Keeping the catch fresh once the fish have been caught has always been a problem and the length of time iced fish will remain in reasonable condition effectively limits the duration of fishing for distant-water trawlers if they rely solely on ice for preservation. The development in recent years of efficient and cost-effective refrigeration and storage systems on board larger fishing boats, has made much longer fishing trips possible. If fish are stored directly in the hold, then dry refrigeration or refrigerated brine spray can be used to lower the hold
temperature. For very high quality, circulating refrigerated seawater tanks may be used.
The most notable advance in fish preservation has been the development of deep-freezing techniques. At temperatures around — 30°C fish can be kept without serious deterioration for long periods, but for good results they must be frozen as soon as possible after capture, certainly within three days. This presents an obvious problem where the fishing grounds are several days' voyage from port unless it is possible to deep-freeze the fish at sea. To do this, some trawlers now carry refrigeration equipment so that some or all of their catch can be frozen immediately, and the duration of their voyage can thus be extended. It is not essential to freeze the entire catch because the later-caught fish can be brought to market in good condition on ice. When fish are frozen aboard, they are often preserved as whole fish which may be thawed later and dealt with through the normal wet-fish marketing channels. These fish are also suitable for smoke curing.
In recent years, various types of factory ship have been developed. Factory vessels can process their catch directly so that the packaged and frozen products are ready for distribution upon arrival back at port. High-value fish such as tuna for the raw fish market, can even be individually and rapidly frozen in low-temperature liquid tanks before storage. Factory ships may be enlarged trawlers, equipped both to catch and to process the fish, or larger ships equipped solely for processing and storing great quantities of fish, and operating in conjunction with a fleet of trawlers which bring their catches to it.
Preservation by freezing has advantages in addition to making possible longer fishing voyages in distant waters. Because frozen fish can be kept for long periods without deterioration, the market supply can be regulated independently of seasonal variations in the quantities of fish caught. Freezing has also opened the market to a wider range of species, because blocks of frozen filleted fish, from species not ordinarily bought by the public, can be successfully marketed.
9.1.2 Demersal fishing
The beam trawl (Figure 9.2) is a tapering bag of netting which can be towed over the sea-bed. The mouth of the bag is held open by a long beam, the ends of which are supported above the sea-bed by a pair of strong metal runners. The upper leading edge of the bag is attached to a strong headrope lashed to the beam. The under part of the bag, which drags over the bottom, is attached to a considerably longer groundrope. As the trawl is towed, the ground rope trails behind the headrope so that fish disturbed on the bottom by the groundrope are already enclosed under the upper part of the net.
The hind part of the trawl net tapers to a narrow sleeve of stronger, finer-mesh netting known as the cod end, within which the captured fish accumulate. The rear of the cod end is tightly closed by a rope, the cod line. When the trawl is hauled up onto the vessel the catch can be released by untying the cod line, letting the fish out of the back of the bag.
Beam trawls have been in use for several hundred years, and were the chief method of demersal fishing in the days when fishing vessels were sail-driven. Although less used in modern commercial fisheries, beam trawls are still operated in Europe from small boats by inshore fisheries where the modern otter trawl is too large. They are used mainly for flatfish and in the north-east Pacific (Alaska) for shrimp. Being easy to handle from small craft, small beam trawls also have applications in biological work.
The size of a beam trawl is limited by the length of beam which it is practicable to handle. The largest nets are sometimes as much as 15 m wide. During the latter years of the nineteenth century, the introduction of steam trawlers capable of towing much heavier nets caused the beam trawl to become almost entirely superseded by the larger, more easily handled and more efficient otter trawl except for inshore use. Recently there has been an increase in the use of beam trawls made more effective by the addition of heavy 'tickler chains' in front of the groundrope. These chains plough up the bottom immediately ahead of the net so that fish buried in the sediment cannot escape by letting the net ride over them. Electrified ticklers are also used, which cause burrowing animals to jump out of the sand, increasing the likelihood that they will be enclosed in the net. These electrified chains, being lighter, do less damage to the bottom.
This is now the chief method of demersal fishing. The otter trawl (Figure 9.3) has a bag of netting resembling that of the beam trawl in general shape, but considerably larger. The sides of the bag are extended outwards by the addition of wings of netting attached to large, rectangular, wooden 'otterboards'. These otterboards are towed by a pair of very strong steel cables, the warps, which are attached to the otterboards in such a way that the pressure of water causes the otterboards to diverge as they move, pulling the mouth of the net wide open horizontally. The under-edges of the otterboards slide over the sea-bed, and are shod with steel for protection.
The headrope, to which the upper lip of the trawl net is laced, is usually some 30-40 m in length, and bears numerous hollow metal floats which keep it a few metres above the bottom. Sometimes, elevator boards known as 'kites' are fitted to the headrope to increase the gape of the net. The lower lip and groundrope are considerably longer, about 40-60 m, and trail well behind the headrope during trawling. The groundrope is a heavy, steel-wire rope, carrying on its central part
a number of large steel bobbins, about 60 cm in diameter, and on its lateral parts several large rubber discs. These help the trawl to ride over obstructions on the sea-bed. A bottom trawl cannot be used, however, on very rough ground or where wrecks may snag the net. Loss or damage is very expensive.
At the junction of the body of the trawl and the cod end, there is a vertical flap of netting, the flopper, which hangs down and acts as a valve to prevent the escape of fish from the cod end if the trawl should stop moving. The under part of the cod end is protected from chafing on the bottom, and a cod line is tied to close the free end. The trawl wings may be joined directly to the otterboards, but nowadays trawls of the Vigneron Dahl type are commonly used, in which long cables about 50-60 m in length are inserted between the wings and the otterboards. This simple modification gives the trawl a greater sweep, disturbing more fish and considerably increasing the catching power of the gear.
The deck of a trawler carries a powerful winch for winding the warps, and the net may be operated either from the side or the stern depending on the design of the vessel. The older method is side trawling. For shooting and hauling the trawl, the sides of the trawler carry pairs of strong gallows supporting the blocks over which the trawl warps run. On British vessels, starboard was the traditional working side for the trawl; but later, trawlers were fitted with gallows on both sides, and carried two trawls, working each side alternately. Stern trawling is a more recent development that is superseding side trawling. Built over the stern there is usually a gantry for operating the net and warps, and many stern trawlers have a slipway in the stern up which the net is hauled. This means the cod end does not have to be lifted over the stern or side. This technique allows greater mechanization, speeding up the hauling of the net and making the trawlerman's life a little less arduous. Although side trawling is likely to remain in use in various parts of the world for some years, few, if any, new boats are being built for side trawling.
When shooting the trawl, the ship moves ahead as the warps are paid out until the otterboards are submerged and the net overboard. The two warps must be of equal length to ensure correct opening of the net. When the trawl strikes the bottom, the drag of the net slows the vessel and the subsequent speed of trawling depends upon the power of the ship. The length of warps is adjusted to about three times the depth of the water, and on side trawlers they are braced in at the stern to keep them clear of the propeller. The strain of towing is taken on the winch brakes.
When the net is to be hauled, the trawler continues ahead as the warps are wound-in by the winch. The cod end usually comes up to the surface with a rush, due to the distension of the swimbladders of fish brought up quickly from the bottom. When the cod end comes alongside a side trawler, it is brought aboard by a hoist from the mast-head and suspended about 1 m above the deck. On a stern trawler the net is usually drawn up a slipway to the deck. The cod line is then untied and the fish discharged on to the deck.
The fish are then sorted and trash fish and bycatch are discarded. The fish are gutted and the heads removed from the larger species such as cod, saithe and haddock before being thoroughly washed by hosing. On factory ships the fish are processed directly. Otherwise they are stored on ice, refrigerated or frozen, before storage.
The Spanish trawl (pareja)
The Spanish trawl (Figure 9.4) is a very large net, similar in general principle to other bottom trawls, but towed by two vessels working together. This trawl has no need of otterboards, the mouth of the net being held open horizontally by the lateral pull of the warps, one of which is attached to each vessel. The technique of using two boats is known as pair trawling and is more commonly used in mid-water trawling.
The net has a wide sweep and great catching power, the headrope sometimes being as much as 100 m in length. The mesh is constructed of lighter material than the otter trawl, and generally produces a catch in rather better condition. In spite of its great size, this net can be towed by a pair of relatively low-powered vessels which individually could not undertake otter trawling. The Spanish trawl can be operated in deep water down to 600 m on the continental slope.
The Danish seine (Figure 9.5) is a light-weight net for taking fish from the bottom, used in shallow water by small motor vessels. The net consists of a central bag
with lateral wings extending 25-40 m on either side. The groundrope is weighted, and the headrope buoyed by small floats.
There are several ways in which this net may be operated. Usually one end of a warp is attached to an anchored buoy, and the vessel steams downtide letting out a great length of warp, sometimes as much as two miles. The vessel now turns at right-angles and shoots the net across the tide. A second warp is attached to the net, and the vessel turns and steams back to the buoy running out the second warp. Both warps are then winched in, dragging the net over the sea bottom. The converging warps sweep the fish between them into the path of the net, gradually drawing the wings of the net together to enclose the catch, and finally the net is hauled up to the vessel.
Alternatively, the warps and net may be laid in a circular course, and then towed behind the vessel for some distance before hauling. This is termed fly-dragging.
The Danish seine carries no devices for clearing obstacles on the sea-bed, and is therefore only used where the bottom is fairly smooth. The condition of the catch is usually better than that obtained by trawl, and because many of the boats make only short voyages the catch is fresh. Fish may be brought up alive and can be carried live to port in seawater tanks.
Line fishing captures fish on baited hooks and can be used for both demersal and pelagic fishes. The latter is described in Section 9.1.3. Both horizontal and vertical lines can be used. The technique is often laborious because usually each hook has to be individually baited, and each captured fish removed from the hook. If the line becomes caught or entangled on the sea-bed, its recovery may take many hours; or the line and its catch may be lost. Line fishing is, however, widely practised throughout the world. The gear is simple and relatively inexpensive, and can be operated from any size of boat which need not be specially designed for this purpose. The capital costs can be kept low by conversion to 'lining' of various obsolescent craft. It is therefore well suited to the needs of communities where little capital is available for investment in fishing. In addition, automated mechanized systems are now available for use by larger boats. Line fishing is very adaptable to local circumstances and to the species sought. To some extent it can be made selective by choice of bait and size or shape of hook. Line-caught fish often fetch a high price because, being individually caught and handled, they can be brought to market in specially good condition.
For long-line fishing on the bottom the fishing gear comprises a very long length of strong line bearing at intervals numerous short lengths of lighter line, the snoods, which carry baited hooks (Figure 9.6). In laying the line, one end is anchored to the sea bottom and its position marked by a buoy. The liner steams along a straight course running out the full length of the line along the bottom, and the other end is marked by a second buoy. As the line is laid, the hooks are baited with pieces of fish or squid. A 10 m or so long vessel with one crew may fish 2500 hooks a day and double that with two crew. Mechanized long-lining systems may fish up to 40 000 hooks a day. Each line is left on the sea bottom for a few hours before hauling. As the line is brought aboard, the fish are removed from the hooks and the line carefully coiled for use again.
This technique does not generally compete directly with trawlers, because it can particularly exploit fishing grounds where trawls cannot be operated and it
concentrates on valuable species such as halibut, which are not caught in quantity by trawl. In continental shelf areas and offshore banks, species targeted include cod, haddock, whiting, dogfish, skates and especially halibut. Its use by British vessels is confined mainly to areas where the sea bottom is too deep or too rough to be suitable for trawling. Line fishing vessels, termed 'liners', operate in the northern part of the North Sea and down the continental slope to the north and west of the British Isles.
Various miniature versions of long-line fishing, for example haddock lines, are used in inshore waters around the British Isles, using shorter lines and small hooks suited to smaller fish.
9.1.3 Pelagic fishing
Drift nets (gill nets)
Drift nets can be used to catch fish which form shoals near the surface, for example herring, mackerel, pilchards and sometimes sprat. A drift net consists of a series of rectangular, light-weight nets joined end-to-end to form a very long vertical curtain of netting which hangs loosely in the water. The top edge of the curtain bears floats; the lower edge is weighted by a heavy rope, the messenger, by which the net is attached to the vessel. At the junctions between the individual pieces of netting are buoy-ropes, or strops, by which the net is suspended from a series of surface floats (Figure 9.7). The size and mesh of net, and the depth at which it hangs in the sea, are chosen to suit the particular fish sought.
For most pelagic species, drift nets are usually shot shortly before darkness. The fishing vessel, known as a drifter, moves slowly down wind while the net is paid out until the full length has been put into the water. The messenger rope from the foot of the net is brought to the bow of the drifter, which swings bow to wind and simply drifts attached to its long curtain of netting. During darkness, shoals of fish ascend into the surface layers, and become entangled as they attempt to swim through the net. The mesh size is selected so that the head of the fish passes easily through the net but the larger middle part of the body will not go through. When the fish try to wriggle out backwards, the net catches behind the gillcovers so that the fish are unable to escape. This method of capture is fairly selective, retaining only fish within a particular range of sizes. Smaller fish can swim right through, while larger fish may not pass sufficiently far into the mesh for the net to slip behind the gillcovers.
The net is hauled a few hours before dawn. The strain of hauling is taken on the strong messenger rope, which is wound in by a capstan or winch on the deck of the drifter. The net is carefully drawn in by hand over special rollers fitted on the side of the drifter. As the net comes aboard, the fish are shaken out and fall through hatches into the hold.
Drift nets have largely been superseded by purse seines, ring nets and pelagic trawls for the capture of herring and most other shoaling fish. In the North Sea the use of herring drift nets had ceased by 1970. They are still used for certain species which follow particular migration routes at particular times, notably for tuna. Salmon are also caught in this way, both on their feeding grounds at sea and in inshore waters as they approach the rivers for spawning. Modern drift nets are mostly nylon and monofilament which means they are very light and relatively simple to handle even though they may reach lengths of several kilometres. The maximum permitted length in the EC is 2.5 km.
There is now considerable concern over the use of monofilament drift nets in the open ocean. This is discussed further in Section 10.4.2. They are effectively invisible to animals using sonar and are responsible for the deaths of many dolphins and porpoises each year. In the eastern tropical Pacific, yellowfin tuna live alongside various dolphin species. Fishermen use spotter planes to locate the schools of dolphin on the surface, and, knowing the tuna will be below, they set their nets in the area. This leads to many dolphin deaths. Turtles and diving birds can also become ensnared earning drift nets the nickname 'walls of death'. A further problem is the virtual indestructibility of nylon nets. Lost nets may carry on 'ghost fishing'. The nets ensnare and entangle marine life, and sink to the bottom where the catch rots away or is eaten. The net then rises up and starts to fish again.
A ring net is a curtain of fine-mesh net hung vertically in the water to encircle surface shoals of fish. Around the British Isles it has been used mainly for capturing herring, although the technique is applicable to many other species which form shoals near the surface. It is specially suitable for inshore and enclosed waters such as sea lochs and estuaries where the enormous lengths of net used by drifters are unmanageable.
The size and mesh of ring nets are selected to suit local conditions and the species sought. Herring ring nets are up to 200 m in length and 30-40 m deep. The top of the net is buoyed by corks and floats at the surface. The lower edge of the net is attached to a weighted sole rope (Figure 9.8). A fine-mesh net is used because the fish are not entangled, as in a drift net, but are trapped by being surrounded by netting.
Ring nets are often laid by a pair of vessels operating together. When a shoal
of fish has been detected, nowadays often by echo-location, one end of the net is secured to one vessel while the other steers a semicircular course paying out the net. The two vessels then steam on parallel courses, towing the net, and finally turn to meet and enclose the shoal. A pair of messenger ropes attached to the lower edge of the net are winched in, closing the bottom of the net below the shoal so that the fish cannot escape by swimming underneath. The net is gradually hauled on board until the captured fish are densely crowded within the central part of the net. They are then scooped out of the net and brought aboard by a dip-net known as a brailer, or by a suction hose.
Purse seines are similar in principle to ring nets but much larger, sometimes as much as 400 m in length and 90 m in depth. In the sheltered conditions of the Norwegian fiords they have been used for capturing herring. Mackerel shoals are caught by purse seines in the English Channel and North Sea.
In warm seas purse seines are extensively used to encircle shoals of tuna. As with the drift nets, dolphins are often incidentally caught and eventually drown in the nets. New tuna nets have now been developed to help avoid this problem. The nets have a fine-mesh area, the 'Medina panel', which is visible to dolphins. So as the net tightens, the dolphins can jump over the net rim and may even be given a helping hand by the fishermen.
Pelagic trawling (mid-water trawls)
Mid-water trawls are used for capturing pelagic fish shoals and also for demersal species during periods when they leave the sea-bed. A pelagic trawl is usually a conical net kept open by floats on the headrope, a weighted footrope, and by
various otterboards, elevators and depressors attached to the mouth (Figure 9. 9). The depth of trawl can be regulated by the length of the warps and the speed of the vessel, but its control requires considerable skill. Modern nets may be under computer control which may greatly increase their efficiency. Sensors can show height above the bottom, depth below the surface, vertical mouth opening and data on fish entering the net. Accurate location of the fish shoals is obviously essential. Sonic techniques are used to enable the position of the net and its relation to the fish shoals to be accurately known and this too can be fed into the computer system. Hybrid semi-pelagic nets which can be used either on the bottom or in mid-water have also been developed.
For capturing pelagic species, long lines can be floated a variable distance below the surface suspended at intervals from a series of surface buoys. The long line lies more or less horizontally in a series of sagging connections between each buoy, with numerous short hooked lines attached and hanging below (Figure 9.10). This method is often more effective than nets over deep water for catching certain large species which do not form dense surface shoals. It is widely used in all oceans by Japanese and Korean fishermen for catching tuna, especially in areas where the thermocline lies deep. A single ship may operate over 100 km of line.
Large pelagic fish such as tuna and sharks are also caught on single hooked lines dangled or trolled through the water from strong poles which are either manhandled or may be hinged to the side of the boat and mechanically raised and lowered. The hooks are baited either with artificial lures or more often with live fish. Sardines or anchovies are favoured bait, and numbers of these are first pitched overboard to attract groups of tuna to the fishing boat. The method is used by Breton and Basque fishermen for tuna in the Bay of Biscay, and by the Japanese in parts of the Pacific where the thermocline is shallow. In the western English Channel, Cornish fishermen use hand lines carrying numerous feathered hooks for catching mackerel.
Figure 9.10 Pelagic long line for tuna. Length of mainline between floats is usually 300 m.
Figure 9.10 Pelagic long line for tuna. Length of mainline between floats is usually 300 m.
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Lets start by identifying what exactly certain boats are. Sometimes the terminology can get lost on beginners, so well look at some of the most common boats and what theyre called. These boats are exactly what the name implies. They are meant to be used for fishing. Most fishing boats are powered by outboard motors, and many also have a trolling motor mounted on the bow. Bass boats can be made of aluminium or fibreglass.