|Target Species Albacore |
|A midwater trawl consists of a cone shaped body, normally made of four panels, ending in a codend with lateral wings extending forward from the opening.|
|In the wet-fish trawler the fish is kept in the hold in the fresh/"wet" condition. |
Tuna pelagic trawlingDrawing of tuna midwater pair trawling fishing operation.
|Species EnvironmentMost of the tuna species are characterized by their strong tendency to form schools, at least at certain stages of their life (in equatorial waters, the young small tuna schools can be of very large size, up to ten of thousand of individuals totalling several hundred tonnes in weight). With other tuna species or in different circumstances, the grouping of fish will be limited to smaller schools or even a few individuals. In addition, tuna are swimming and may group themselves at different levels, from the surface to deep waters. The most effective fishing method(s) varies according to the degree of aggregation of the fish as well as its position to the surface.Fishing Gear Midwater pair trawls have roughly similar design as other midwater trawls for single trawling. Midwater pair trawls are, in general, designed for having more horizontal opening than standard midwater otter trawls. Midwater pair trawls can be rigged either with two towing warps from each vessel or alternatively, but less common, with one towing warp from each vessel and a bridle arrangement. In addition to being in a position to use bigger net with larger mouth opening when towing a trawl in pair, one of the main advantages of midwater pair trawling compared to single midwater trawling is the possibility to tow the trawl very close to the surface. The herding effect on fish by the two vessels drag also normally increase the capture efficiency in shallow waters and at the surface. Compared to a standard midwater pair trawl, such a trawl being designed for catching tuna, a "tuna midwater pair trawl" will have the largest dimensions possible according to the power of the engines of the pair trawlers and, for this reason, will have, in general, very large meshes in the forepart. For catching tuna, the experience shows that the towing speed has to be large enough; for this reason, a tendency is to lengthen the body of the trawl.Vessel OverviewAlmost all the pair trawlers targeting tuna are Stern trawlers, 20 to 25 m in length are used with standard deck arrangement. Usually vessels of equal size and power are used. Good maneuverability is necessary, for this reason (and others) large Trawlers are, in general, not really suitable for pair fishing.Handling ModeCatch of tuna trawlers can be either iced, if the vessel's cruise is less than two to three weeks, or frozen.Fishery Production SystemsThe main exploitation form using tuna midwater pair trawling is semi-industrial.Fishing EnvironmentThis fishing technique is used for catching surface swimming small schools or groups of tuna and also bigger individuals usually found swimming in sub-surface waters.Fishing OperationsFor albacore tuna, the midwater pair trawl is towed between the surface and a few tenth of meters. It includes the following fishing sequences:|
|1. Searching for fishThe first part of the operation is dedicated to searching for tuna schools, by visual spotting (sea birds, or behaviour of fish species jumping, breezing, boiling, sea surface) or by means of echosounder and sonar.|
|2. Setting the netOnce a school of fish is located, and its extent and depth determined, the vessels move downwind and one of them pays out the net. This is done very quickly. Once the net is in the water and safely clear of the setting vessel, the partner boat comes alongside and throws or receives a light heaving line. The ends of the bridles are tied to the heaving line and are pulled quickly aboard partner vessel and connected to its warp. Midwater pair trawlers mostly use two warps from each vessel.|
Fishery AreaPelagic trawls are used to catch in the Northeast Atlantic (especially in France) which is almost the only area where midwater pair trawls are used for catching tuna: a French fleet (around 100 vessels in 2 000) targeting albacore tuna.SeasonalityTuna midwater pair trawling is a seasonal fishing technique activity since it depends on the migration patterns of target species.IssuesDiscard In most cases is it a single species fishery, bycatch rates of other species are low. There is no impact on bottom habitat and bottom structure. On few fishing ground the incidental catch of dolphins and marine mammals create some problems.
|3. HaulingOnce the net has been pulled through the school of fish, hauling begins. The two partner vessels haul, each of them, all the length of warp on one side, drawing the two boats gradually closer as they do. When both warp ends are up, one on board each of the partner vessel, a heaving line is throw across and the bridle ends from the assisting vessel are attached to it. The bridles on the assisting vessel side are released to the hauling vessel. Then, the hauling vessel pulls in both sets of bridles together.|