagic species of high individual value. Examples include tuna (albacore and skipjack), barracuda and salmon.
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Harpoon
This method is used for fish having high individual value such as swordfish and bluefin tuna. Harpooning is a completely selective fishery, since the target must be seen before striking, so the size and hence age can be determined and only mature fish taken. |
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(Source: www.vuatkerala.org)
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