|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.|
|These are trawlers on which the fish is preserved by freezing. |
|Species EnvironmentShrimps constitute one of the most valuable groups of marine species resources. Approximately 2 million tonnes are captured annually in world's fisheries. Targets in shrimp otter trawling encompasses a wide range of species in both tropical and temperate waters.Fishing GearTrawls used in otter shrimp trawling encompass a range of designs and sizes. Shrimp trawls are typical made in relatively small meshes, with 20-60 mm in the codend. The mesh size in the belly part of the trawls seldom exceed 80 mm. Some larger shrimp trawls may have larger meshes in the wings, up till 100-200 mm. Vertical opening of shrimp trawls may range from less than 1 till 15-20 meters. Characteristic for otter shrimp trawling is that one or two trawls are towed from the stern of a vessel. Towing with two trawls are described as twin trawling. Two kinds of twin trawl rigging can be identified; use of three towing warps regulated by separate winches (Figure 1) and with two towing warps and a bridle arrangement in front of the otter boards as illustrated in Figure 2 . Twin trawls has become popular in recent years for shrimp trawling as the horizontal catching area increases compared to the use of a single trawl. Development is underway to tow three trawls from the stern of a vessel using only two otter boards to achieve horizontal spread. In order to enlarge horizontally the swept area on the bottom, either certain model of trawl are used (e.g. flat trawl with long wires, tongue trawl) or particular rigging (e.g. twin trawling).|Vessel OverviewThe vessel might be of any size open boats to large trawlers 50 m L.O.A. Generally otter trawlers are equipped with two stern gallows with towing blocks through which the towing warps are running. Otter trawlers have normally two towing warps that are regulated from two winches. Medium sized and large trawlers are often fitted with a stern ramp, on which the trawl is hauled onto the deck. Specialized industrial shrimps otter trawlers with advanced equipment are used in temperate to cold waters.Handling ModeA range of processing of captured shrimp can be identified, depending on the size of the vessel, 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 and cooking.Fishery Production SystemsOtter trawling is, by far, the most common fishing method for industrial shrimp fisheries.Fishing EnvironmentShrimp otter trawling is conducted in depths ranging from a few meters down to around 500-800 m.Fishing OperationsTrawling for shrimp is done with relatively low towing speed, between 1 and 3 knots, normally 1.5-2 knots. Because of the size of target species and their non-directional avoidance reactions the mechanism of capture is predominantly by filtering, which require relatively small meshes in the belly as well as in the codend of the trawl to retain the catch. The small mesh sizes used in the codend also retains small fish that also occur on most of the shrimp fishing grounds. As many species of shrimp stay close to the bottom, good bottom contact with the ground gear is a requirement for efficient capture. Some shrimp species, however, might occasionally migrate off bottom, and in such situations high opening trawls will be the most efficient trawl design. Towing of two trawls between two otter boards increases the catching width of shrimp trawls, and the development towards multitrawl riggings are typical for modern shrimp trawling (twin trawling). An echosounder and navigational instruments are minimum instrumentation. Larger vessels might have winch control systems that regulate the length of towing warps to equal tensions when towing. Shrimp trawls might also be equipped with acoustic instruments that record vertical opening, door spread, catch amount and temperature in the trawl depth. Otter twin trawls are often equipped with a symmetry sensor that regulate the length of the three warps so that the trawls have symmetry during towing.Fishery AreaOne boat stern trawling is the fishing method most used in the countries of the Bay of Bengal region (Thailand, Malaysia, India, Sri Lanka) for inshore shrimp fisheries. Simple rigging with a single trawl is used inWest Africa and in many Northern European countries.SeasonalityTrawling for shrimps can be conducted year around or seasonally.IssuesDiscard The major negative impact of bottom otter trawls on the biological environment is related to the capture and frequently discarding of non-target sizes and species both of fish and non-fish species. Non-target species can be reduced by the use of selective devices, like the Turtle Excluder Device (TED) in tropical shrimp trawls and the Nordmore grid to reduce capture of fish in the northern shrimp fisheries. In tropical shrimp fisheries devices that can reduce capture of non-target fish are under development. Some of these have been introduced in commercials fisheries in the US and in Australia.
Examples of twin trawls Figure 1 and 2