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| Home | Submarine cable network | Laying, burying and maintenance cables | Fishing activities | Fishing vessels |
Fishing vessels
Fishing vessels are classified according to various criteria :
- The distance travelled out of port to reach the target fishing zone
· Coastal fishing
· Fishing on the high seas
· Long distance fishing expeditions (possibly a full-scale commercial fish harvesting operation) - The fishing equipment or gear on board the vessel
· Active machines: trawl net, pelagic trawl, seine, dragnet, certain types of net, trawl beam.
· Passive machines: bow nets, racks, trawl lines, lines, nets.
· Tuna, sardines, cod, flatfish in general, shellfish, mackerel, hake.
- Ships fishing with nets
· Bottom trawlers
· Pelagic trawlers
· Seiners
· Drifters (drift nets on the surface)
· Shrimp trawlers
· Crayfish trawlers
· Coastal vessels - Ships fishing with lines
· Line fishing vessels
· Tuna line-fishing boats
· Long-liners
· Surface line-fishing vessels - Ships using diverse tackle
· Lobster and crab fishing vessels
· Dragnet vessels
THE MOST SIGNIFICANT RISKS ARE POSED BY TRAWLERS, DREDGERS AND DANISH SEINES.
Trawlers
· Bottom trawlersThis last type of ship uses trawl nets slung between two vessels, and so poses no threat to cables except in exceptionally rare incidents.
· Boom trawlers
· Bottom ox trawlers
· Shrimp trawlers
· Pelagic trawlers, single or paired
Note: Crayfish trawlers, whether they are coastal or ocean-going vessels, are in fact bottom trawlers.
The size of these ships varies from 12 metres for the small coastal variety, to more than 50 metres for the industrial trawler. Factory ships can be up to one hundred metres long.
The fishing zones for the different types of vessel are very distant from each other, and they never (or almost never) fish together. The fishing techniques used, although on different scales, are the same.
The trawl
A trawl comprises a large bag-shaped net (trawl) held open vertically by the displacement of water caused by the forward thrust of the ship, and by floats.The warpsThe opening in the horizontal direction is formed with PANELS, (doors, or otterboards) which act like blades operated by the force of the displacement of water over their surface.
As the vessel's speed increases, the resisting surface of the panels increases, and so they are increasingly separated one from another. This is referred to by the professionals as "squaring ".
One of the parameters vital to the calculation of the resistance of a body as it advances through water, is the square of the displacement speed.
The otterboards, lying in grid format to form a blade spacing system over the warp or towing cables, with the trawl or towing gear connecting the panels to the trawler, thus work ultimately in a position perpendicular to the seabed (or almost perpendicular, depending on the particular circumstances and the intended prey) and at an angle of between 30 and 40 degrees in relation to the direction of the ship's direction of movement.
The diameter of the towing cables, or warps, varies according to the size of the ship. There are generally two cables towing the trawl. The diameters range from 13 mm for small ships to 27 mm for large trawlers.The cross-linking gear (links or pendants)The length of the nets trawled depends on the depth of the water. It is generally reckoned that this should be about three times the water depth. For a depth of 100 metres, there will be 300 meters of net in the water. In shallow waters, the ratio is greater than 1:3. So, at a depth of 30 metres, one can have a TRAWL of 150-200 metres.
Conversely, in deep waters, the ratio decreases so that at 1,000 metres depth less than 2,000 metres of warps will be trailing. This is due to the total weight of the warp being in the water, but also because of limitations to the quantity of trawl net that can be stored on board. The lengths of trawled warps must be exactly the same either side of the ship. On this depends the efficiency of the trawl.
Panels are sometimes directly woven into the trawl, but in general, there is an ARM (cross-link or pendant) linking them together. These cross-links are generally made of steel wire rope, with a diameter either the same as, or slightly less than, the warp. In certain cases, the diameter of the cross-links can be greater than that of the warp. Mixed material linking gear might be used (steel and textile) or even chain. The length of the cross-links can be up to 200 meters, with an average value of between 50 and 100 metres.The panelsMixed material linking gear might be used (steel and textile) or even chain. The length of the cross-links can be up to 200 meters, with an average value of between 50 and 100 metres.
The shape of the panels varies depending on individual ship owners, and the customs and practices of the area. They may vary from being round, through oval, to rectangular panels. The weight of the panels varies according to the size and the power of the ship.Opening of the trawl and spacing of the panelsSmall ships have panels of approximately 50 kgs, while large trawlers may use panels of more than 2 tons. A trawler of 24 metres and 600 CV generally has panels weighing between 400 and 750 kgs.
The panels can be made partly of wood, or a synthetic material (ostalene), with their undersides and framework made out of steel, or they may be made totally in steel. The underside scrapes along the sea bed, and are therefore subjected to considerable wear and tear. To increase the life of the panel undersides, and so lengthen the working cycle of the vessel (before having to unload the vessel for re-conditioning the panels) they are reinforced with tungsten steel and manganese. If the trawler is obliged to suddenly stop fishing, the panels fall flat on the bottom, as they are no long held in the balanced state resulting from the traction of the trawler on the one hand and the tension of the trawl, on the other.
If the bottom is muddy, a suction effect of the gear on the bottom can make it difficult to turn. This sometimes results in the vessel being driven off course. As soon as the boat gains speed again, the panels resume their normal position.
How widely the trawl net opens will depend on its construction, the model, the speed of the ship and the surface area of the panels.Example:
For a trawl having a spinal central line (a boltrope high up on the upper front part of the trawl on which the floats are secured) 50 metres long, with cross-links of 70 metres, a probe (((depth sounder?))) of 180 meters, and so a trawling gear overall of 550 metres, the spacing between the panels will be approximately 70 metres.
Beam trawlers target only bottom-dwelling fish such as monkfish, skate, dab, sole and plaice, and trail two trawls simultaneously.
- One on the port side,
- the other to starboard.
The trawl net comprises:
a "pole", giving the vessel its name, made of a hollow metal tube with a large diameter of anywhere from 150-250 mm, and depending on the size of the ship, being used to form the horizontal opening.Pair trawlersThis pole has a series of (((semelles) which form the vertical opening, and limit friction on the bottom.
The higher part of the trawl is made up of nets. The lower part, apart from the pocket where fish collect, is made up of a series of chains.
Each trawl net is towed by only one warp or cable.
The rigging on a trawling vessel is very heavy and requires considerable traction power.
The trawl is constructed in such a way that speed does not affect the quality of the catch, and therefore these ships can move very quickly while fishing.
The average speed of a traditional trawler is about 3 to 4 knots, whereas a beam trawler moves at double this speed.
The risk of this happening is of course only slight, because so many variables are involved.
Many ships use twin trawling nets called TWINS.Twin wheelsInstead of towing just one trawl, they drag two trawl nets simultaneously. Different rigging is required for this, incorporating chains of large diameter, on the front bit of the trawls. Some ship owners even use triple trawls.
In order to prevent tearing the part of the trawl net lying close to the bottom (the "belly" and lower "wings"), the lower rope (a sort of fold at the lower front edge of the trawl net, held in place by chains and rubber discs) is often protected by diabolos or twin wheels.Rock HopperThese TWIN WHEELS are made up either of steel spheres with a diameter of 400 mm, or of rubber discs with a diameter of 100 to 350 mm interconnected with a metal reinforcing rod. Originally, these twin wheels were designed to roll along the bottom, dispersing or riding over any obstacles encountered.
Today, a different technique is sometimes employed : the ROCK HOPPER method.CatchesThe rollers or twin wheels on the Rock hopper are attached by a chain which prevents them from turning, and are tightly assembled on the cable or the support chain (as closely packed solid entities).
When these twinned wheels meet a rock, instead of following the contour of the rock, which might tear the lower part of the trawl, they " jump" over it because of their rigid construction, obliging the belly of the machine to follow.
This makes trawling possible now in areas where before it was considered too dangerous or risky.
The trawlers bring back all the species normally found on the sea bottom or near the bottom :
- gadides, skate, shark, monkfish, flatfish, crayfish, etc...Gadides : hake, julienne, pollack and black cod, whiting, eaglefin, pout.
This technique of trawling, little used in France, requires two ships, towing the same trawl. This method was employed in particular by ships without sufficient horse-power to trawl on their own.
The distance between the ships conditioned the opening of the trawl, and so panels were not required to fulfill this function. The main risk of hooking on to objects on the seabed with the panels was thus removed. However, in the absence of the weight provided by the panels, compensating ballast consisting of old mooring chain were used, and this scraped the bottom.
These trawls could be enormous. The warps were longer so as to compensate for the lack of panel weights. The two ships working together were a great deal less manoeuvrable than a single vessel.
These ships trail two trawl nets simultaneously and employ booms, like the beam trawlers, to keep their equipment away from the side of the boat.
They open their trawls, as well as the traditional bottom trawl nets, by means of panels, which are significantly longer than usual, and are sometimes punctured by a series of slits.
The trawls are relatively small, as are the panels, which have a reduced weight and surface area. They also trail a third, small-sized trawl net as a control trawl for assessing when to stop trawling and transfer the catch to the hold.
These trawlers are after a specific catch, and so they operate only in very certain places: gulfs, estuaries with a particular set of saline and temperature qualities favourable to shrimps.
This type of ship is specialised in fishing scallops. To this end, it trails shellfish dragnets in varying numbers depending on the site and the legislation in force.
The dragnets are constructed around a metallic structure of about 1m20 to 2 metres wide, which creates a small vertical opening in the net.
The lower part of the dragnet is made up of a kind of net having round metallic meshes stacked up one upon the other. The lower front part of the dragnet is equipped with teeth, with a precise length and spacing, again depending on the legislation in force, which serves to get rid of the sand attached to the scallops. These teeth rake through the substrate to a depth that matches their length (generally about 10 centimetres).
The dragnets are towed using relatively small-diameter cables (only one per dragnet).
As the harvesting areas for scallops re very localised, a large flotilla of fishing vessels can turn up in a small sea area.
Fishing scallops is stringently regulated and supervised.
Bottom pelangriers, or Long-liners
These ships fish with long lines several kilometres long that can reach the ocean floor.
On the principle line or "mother" line, hooks are fixed at variable intervals by means of short lines running perpendicularly to the mother line. The bait is attached to these hooks.
The line might be textile or synthetic. The short lines might be textile, synthetic or in steel when fishing for sharks. The bait can be attached by hand or automatically on the larger long-liners. The bait varies according to the type of fishing, and might be sardines, mackerel, squid.
Some long-liners lie directly on sediment, or are semi-floating with one part on the bottom and the rest floating at a variable distance depending on the operation.
The danger of the long-line vessels for cables lies in the fact that under certain circumstances the hooks can catch on the protective sheath surrounding cables, and subsequently penetrate it. Water can then get in and damage the conducting wires.
These ships fish for hakes, julienne, conger eels, shark, skate, sea-bream.
Drifting trawl lines
Called by the Anglo-Saxons " long liners ", these ships trawl extremely long surface or sub-surface fishing lines.Tuna line-fishing vesselsThey fish mainly for tuna, sword fish and shark.
These boats are characterised by their distinctive antennae or poles used to support the lines. They only fish at the surface of the sea and move relatively fast.The small linersThey are very manoeuvrable, and can easily avoid other ships.
These are small fishing vessels working close to the coastline, and specialised in fishing fine fish (bass, pollack, sea-bream) fishing with several lines or just one line.The shell-fish trawlersThey are often to be found in waters that are too rough for cable ships.
These boats are specially designed for fishing shellfish (crab, spider crab, lobster, shrimp, possibly cuttlefish and whelks).They fish using pots or cages, different ones being used to catch the different species. The pots are attached to a mother line, at variable distances depending on the depth. The bait is fixed inside them. The pots pose little or no risk to cables. In the event that a pot became entangled in a cable, the ship would be obliged to break or cut its mother line, and would in this case lose perhaps ten sets of gear.
Each ship will be equipped with several hundred pots of this kind.
These ships characteristically employ semi-passive and semi-active fishing techniques. The design of the fishing gear is comparable to that of the trawler, but it is used differently. The ship moors an anchor to a flagged marker buoy. To this buoy is attached a very long rope which can touch the sea-bottom.
Then the ship trails out the seine net, returning to the marker buoy in a wide encircling route so as to encircle as many fish as possible on the sea-bed.
When the ship gets back to the buoy, it lets off simultaneously the two buoyropes as it brings the seine net on board the boat. The catch can then be transferred to the hold, or held in the net for the journey back to port. This fishing technique is little used by the French.
Fishing practices posing no threat to submarine cabling
Pelagic trawlers
These ships work alone or in pairs, trawling a very large net in search of pelagic fish :Seinerstuna, mackerel, anchovy, bass, etc...
Their trawling gear is very large and relatively fragile, and must not touch the bottom at all if the considerable cost of damage to the trawl is to be avoided.
There are two types of seiners :Drift netsThe seine is a large pocket-shaped net that encircles a shoal identified beforehand with the help of radar or an echo-sounder.
- Sardine fishing boats fishing inshore, and in bays in particular,
- Ocean-going tuna boats, always found on the high seas.
The lower part is ballasted with chains, and the upper part is maintained at the surface by large floats.
Once the circling manoeuvre is complete, the purse net is closed with a drawstring arrangement (a line or cable threaded through a series of rings). The fish are then held within the net, and cannot escape.
Seine nets are very large indeed, especially when used by ocean-going tuna fishing vessels, and are relatively fragile. Touching the bottom is to be avoided at all costs, or the nets may be damaged or lost. The largest seiners are to be found fishing for tuna in the deepest parts of the ocean. Tuna fishing with seine nets is practised in the Gulf of New Guinea, the Indian Ocean, and the Pacific.
Several species are fished for with drift nets :- tuna, salmon, swordfish. In the past : sardines and mackerel.
These nets float on the surface or just below it, and drift with the prevailing currents. The ships are obliged to follow the nets in order not to lose them. The nets are not attached to the seabed at all.
The only problem posed to cable ships can be the presence of a large group of boats working together in one area. This kind of fishing fleet moves rapidly to follow migrating fish shoals.
