|A bottom otter trawl is a cone-shaped net consisting of a body, normally made from two, four and sometimes more panels, closed by one or two codends and with lateral wings extending forward from the opening. A bottom trawl is kept open horizontally by two otter boards. A boat can be rigged to tow a single or two parallel trawls from the stern or from two outriggers.|
|Basic equipment of the vessels are the outriggers, on which the gears are towed.
Shrimp otter trawlingShrimp otter trawling using outriggers
Shrimp outrigger trawlingExample of a Outrigger shrimp trawler (USA)
|Species EnvironmentPenaeid shrimps are the main targets in these shrimp trawl fisheries, but the technique is also used to catch other and smaller species in tropical fishing areas.Fishing GearThe trawl designs used by outrigger trawlers have a low opening (less than 2 meters), with the wings attached to the upper and lower edge of rectangular otter boards that traditionally are made of wooden planks spaced by slots. The trawls will have mesh sizes ranging from 60-80 mm in the front part down to 30 mm in the codend. As such trawls always are used on flat and smooth bottom they do not have much of a ground gear for bottom protection. The flat Gulf of Mexico trawl is a model of traditional shrimp trawl. It is a four-panel trawl. The upper and lower panels consist of three sections - one large main section and two small triangular sections called "jibs". The central belly section is shorter than the top section by some 10 mesh lengths, allowing a slight overlap (between 0.6 and 1.0 m, depending on the size of the trawls). The sections forming the belly triangles are larger than the top section so as to form the two sides of the trawl. The height of the sides decreases from front to back. Other trawl types are obtained by altering the cut of the sides, which may also be rectangular. Depending on the, cut, the triangles may also be made and assembled differently; the mesh direction may be changed to obtain a better distribution of tension (in the direction of the course of the yarn). It is essential that these sections be strengthened because they are particularly vulnerable. Flat trawls modified in this way have become "Western jib trawls". In some area, trawl with higher vertical opening are used in order to catch more demersal fishes in addition to shrimps. In general, a small "trap net" is also used to assess the potential catch of shrimp.Vessel OverviewOutrigger trawlers normally have the superstructure forward with the working deck aft. The principal characteristic of these vessels is the presence of outriggers which are usually fastened to the mast or at the foot of the mast and extend out over the sides of the vessel during fishing operations. Two trawls (one of each side) or four (two of each side) are towed at the same time.Handling ModeA range of processing of captured shrimp can be identified, depending on the captured species, duration of fishing trip and market requirements. Such processing may include chilling with ice for fishing trips of shorter durations, or freezing the shrimps, beheaded or not, and packaging in boxes.Fishery Production SystemsOutrigger trawling with two or four otter trawls is, by far, the most common fishing method for industrial tropical shrimp fisheries.Fishing EnvironmentFloridian-type outrigger trawling was introduced to American fisheries in the Gulf of Mexico around 1950. Since then this fishing technique has spread to most of tropical fisheries exploiting Peneaid shrimp species all over the world.Fishing OperationsShrimp outrigger trawling operation can be divided into the following main sequences:|
Shrimp outrigger trawlingTowing the nets
|1. TowingThe two (or four) trawls are towed from the ends of two outriggers, on port and on starboard side of the vessel, and the net(s) is towed by a single warp terminating in in a crow-foot. The outrigger booms are at an angle of between 20° and 30° from horizontal. As far as efficiency is concerned, this type of rigging, using two small trawls, results in a catch rate 15 to 30 percent higher than with single rigging using a single trawl with a similar drag. The reason for this is that: (1) two small trawls have a wider horizontal opening than on single trawl, with the same drag; (2) two small trawls work better on the bottom than one large trawl. Moreover, it is easier to manoeuvre two small trawls. The same reasons also fully justify the use of twin trawls.|
Fishery AreaDouble rig otter trawling is adopted for shrimping in many countries in North America and Central America, the Arabian Sea and the Gulf of GuineaSeasonalityFishing season may be year around or in seasonally, depending on the regulations in place in various countries. Such regulation might aim to limit total fishing effort and to reduce fishing effort during spawning periods.IssuesSelectivity The major potential negative impact on the living resources and environment of the this fishing technique is as for any other bottom otter trawling: Environmental Scraping of the bottom by groundrope and otter boards.Discard The capture and frequently discarding of non target sizes and species both of fish and non-fish species and occasionally, the incidental catch of endangered species.Selectivity Use of Bycatch Reduction Devices (BRD) is considered a useful measure to reduce bycatch. They are installed in a shrimp net that provides a small opening in the top of the shrimp trawl for non target size of fish or species. BRDs had a varied effect on unwanted fish bycatch. Exclusion rates depends on the design of the BRD, the composition and quantity of bycatch, and whether trawling is undertaken during the day or night. In most cases, bycatch reduction average about 20% during night trawling and about 40% during day trawling. The data collected suggest that BRDs have little impact on prawn catches. The following BRD can be used: square mesh windows; fisheye; cones and rigid grid (NAFTED type or simply TED, Turtle Excluder Device). The last one prevents catch of endangered species. Possible solutionsSquare mesh window is the simplest BRD. The windows allows fish to escape upwards through the large square mesh, while the shrimps that do not swim as well as fish pass into the codend. It is normally located in the trawl extension immediately ahead of the codend.Possible solutionsFisheye is a simple steel frame attached to the codend to provide a small elliptical opening for fish to escape through. It faces forward in the top of the codend. Animals pass into the codend and must turn to swim forward to escape through this device.Possible solutionsThe cone is not a BRD but has been designed to increase the efficiency of BRDs to exclude fish from trawl. Developed by shrimp fishermen in the Gulf of Mexico, it is simply two panels of netting attached to a small wire hoop and inserted behind a BRD such a fisheye, square-mesh window (or RES, Radial Escape Section). The cone impedes the passage of fish into the codend and "stimulates" them to swim forward and through the escape openings.Possible solutionsThe rigid grid, NAFTED type was originally designed by to exclude large animals such as sharks, stingrays and turtles from the trawl (however, the experience shows that, in general, small fish are also excluded). The NAFTED BRD features a grid, with a bar spacing appropriate to local conditions, secured to the trawl at 45 degrees. A panels of netting guides all animals to the bottom of the codend and prevents shrimp loss through the escape opening in the top of the codend. Large animals are physically guided by the grid through the escape opening while shrimp and other animals pass through the bars and into the codend.Possible solutionsThe rope BRD consists in a window including parallel ropes extending lengthwise along the trawl (in place of square mesh). It is normally located in the same position as the square mesh window.Possible solutionsIncentives for shrimp fishing with more responsible fishing practices. Under this alternative, positive incentives for switching to alternative and more friendly fishing techniques would be created, probably in the form of subsidies for purchasing or constructing gear designed with BRDs.
Shrimp outrigger trawlingHandling the nets
|2. Handling the netWhen handling, the otter boards (and the sledge when twin trawls are used) and the nets remain suspended at the ends of the outriggers and the codends alone are taken aboard. This operation may be done by three or four men depending on the size of the trawls. Regarding work load, it is often easier to repair small trawls than a single large one. Such trawling with outriggers allow higher yields and therefore more income with less work for the crew and explain why such rigging was quickly adopted, first of all in the Gulf of Mexico and then in other tropical fisheries throughout the world. Usually, a small try-net is towed from the stern of the boat at the same time as the other trawls. It is used to give a quick estimate of the quantity of shrimp available both before and during the tow.|