Reti a strascico doppie
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Le reti a strascico doppie funzionano in modo simile allo strascico semplice, ma sono generalmente molto più grandi. Dato che l’attrezzatura viene trainata da due imbarcazioni, l’apertura viene mantenuta durante il traino dalla distanza fra queste ultime, eliminando dunque la necessità di divergenti e riducendo l’attrito. Permette inoltre il traino di una rete grande da parte di due imbarcazioni con un consumo energetico relativamente modesto.
Le ralinghe rinforzate con dispositivi o elementi in gomma di protezione servono a proteggere la rete da eventuali danni causati dal fondale e l’attrezzatura viene di solito calata e salpata dalla poppa tramite un tamburo avvolgirete. Una lunghezza di cavo e/o catena è inclusa nell’attrezzatura tra il cavo di traino e le briglie, al fine di assicurare un adeguato contatto con il fondale. Dopo aver lanciato la sua rete, il primo peschereccio passa le briglie al compagno, tramite messaggero, che le connette al cavo da traino principale. Entrambe le imbarcazioni rilasciano i cavi mentre avanzano per assumere la posizione di traino. I pescatori scozzesi solitamente praticano lo strascico doppio a distanza di 0,15 e 0,4 miglia nautiche, a seconda della profondita dell’acqua e delle condizioni del fondale. Al termine delle operazioni di sollevamento, le imbarcazioni si riavvicinano per trasferire la briglia nuovamente al primo peschereccio, affinché possa salpare la sua rete.
- 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.
