La pêche de fond au bœuf
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Le chalutage de fond en bœufs utilise des filets identiques à ceux du chalut simple, mais ils sont généralement beaucoup plus larges. Deux bateaux remorquent l’engin à une certaine distance l'un de l'autre, ce qui permet de maintenir l'écartement du filet, de ne pas recourir aux panneaux de chalut et de réduire la résistance. Ce système permet également aux deux bateaux de remorquer un grand filet sans trop consommer d'énergie.
Les bourrelets "rock-hopper" ou les ralingues de fond caoutchoutées sont employés pour protéger le filet des dégâts de fond ; l’engin est généralement filé et tiré au-dessus de la poupe à l’aide d’un enrouleur de filet. Une certaine longueur de câble et/ou de chaîne est prévue dans le gréement entre la fune et les brides du chalut pour assurer un bon contact avec le fond. Lorsque le premier bateau vire son filet, il passe les brides du chalut au second bateau à l’aide d’un tournevire, le second bateau fixe les bride aux funes. Les deux bateaux déroulent les funes tout en avançant pour se mettre en position de remorquage. Les chalutiers-bœufs peuvent remorquer à une distance l'un de l'autre de 0,15 à 0,4 milles marins, selon la profondeur de l’eau et les conditions du fond. La pêche terminée, les deux bateaux se rejoignent et la bride de chalut retourne sur le premier bateau pour effectuer les opérations de remontée du filet.
- Click to view environmental impacts of Demersal Pair Trawling
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Potential Impacts (Biological/Environmental) Gear Selectivity Regulation Biological
• Removal of and damage to sedentary marine organisms such as seaweed/corals
• Capture and removal of small sized marine organisms and non-target species
• Capture/discarding of undersized target species
Environmental
• Damage to seabed strata
• Disturbance of bottom sediments
• Less overall disturbance due to the omission of trawl doors.
• Mesh size
• Use of square mesh panels
• Trawl door design
• Control of headline height
• Design/construction of footrope materials
• Minimum mesh size
• Minimum fish sizes
• Inclusion of square mesh panels
• Maximum twine thickness
• Net attachments must meet specific regulation conditions to prevent obstruction to net meshes, eg use of bag strengtheners
• Maximum _ of meshes in bag circumference
• % of catch mix retained on board
Improvement measures and initiatives taken by Scottish Fishermen
• Progressive increases to mesh size have reduced undersized capture and discards. Cod end mesh size has increased to 120mm for basic towed gear. This has increased the age of capture of most demersal species of fish.
• Introduction through regulation of compulsory square mesh panels (SMPs) has greatly improved selectivity. Further research into different mesh size SMPs and alternative positions in net configuration were carried out in 2006 with Scottish industry and are ongoing.
• Towed gear SMPs have a minimum dimension of 80mm or 90mm depending on trawl type. Many Scots fishermen now operate voluntarily with SMPs of up to 120mm, which increases the escape capacity of the panel.
• Twine materials have improved, allowing net manufacturers to construct trawls from thinner twine, which in turn makes them lighter and gives less drag under tow, hence interaction and damage to the seabed is reduced.
• Many vessels now use footropes equipped with larger diameter discs, which raise the footrope from the seabed. This reduces seabed damage and can acts as a selectivity aid to allow bottom-dwelling fish species to avoid capture.
• Pair trawling does not require trawl doors to be rigged into the trawl gear. This lightens the demersal trawl considerably and reduces drag. It also means less interaction with and consequent damage to the seabed. Less drag also means that boats save energy (fuel) and reduce carbon emissions.
