
Active Fishing: Trawls | Selective Trawls | Seine Nets | Dredges | Other |
Passive Fishing: Nets | Fishing Lines | Long Lines | Traps and Pots |
Trawl (fr. Chalut) funnel or cone shaped net which is towed through the water by one or two boats (Pair trawl net; fr. Chalut boeuf)
Midwater / Pelagic trawl (fr. Chalut pelagique) trawl towed through open waters. The depth at which they are towed is adjusted to the targeted species or fish group. These nets are much larger than the ones used in bottom trawlers. Large meshes cover the anterior end of the net to direct fish towards the cod end of the net, where the smaller mesh size traps the fish.
Pros
• Pelagic trawls have a fairly high interspecific selectivity since the species fished often live in homogenous groups.
• The nets do not harm the ocean floor.
Cons
• The fine meshed trawls incidentally catch high levels of non-target species (bycatch). The larger ones often trap turtles and cetaceans.
• This fishing technique is only economically viable for schooling or shoaling fish.
Bottom / Demersal Trawl (fr. Chalut de fond) trawl that is towed along the ocean floor, by one or two boats
Pros
• Bottom trawlers can capture most benthic and bentho-pelagic commercial species.
• It is easy to monitor since the nets are visible on the boat and in the seaport, and the dimensions of the trawls are adapted to the power of the boat.
Cons
• Low selectivity: bottom trawls trap large amounts of non-target species, which even once removed from the net and returned to sea, have almost no chance of survival. This is an especially grave issue when juveniles are caught, since the survival of juveniles is crucial to the well-being and persistence of a species.Bycatch is sometimes accidental, but other times inevitable: when a species lives in close proximity to the targeted species (e.g. bycatch of hakes in the European lobster fisheries)
• Bottom trawls cause physical damage to benthic ecosystems, disturbing a wide range of non-targeted organisms (e.g. seaweed, corals). The steel trawl doors dig deep furrows as the net is dragged along the ocean floor and the footrope, (the part of the net that stays in contact with the bottom) alters benthic ecosystems by displacing large rocks and reworking sediments.
• In sandy and muddy bottoms, these engines stir up the sediments, creating large amounts of suspended matter, which decreases the amount of light available on the ocean floor and reintroduces persistent pollutants into the water column.
Levels of by-catch in bottom trawlers can be lessened by using methods of selective trawling. New techniques are used to increase the intra- and inter-specific selectivity of trawling.
Intra-specific selectivity: the capacity of the gear and/or fishing technique to capture individuals of a certain size for a given species.
Inter-specific selectivity: the capacity of a gear and/or fishing technique to capture a certain species over another.
Selectivity depends on the:
• Mesh size: the size of the mesh located on the cod end of the net must be the appropriate size for the targeted species.
• Size of the cod end (length and circumference.
• Rigidity/ elasticity of the material used: when the trawl is first submerged, the mesh stay relatively closed but once several pounds of fish are caught, the weight of the catch causes the mesh to stretch out and mesh size to increase, making it possible for organisms escape.
• Presence of material blocking the cod end.
Selective gear:
Grids : grids can be placed at the anterior end of the net so to let smaller organisms out. Larger animals who come in contact with the grid slid into the cod end of the trawl (e.g. for capturing shrimp while letting cods free).
Square mesh panels : square mesh panels can be placed in the superior part of the cod end, They stay open during the tow so that juveniles can escape.
Large mesh trawl : uses large mesh size at the anterior end of the trawl so that selectivity occurs before the fish get trapped in the cod end (only useful for small trawls since, in larger trawls, fish swim in the middle of the opening, far from the lateral netting).
Selective panels : help separate species according to their morphologies: since fish tend to swim in the center of the trawl and benthic invertebrates tend to stay near the bottom, each group can be lead into separate cod ends. This way, in an invertebrate trawl fishery, fish can be directed towards an open cod end via which they can escape. Moreover, since fish tend to swim upwards in order to escape trawls, if escape panels are placed on the superior part of the codend, unwanted species can readily escape.
Selective Trawls: Still Room for Improvement
Even though great advances have been made to increase selectivity of trawling fisheries, it is important to note that the escape does not assure the survival. Once individuals enter the cod end, the risk of injury is extremely high and escapees rarely leave unharmed. More research must therefore be put into creating fishing gear that minimizes the stress and physical harm endured by untargeted species so to increase the escapee’s chance of recovery. Areas known to have high densities of juveniles and non-target species must be avoided to minimize unwanted mortalities.
Seine (fr. Senne) long rectangular net rigged with floats and weights that hangs vertically in the water. Target species: sardines, mackerel, anchovies, herring, certain species of tuna.
Purse seine (fr. seine tournante) seine net that has a series of rings lining the bottom of the net theough which a rope is threaded. The purse seiner encircles the school with the net pulls the rope to close the bottom of the net like a drawstring purse.
A similar but smaller version of the purse seine (fr. la bolinche) is used for coastal fisheries.
Pros
• Seining is a somewhat selective since targetted fish swim in large schools of conspecifics.
• The quality of the fish is preserved.
• Seines do not physical impact ocean floors.
Cons
• Seines can only be used on pelagic species.
• Since seiners often spot their catch by spotting the fish, birds or mammals predating on the school, the predators often get caught in the net as bycatch.
• When the vessel capacity is filled, too many non-target species have been caught, gear problems occur and/or the catch quality is deemed insufficient, fishermen release the catch back to sea. This process is called slippage and causes unnecessary stress and harm to fish, and is likely to cause mortality of a portion of the catch.
Dredge (fr. Drague) metal basket dragged along the ocean floor by a boat. It that has a rigid opening on one side and the bottom ridge of the basket is lined with a scraping device. inferior portion of the the basket is lined with a scraping device (smooth blade or blade with teeth).
Pros
• Dredges are fairly selective since their dimensions are made to fit the catch and larger openings on the armature enables the escape of non-target individuals.
• Their use is well regulated..
• Dredges nearly always yield successful catches.
Cons
• Dredges cause physical damage to benthic ecosystems, disturbing a wide range of non-targeted organisms. The blades and teeth on the device scrape epibenthic organisms out of their habitats and disturb the sediments, which decreases the amount of light available on the ocean floor and reintroduces persistent pollutants into the water column.
• They are only useful for mollusks.
Fishing on foot (fr. Pêche a pied) fishing in the intertidal zone using a hand dredge, Dipnet (a small mesh bag, sometimes attached to a handle, shaped and framed in various ways. It is operated by hand or partially by mechanical power, to capture the fish by a scooping motion. Also called: scoop, scoop net) or bare hands. Both feet must be touching the ground.
Target species: bivalves.
Free-diving spearfishing (fr. Pêche en apnée) apnea diving to catch fish with some form of spearing device. The use of oxygen tanks is forbidden for most species but derogations may be obtain for the fishing of corals and ormeaux. Scuba diving spearfishing is tightly monitored.
Target species: sea urchins, palourdes, ormeaux,...
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Gill net (fr. Filet maillant) curtain of netting rigged with floats and weights and vertically suspended in the water column. These nets are practically invisible underwater. As the fish swim into them, their gills get entangled in the meshes. Single nets are linked into fleets of nets.
• The use of pingers (acoustic deterrent devices that produces ultrasound to deter cetaceans and porpoises from gillnets) are mandatory and help reduce marine mammal bycatch
- Bottom gill nets (fr. Filet maillant de fond) bottom-set gillnets. They may be up to 50 kilometers in length.
Target species: benthic and demersal fishes e.g. cod, flat fish, snappers, hakes, Pollack, ling, seabreams
- Pelagic gillnets/Set nets (fr. Filet maillant pelagique) gillnets that are kept at or right below the surface.
- Drift-nets (fr. Filets dérivants) gillnets that are allowed to float freely with the waves.
Target species : tuna, mackerel, salmon, calamari and herring.
Trammel (Fr. Trémails) net set vertically on the ocean floor made up a central small mesh panel and two lateral large mesh panels. These nets are set upright on the bottom of the ocean. When fish come in contact with trammels, they push the small mesh panel through the large mesh net and gets entangled (see figure).
Target species: Pelagic and demersal species e.g. Sole, European Plaice, mackerel, herring, tuna, Pilchard, Sea bass, rays, brabs, lobsters, turbots.
Bag nets – Scoop nets (smaller) and Stow nets (larger) (Fr. Tamis) finely meshed bag-shaped net kept open by a wood or metal frame, their shape and size vary according to the region and target fisheries. May be operated by hand, held on stakes to the sides of a boat or anchored in a river bed to catch migrating or drifting fish.
A large variety of nets are used in coastal fisheries, all of which are slight variants of the nets above described.
Pros
• Low or no impact on ocean floors.
• Mesh size is adjusted to catch target species (except for trammels).
• No baits are used, which reduces the risk of catching birds while the net is being set.
• Pingers are mandatory and help reduce marine mammal bycatch.
Con
• Catch levels of non-target species are still are high (especially in trammels).
• Pingers have proven themselves to be very efficient in deterring the common dolphin, yet a number of cetaceans and porpoises still get entangled and drown despite the presence of pingers.
• Ghost netting. Ghost nets are nets that have been lost or abandoned at sea. They drift through the waters, catching anything that crosses their path. Once the weight of the catch exceeds the buoyancy of the floats, they sink to the bottom and either get entangled and continue to catch benthic organisms or their catch is consumed by bottom dwellers and once the buoyancy of the floats exceeds the weight of the catch, they return to the water column and resume their hunt.
• The fishing effort of net fishing is difficult to measure accurately for it is impossible to know how long nets are being left out. Since nets can be left out at sea for several hours or several days, a 50 km long net that should normally be left out for a day is acts as a 100 km long net if left out for 2 days and a 200km long net if left out for 4.
Fishing Line (Fr. Ligne) line with hook(s), baited with lures or bait fish. The line may be operated by hand or fixed to the end of a fishing rod. They are used by small coastal vessels to fish epipelagic species and in open waters to fish Thonidae
- Hand-line (Fr. lignes à mains) single fishing line usually equipped with fishing reels and attached to a rod that is held by hand.
Target species: sea bass, pout, sea bream, hake, mackerel, pollack,...
- Trolling with spinnaker poles (Fr. Lignes de traîne avec tangons) several fishing lines equipped with fishing reels are fixed to spinnaker poles and drawn through the water at a speed of 5 knots or higher.
Target species: sea bass, tuna,…
Pros
• No impact on the ocean floor.
• Very high selectivity : bait is chosen so to catch the appropriate fish.
• Non-target species or individuals are usually returned live to the sea.
• Since animals are brought back to the boat alive, this technique provide high quality products
• Low energy requirements.
Con
• Expensive if catch levels are low (price of bait).
Long line (Fr. Palangre) a gear consisting of a main line supporting a number of short vertical lines (called snoods), carrying baited hooks.
- Pelagic long line (Fr. Palangre flottante) a long line rigged with floats and weights that hangs vertically in the water.
- Drifting long line (Fr. Palangre dérivante) pelagic long line that is allowed to float freely with the waves.
Target species: sea bass, tuna, swordfish,…
- Bottom / Demersal long line (Fr. Palangre calée de fond) Bottom-set long line.
Target species: sharks, rays, congers, lings, sea breams, gurnards,…
Pros
• Low or no impact on ocean floors.
• High selectivity (not so much the case for drifting long lines): the bait is chosen so to catch the appropriate fish.
• Since animals are brought back to the boat alive, this technique provide high quality products
Cons
• The fishing effort is difficult to measure accurately.
• Baits often attract oceanic birds that then get caught and drown.
• Expensive if catch levels are low (price of bait).
Trap/Pot (Fr. Nasses et casiers) a basket-like wicker trap with a rigid structure covered by a netting for catching fish. The opening of the trap is shaped and positioned in such a way that when the animal falls into the trap, it is very difficult for it to come back out. A bait may be placed inside to attract specific species. Traps are often set out in large numbers and linked to one another via a long rope so as to ease their deployment and retrieval.
Target species : mollusks, crustaceans and gastropods.
Pros
• They are generally selective (especially when a bait is used).
• Non-targeted species or individuals can be returned live to the sea.
• Low or no impact on ocean floors.
• Since animals are kept alive in the traps, this technique provide high quality live mollusks, crustaceans and gastropods.
Cons
• Traps are almost only used to catch mollusks, crustaceans and gastropods.
• Lost or abandoned pots and traps act as self-baiting traps that go on catching animals year after year.
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Sources:
Seafood Choices Alliance Produits de la Mer. Guide des espèces à l’usage des professionnels. (2008)
http://www.seafoodchoices.org/resources/documents/FrenchSpeciesGuide_RevisedFinal.pdf
Food and Agricultural Organisation. Profils des peches et de l’aquaculture par pays : France. (2005)
http://74.125.113.132/search?q=cache:XdMinVgsHioJ:www.fao.org/fi/website/FIRetrieveAction.do%3Fdom%3Dcountrysector%26xml%3DFI-CP_FR.xml%26lang%3Dfr+coquille+saint+jacques+peche+francaise&cd=10&hl=en&ct=clnk&gl=us&client=safari
Ifremer.
http://www.ifremer.fr/francais/index.php
Images:
http://www.afma.gov.au/information/students/methods/default.htm
http://www.ifremer.fr
http://www.greenpeace.fr