The trawler means any vessel that pulls a bag net through the water. There are many types of trawlers varying in size from open boats, powered by outboard engines to huge factory ships, which can fish in the most distant waters.
Two requirements are common to all trawlers. One is the need for towing power and the other for winch or mechanical hauling system. In order to have a good towing power a trawler should have a reasonable draft or, displacement and should have a large slow-turning propeller. Many trawlers are fitted with propeller nozzles to give them more towing power. The winch or hauling machinism is the next important requirement for a trawler. Hauling or pulling power is usually expressed in tonnes, kgs. per speed (feet or metres per minute) for bare drum, or full drum.
The use of otter boards to open the trawl net month horizontally in preference to a beam was introduced over 100 years ago, and it is the single most productive method of fishing. Although otter trawlers are of three categories e.g. side trawlers, stern trawlers and double rig trawlers: there are actually a large number of variations in the gear used. They are shrimp trawls, prawn trawls, combination trawls, wing trawls, bobbin trawls, herring trawls, semi-pelagic trawls and mid-water trawls. All but the smallest of otter trawlers have gallows, derricks to handle the heavy otter boards.
The distinguishing feature of all side trawlers is the two steel gallows frames, one place aft and one forward on the starboard side. The starboard otter door is hauled up to the forward gallow and port otter is hauled up to the aft gallow block.
Fishing over the stern can be a very efficient way of trawling. It is particularly successful on the large factory trawlers and freezer trawlers, which could accommodate a stern ramp and a fish deck below the main gear deck. Setting and hauling the gear from a stern trawler is an easier operation than from a side trawler. The main points to be decided, when designing a stern trawler layout are the positions of the bridge, engine room, winch and whether the vessel will have a stern ramp.
The double-rigged "Mexican" type used in 1955 is the best known. The features of shrimp rig have been developed to suit a particular type of low-head line net with long otter boards attached close to the wing ends. One such net is pulled from each of the side booms, which are extended during fishing operation. A small Try-net is sometimes also operated over the stern.
Beam trawling is not used much today except in some shrimp and sole fisheries. Before the invention of the otter trawl, most bottom trawlers used a beam trawl. Beam trawls are cumbersome to use and are always fished over the side. Modern beams are made of steel. Several tickler chains are added near the foot rope to increase the weight of the gear.
In many ways scallop draggers resemble beam trawlers. The scallop draggers are small steel beam trawls with metal ring instead of netting on the underside. They scrape up scallops and other shellfish from the sea bed. Large scallop draggers may work three dredges, on each side, from booms as on double rigged beam trawlers.
Pair trawling fleets operate in Europe, Scandinavia, North America, Japan, Taiwan, Korea and Thailand of fish shrimp, hake, herring, cod, flatfish and also for miscellaneous fishes. Pair trawls are towed by two boats. The boats maintain a distance apart equal to about half the warp length. No otter boards are needed for pair trawling operations. This reduces the overall drag on the gear and permits the use of larger net. Long heavy cables are inserted between the net and the warp end at which point a weight and a swivel are usually attached. For most pair trawlers, a stern trawler or sade trawler deck arrangement is appropriate. The larger pair trawler fish over the stern and the smaller ones mostly on the side. If one vessel has more power than the other, then the stronger vessel will take the starboard side of the gear since it requires more helm.
Another type of bottom trawler is the Danish seiner used to catch demersal fish like cod, haddock, hake, pollock, flatfish, skate, sole and prawns. This vessel used no otter boards, but instead it sets over a mile of rope in an arc or L shape on each side of the net. The net itself resembles a large light trawl with long wings. The rope warpe scrape slowly over the seabed, all the while herding fish in towards the advancing net. Ones the ropes are almost closed or parallel to each other, the vessel speeds up to drive the remaining fish into the net and then the warps are winched quickly. The whole operation takes one to two and a half hours and in that time an area of the sea bed measuring over one mile by half a mile will have been "Swept" clean of fish.
Surround nets of many types have been used the world over to catch surface swimming fish. Ring nets, lampara nets and tuck seines are the best known of the numerous kinds of surround net that preceded the best known and most efficient, their use has seriously depleted stocks of herring, anchovy, pilchard and mackerel in different parts of the world. The chief distinguishing feature is that a pursue seine has purse rings fitted all the way round the foot. A ring net or lampara may have a few rings or none at all but a purse net must be rigged with a continuous series of rings so that it may be "pursed" or closed in a single winch operation.
The common features to most pursers are:
j. Ring net boats: -
Ring nets or lamparas are small surround nets, which if they are rigged with purse rings, only have a few of them located usually at the central bunt. Lampara nets are tapered towards the wings ends and have relatively long wings. Both ring netting and lampara fishing have been largely displaced by purse seining. Ring net boats are small and lightly powered. Most ring net boats work on two-boat system, which is much handier when the net has to be towed. Both ring netters and lampara boats tow their gear some length before closing.
Many purse seine boats use lights of 500-1000 watts to attract fish before setting the net. Light attraction is important in the tropical bait and sardine fisheries and in squid-jig fisheries. Fish attraction by lamp is not possible on moon lit nights or in areas of very strong currents.
Lines are still used effectively today to catch tuna, skipjack, mackerel, squid, snapper, cod and dog fish. There are many methods or line fishing, such as tuna long lines, bottom set long lines, pole and line gear for skipjack, trolling lines, squidjigs, mackerel jigs. Line fishing involves lots of labour as fish are hooked, on by one and lines need to be baited, cleaned, untangled and recoiled for setting as well as to be attended during fishing as in case of jigs and hand lines. Most line boats are small, but some measure over 200 tonnes with over 500 h.p.engines.
These are mostly large vessels, ranging from 90 feet (27 metres) 400 h.p to around 180 feet (55 metres) 1500 h.p which traverse the ocean in search of the migratory tuna. Because of the long distances they must travel, the catch needs to be frozen and the vessel requires large fuel and water capacities. Tuna lines are the "longest" long lines in the world. Many miles of line may be set with the baited hooks hanging from 100 or even 150 fathoms below the surface. The line hauler on the deck coupled with a coiler is used to coil mainlines.
Bottom set long lines are used in tropical and sub-tropical waters to catch snapper, grouper, ray, shark, eels and various reef fishes. These long lines carry hundreds of baited hooks rigged close together on short branch lines or snoods. The lines are anchored at each end with a buoy line attached to each anchor.
Trolling line boats tow lines extending on either side to catch skipjack, yellowfin, salmon, mackerel and other surface swimming pelagic fishes using unbaited hooks. Tuna trollers tow at up to 6 knots speed but usually in the range of 3 to 5 knots. They prefer to troll with the sun behind them, just after dawn, or before sun-set. When they pass through a school of fish, the crews are hard-pressed to haul in and clear the lines, the fish strike so rapidly. Trollers may be small open boats, or refrigerated vessel of 25 or 30 metres length.
Hand line boats operate all over the world, some in shallow waters and some in seas up to 200 fathoms deep. Traditional hand liners use no winch. One needs a strong back and tough hands to pull in lines from those great depths. While hand lines are usually baited and carry only one or two hooks, jigging lines carry only artificial lures but may have dozen of hooks in one line or jig. They are used to catch all kinds of demersal fish. Special fishing reels developed for hand liners have been proved to be most beneficial. These reels may be electrically or hydraulically operated.
Jigging is done by using a hand reel, which is oval shape to create the erratic jigging motion. Jigging is tried at various depths, until the fishermen determine, where the main squid concentrations are located. All the jigs are then set for that depth. The squid are attracted by means of powerful lamps strung on lines stretched between the masts.
One of the most modern and productive methods of line is fishing. Pole and line vessels are fitted with a narrow platform protruding all around the vessel at deck level, outside the rail or bulwarks. The platform extends forward from the stern to forend like a bowsprit. The crew stands on this platform with their backs to the rail when fishing with the polers. Successful pole fishing depends on the use of live bait to attract skipjack and albacore and seawater spray to excite and keep the fish close by. Schools of skipjack swim very fast, chasing small fish, which they feed on voraciously. When the boat sights a school it approaches the fish and the crew casts live bait into the sea beside them, Once the skipjack begin to attack the bait fish, the sea water pumps water from perforated or nozzled pipes extending fore and aft on each side and across the stern. At this point the crew begin to cast their lines among the fish. The poles the crew begins to cast their lins among the fish. The poles are made of bamboo or fiberglass, the lines are of bylon and hooks of hooks in their excitement and are hooked and swung on board. Ten, twenty or more crewmen are employed at one time using the fishing poles. When fish are very large, two poles are attached to one line to enable the crewmen to swing them on board. As the hooks have no bards, the fish fall off the hooks as they strike the deck.
Gill nets are used the world over to catch every kind of fish in the sea. The nets may be bottom set, surface net, anchored of drifting but these variations of not affect the vessel layout much. The nets are set over the side or stern, and hauled in over the bow. The main problems with all gill nets are that of preventing entanglement of fauling. Most gill nets used to be hauled by hand. The fish are picked out or shaken out as the nets come on board. Modern gill net haulers are powered sheaves, somewhere between a line hauler and a power block in shape.
Lift nets and stick-held dip nets are used to catch sardine, anchovy and baitfish, where lights are used to attract fish to the surface. The lamps are hung from booms, which can swing out over the dip net. The net is held by strong bamboo poles, which are made secure on the vessel and have tackle lines extending from the outer ends up to a block on each mast. The net is a stocking shaped bag attached to an iron ring and pole. The hoop is passed through the fish and taken up on deck. The end of the stocking bag is lifted up and the fish slide out through the ring on the deck.
Small fish traps are used to catch lobsters, crabs shrimp, octopus, eels and all kinds of reef fish. A small vessel using 20 or 30 traps and landing 20 to 30 kgs. of fish or shellfish per day may make a profitable living. Traps are always baited with flesh or artificial baits. The traps are set on the bottom in series with a small anchor buoy at each end.
A Mexican inventor has developed a bicycle-type under water vehicle, suitable for fishing, known as "Bucicleta". The vehicle is shaped like a small aeroplane. The fuselage is a plastic tube measuring 10-16 cm. In diameter and 2.20 m., in length. The wings are shaped like shark fins and the tail is an aerodynamic T-shaped. It is run either by pushing or by pedaling. The vehicle can also be built with a propeller.