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Food and Agriculture Organization of the United Nationsfor a world without hunger
Equipment Class: Fishing technologyEquipment Type: Environmental measures
Backdown manouvre  is the most important, and in a way culminating, stage of the porpoise release operation. Blacking down is a process whereby by running the seiner in reverse the floatline of the purse seine can be submerged and pulled from under the porpoise.
Backdown manouvre
Backdown manouvre

The area where the floatline sinks and the animals swim or spill over is called the blackdown area. The backdown stage is executed after about 1/3 of the seine has been hauled in. At this point, the floatline is made fast to the vessel's bow and stern at two respective floatline 'tie-down pints', one at the outermost bow bunch and the other at an appropriate spot at about 1/3 of the seine's length.
Vessels whose purse seines are fitted with long small-mesh porpoise safety panels and no aprons have more flexibility regarding the location of the tie-down points, since there is no apron to be aligned at the apex. Thus if most of the backdown channel is adequately covered with the small-mesh netting, such vessels can do some hauling, then backdown then haul again, and so on for several times and still have enough small mesh at the channel apex on every backdown. The advantage of this method is the reduction of porpoise crowding at the apex.
The tie-down points must be so chosen that when tha backdown channel is formed, the small-meshed Medina panel and the super-apron, if any, are located at the apex of the backdown area, so that porpoise can spill over the floatline without being caught, by entrapment or entanglement, in the large tuna netting.

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