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Fishing Techniques

Tuna Purse Seining

Main Components

Aquatic species

Target Species
Tuna market species

Target Species
Tuna like speciesTarget Species
Bonitos

Gear types: Purse seines

Purse seines

A purse seine is made of a long wall of netting framed with a lead line of equal or longer length than the float line.

Vessel types: Tuna purse seiners

Tuna purse seiner

These vessels are large purse seiners with the same general arrangement as the American seiner, equipped to handle large and heavy purse seines for tuna.

Characteristics

Tuna purseining
Drawing of tuna purse seining fishing operation

OverviewTuna purse seining involves surrounding tuna shools with a net, impounding the fish by pursing the net, and drying up the catch by hauling the net so that the fish are crowded in the bunt and can then be brailed out. Species EnvironmentAs the purse seine catches fish above the thermocline, fish tuna and tuna-like fish inhabit in the surface and sub-surface zone (mixing area) are the target of this technique. They are, in tropical waters, juvenile yellowfin, juvenile bigeye and skipjack, together with some small tuna-like fish such as frigate tunas, bonitos, etc. Those fish are generally feeding baitfish near surface. In temperate waters, they catch either small juvenile bluefin tuna when they are feeding on baitfish or large bluefin tuna while they surface for spawning activities. Also in temperate waters, purse seine is occasionally used to harvest albacore, generally during night when fish come to surface for feeding.Fishing GearThe industrial purse seine gear is a large net which is used to encircle the school of tunas and closed at the bottom to entrap them. The synthetic net measures 1 500 to 2 000 m long and 120 to 250 m depth. The size of the mesh is usually around 120 mm stretched mesh. The top of the net is mounted on a floatline and the bottom on a leadline which, usually consists of a steel chain with steel rings, known as "purse rings", are attached below the chain; the purse line which run through the purse rings is made of steel and allow the pursing of the net. There are smaller purse seine gears used for less industrialized fishing near coastal areas, mostly targeting small tuna-like fish, such as frigate tunas, and bonitos. Those fishing is seasonal and the same gear is used for other pelagics such as sardine and anchovies.Vessel OverviewTuna purse seiners vary considerably in size. Industrial tuna purse seiners are usually large vessels which length ranges between 45 and 85 m, sometimes over (up to 100-110 m). Those seiners are facilitated with a large skiff, often with a few speed boats, and with a helicopter. Also equipped with a brine freezing fish wells. In coastal areas, smaller purse seiners also operate, sometimes on juvenile tuna species or tuna-like fish. The same boats are multi-purpose, including fishing for small pelagics (sardine, jacks, mackerels etc.) and seasonally fish for tunas. The size varies from 20 to 50 meters. All purse seiners are equipped with a power block to purse the lead line after fish are inside of the net. In some area, paired seiners were used of the size of about 300 to 400 GT. But this fishing methods has been reduced if still exist.Handling ModeStorage of tuna once it is caught presents a problem, for the size of the fish is large. Some vessels are equipped to bulk-freeze the catch, but the commonest method is to keep the fish in refrigerated brine tanks (brine at 0 °C), which form much of the lower parts of the hull and are equipped with batteries of seawater pumps for circulation. In the industrialized purse seiners, the tunas are preserved in wells of 20 to 40 metric tonnes each, (total 800 to 2 000 metric tonnes) with brine freezing at -20 °C. In the more artisanal purse seiners, tuna are generally kept in iced seawater.Fishing EnvironmentIn general the tuna purse seines are used to capture (large pelagic) fish aggregated and swimming not too far from the surface (mixing zone above thermocline) in the high-sea waters, as well as in the near coast waters. Aggregated resources up to a maximum of 300 m depth, but practically at depth less than almost 150 m depth and mostly less than 60-70 m are targeted.Fishing OperationsThe fishing operations realized by a purse seiner to catch tuna, from the beginning of the shooting of the net up to the end of the hauling, back on board of the seine (ready for a new shooting), are called a "set". There have been so-called dolphin set, in which case the net is set and fish schools associated with certain species of dolphins are herded into the net using 2-3 speed boats. When the school is inside of the net, the end of the net is closed and the bottom is pursed.

1. When a school is detected, the vessel places itself on the right side of the school (most of the tuna purse seiners have deck arrangement for operating from the left side, but not all the vessels have the same). For the correct positioning of the vessel versus the fish school, the direct observation from the crows nest, at the top of the mast, is essential (as it was also before for the detection/location of fish from signs at or over the surface of the sea, including seabirds). The skiff, a highly powered annex, attached to one extremity of the purse seine and having one extremity of the purse line cable (whereas the other extremity of this cable is attached to the winch on the purse seiner) which is kept on the top of the net at the sloped part of the stern of the vessel, is released.

2. The vessel then encircles the school at maximum speed. Usually, all the purse seine is set and the circle is closed within 4 to 8 minutes. In the Mediterranean Sea, purse seiners fishing northern bluefin tuna often do not shoot all the length of the seine and close the circle with only a portion of the net. At this time, tunas can change their direction and escape before the circle is closed. In the eastern Pacific Ocean, purse seiners often set on tuna school associated with certain species of dolphins. In this so-called dolphin set, the net is set across the direction where the school is moving towards and fish schools associated with dolphins are herded into the net using 3 to 5 speed boats. When the school is inside of the net, the end of the net is closed and the bottom is pursed. In the last several years, the use of fish aggregating devices (FAD) is getting very popular in all the oceans. Each vessel set several FADs in the ocean with radio buey to locate it. Some of them even have fish detecting device and transmit the information of the conditions gathering the fish under the device to the fishing vessels. When enough fish are gathered, the boat sets the net around the FAD to catch everything under. Frequently drifting baitboats are used in place of FADs.

3. Once the encirclement is finished, the extremity of the net that stayed attached to the skiff is transferred aboard the purse seiner and the two extremities of the purse line cable are hauled with the winch as quickly as possible in order to close the net at its bottom (this is called "pursing" because it is similar to pulling the draw string of an old-fashioned purse). It is worth observing that, until the purse seine is not closed, the tunas can still dive below the net or the purse seine vessel and escape. If the net stays from the surface all the way down to the thermocline, the chance of fish escape from the bottom would be minimal. During the pursing, and especially when there is current, in order to prevent the purse seiner from drifting over the net, the skiff is attached to the starboard side of the vessel and pulls it away from the net. In the Eastern Central Pacific area, when the set is made on the school associated with dolphins, a special operation, known as the "backdown operation" is realized at this time in order to let a par of the float line submerge in the water so that dolphins having been trapped in the purse seine can be released through that section. Speed boats are used to hep release operation. The pursing may take for large purse seines around 15 to 20 minutes.

4. The net is then pulled aboard the purse seiner with ahydraulic power block which is, on board industrial purse seiners, attached to the end of the boom and hanging above the deck, on medium size vessels operated at the extremity of a crane fixed on the deck at the aft part. Under the power block, the net is stacked on the stern of the boat by fishermen in such a way that it will come smoothly off the stern at the beginning of the next set. As a whole, this operation will, if there is no incident, take around one hour (minor incidents are not uncommon during the fishing operation) or even longer, depending on the size of the net and catch.

5. When most of the purse seine has been retrieved, the tunas have been grouped within a restricted area along the portside of the vessel. Then the fish are harvested from the purse seine using a large scoopnet called the "brailer" (brailing operation); several tonnes of fish are taken on board each time. The duration of this operation will obviously depend upon the quantity of fish in the net. In the bluefin tuna fishing in the Mediterranean Sea, and southern bluefin tuna fishing off Australia, the pursing is stopped a half way, where the fish are not yet so much aggregated. This is to keep the fish well alive, as most of the live fish caught are transferred to transport cage for tuna farming.

6. The tunas go towards fish-wells through trays and tubes arranged in the deck. In the fish-well fish are in brine which cools the fish without delay and freeze it for long conservation at -20 °C or even lower.

Fishery Production SystemsPurse seining for tuna is a fishing technique used all around the world by the industrial and semi-industrial fleets of several countries.Fishery OverviewThe industrial fleets worldwide have become increasingly mobile, with vessels changing ocean areas in reponse to changing catch rates, market prices and operating costs. These movements are typical for super seiners which for example, after a successful fishing season in the west Indian Ocean, can return to the eastern Atlantic Ocean to continue fishing off the West African coast, or to the Pacific and the east Indian ocean to fish off Japan or in the EEZ waters of the Pacific island nations. The introduction of the extended economic zones (EEZs), confronted the tuna fishery with the necessity of purchasing fishing rights. Access to some of the EEZs had been free, but now had to be paid for.
Fishery OverviewPacific Ocean
Several Latin American countries started developing tuna fleets, either their own or flying their national flags, to fish the Eastern Pacific tunas. This development has apparently been motivated by the proclamation of the 200 mile EEZs and possibly by a series of conservation and labour regulations imposed on vessels flying the US flag. Among these countries are Peru, Chile, Ecuador, Panama, Venezuela and Mexico. Perhaps the most modern fleet of tuna purse seiners in the Eastern Pacific Ocean is that of Mexico, where in the late 1970s a large investment was made on the development of this fleet. The US tuna purse seine fishery had developed exclusively in the eastern Pacific. Many tuna fisheries are present in Central and west Pacific Ocean. The US tuna purse seine fishery had developed exclusively in the eastern Pacific. But the 1970s and 1980s it had expanded all over the Pacific. However the use of traditional California tuna purse seines for skipjack fishing in the central and western Pacific was not an unqualified success. Japanese purse seiners fishing long, deep and light weight purse seines outfished many American seiners. Giant seines with length of 2 000 m and more have been used by Japanese tuna purse seiners in the western Pacific Ocean. With the development and increasing sophistication of fishing with FADs the need for such huge nets has diminished and the more conventional net is now about 1 600 m long and 300 m deep. The Spanish fleet composed by large sized vessels is distributed among Pacific Ocean, Eastern and Central Atlantic.The tuna purse seining fleet of France was developed during the 1960s and nowdays is operating in Southern Pacific. At the same time Taiwanese and South Korean seiners began vigorous operations in the south west Pacific waters. In the Philippines, tuna fisheries have achieved a remarkable rate of development during the 1970s and 1980s, partly as a result of the successful introduction of large-scale purse seining operations in association with FADs and log fishing. In this country most of the purse seine industry is based on a combined use of FADs of steel or bamboo construction and light attraction techniques. The method combines indigenous (FAD and light attraction) and modern (mechanized purse seining) technologies, which when operated separately would not be as productive. The FADs are set in ocean waters from 20 to 60 miles offshore and some 7 miles apart.

Indian Ocean
Before 1979 tuna was fished in the Indian Ocean mainly with longlines and pole and lines, but purse seining for tuna expanded considerably during the first half of the 1980s. The bulk of the catch is composed of more or less equal amounts of yellowfin and skipjack tuna. A large proportion of the catch is taken by vessels from outside the region. French and Spanish fleets for example, alternate their operation between the area of Seychelles and the Atlantic Ocean, depending on the prospects.As result of theses changes and trends, several new countries fishing in the Indian Ocean, such as Thailand, Philippines, Indonesia, India, Sri Lanka, Seychelles, Maldives, are developing their own tuna fleets and processing industries.

Atlantic Ocean
Since 1948 Norwegian fishermen have fished bluefin tuna with purse seines in North Atlantic.This is a seasonal summer fishery which depends on the annual feeding migrations of the fish from the Mediterranean. In tropical Atlantic, Eastern and Central Atlantic some West African ports became important bases for the distant water tuna fleets. In addition to the tuna pole and line and longline vessel, tens of Spanish, French, Ivorian Coast, American (USA), Mexican, Japanese, Russian and other large purse seiners use facilities and deliver fish to markets and canneries at Dakar, Abidjan, Tenerife, Cape Verde Islands at the east side, and in Puerto Rico, Venezuela and other American countries at the west side of the Atlantic. Other participants in the West African purse seine tuna fisheries are Ghana, Senegal and Republic of Congo. Russia and the USA, in addition to direct operations, have joint ventures at both sides of Atlantic. Relative newcomers are Venezuela, Panama and Mexico, with fleets fishing also in the eastern Pacific.

Mediterranean Sea
Vessels operating in the Mediterranean Sea from Algeria, France, Greece, Italy, Malta, Spain, Morocco, and Turkey are very specific and they can be classified as Mediterranean purse seiners. These are medium sized vessels, the largests not exceeding 45 m in length. Aboard these vessels the traditional mast with aft-facing derricks has been replaced with a hydraulic crane. In the case of the purse seine used in the Mediterranean Sea to catch northern bluefin tuna, the mesh in the body and the bottom part of the net are slightly larger (up to 200 mm) while in the bunt the meshes are around 120 mm. The northern bluefin tuna are fished following their migrational patterns particularly in the Western Mediterranean (Tyirrhenian, Ligurian, the Strait of Sicily, Aegean, Marmarra).Tuna are identified by using airplanes which transmit by radio to the fishing vessels the exact position of the fish schools.IssuesEnvironmental Since catching performance of purse seine are not very selective (in terms of both species and sizes of fish) there could be important by-catch of non-targeted species or very small fish of target species. This is particularly true when the set Is made with FAD (fish aggregating device). However, the impact of such catches on stocks has to be carefully evaluated. For example, by-catching frigate tuna may have no impact on that resource as the stock size is huge and the species is not fully commercially utilized. On the other hand, by-catching small bigeye tuna definitely reduces the yield per recruit of that stock. Every effort has been taken to reduce or avoid incidental mortality of dolphins by large tuna purse seiners fishing in eastern Pacific. Special techniques have been developed to reduce by-catch of dolphins; a "Medina panel" included as part of the seine used in conjunction with a "back down" manoeuvre proved to be a very effective method, which secure that encircled dolphins are released alive.Discard Possible incidental catches of species such as certain sharks or turtles are also worth mentioning in certain areas. However, it has been found that most of the sea turtles in the net are alive and released to the sea before the net is pursed. Pursing may entangle the turtle on the net and may cause mortality. Most of the sharks are pelagic species and not endangered. The increasingly used practice of encircling floating objects, including man-made FADs increases the capture of small sized and immature fish aggregating around such devices.Source of information

M. Ben-Yami 1994 “Purse seining manual” FAO and Fishing News Books Ltd.

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