May 6th, 2010 
molitva leyli COMPLETE PURSE SEINE TUTORIAL AND GUIDE
Purse seine Tutorial, Guide and Manual : Vessel type, Vessel Overview, Tuna purse seine, Species Environment, Fishing Gear, Handling Mode, Fishing Environment, Fishing Operations.
Tuna purse seining involves surrounding tuna schools 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.
Vessel type
Tuna purse seine
Species Environment
As 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.
World tuna like fish species
Fishing Gear
The 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.
Purse seine Power Block
Tuna purse seine winch
foot-rope depth sensors
Fish Aggregating Device, sea kite FAD design from McIntosh
Purse seine power block and net
Vessel Overview
Tuna 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.
purse seine vessel
Purse seine vessel profile
Handling Mode
Storage 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 Environment
In 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. Less than 100 m, the target is skipjack tuna and juvenil big eye tuna and bluefin tuna. For large tuna species like adult big eye tuna and bluefin tuna the depth is more than 150 at the day light.
Range depth – diving behaviour of target species from ”longline range and bycatch target species” , bycacth species above 100 m include sea turtles, sharks and some billfish; target species below the line at 100 m include bigeye tuna and day swimming broadbill swordfish.
Depth record for tracked skipjack no 8 (fork length= 51 cm) from 18.oo – 08.00 morning. Picture from; ”Behaviour of yellowfin tuna (Thunnus albacares) and skipjack tuna (Katsuwonus pelamis) around fish aggregating devices (FADs) in the Comoros Islands as determined by ultrasonic tagging”. Patrice Cayré
Depth record for tracked yellowfin tuna. Picture from; ”Behaviour of yellowfin tuna (Thunnus albacares) and skipjack tuna (Katsuwonus pelamis) around fish aggregating devices (FADs) in the Comoros Islands as determined by ultrasonic tagging”. Patrice Cayré
picture from; ”Deep diving behaviour observed in yellowfin tuna (Thunnus albacares)”
Laurent Dagorn1,a, Kim N. Holland2, Jean-Pierre Hallier3, Marc Taquet4, Gala Moreno5, Gorka Sancho6,David G. Itano7, Riaz Aumeeruddy8, Charlotte Girard1, Julien Million9 and Alain Fonteneau10
Fishing Operations
The 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.
Japanese purse seiner
large tuna species, bluefin tuna cacth
Source of information; M. Ben-Yami 1994 “Purse seining manual” FAO and Fishing News Books Ltd.
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