Trawling is one of the common methods of fishing. It involves pulling fishing nets through the water behind one or more boats. The net used for trawling is known as a trawl (often called dragged or towed gear).
Trawlers catch prawns by dropping a trawling net to the bottom of the sea and then dragging it through the water. Prawns are caught in the net as it moves, and they are unable to swim out if the trawler moves quickly enough. Many other items are caught using this indiscriminate fishing method.
In the article below, we explore some of the general information regarding trawlers and their associated fishing method.
Trawling Nets
Trawling nets are designed to be towed through the midwater trawl (water column) or along the bottom trawl (seafloor). Trawl nets are shaped like a funnel or cone with a wide gap to catch crustaceans or fish and a narrow closed end. Trawls can be used at different depths, and nets vary by their mesh size. Trawl nets are drafted differently depending on whether the target species is fish or prawns. In addition, they have rollers or bobbins on the ground gear that minimizes bottom contact and allows the net to move over the seafloor without snagging. It is crucial to remember that both midwater and bottom trawls use trawl doors or otter boards to keep the net’s mouth open.
Types of Trawler
Boats used for trawling are called draggers or trawlers. They vary in size, from small boats to large boats. One or two trawlers can carry out trawling.
Trawlers can be classified by the type of fishing method used, architecture, the type of fish they catch, or the geographical origin. We have used a classification that follows the Food and Agriculture Organization of the United States (FAO). The organization classifies trawlers by the gear they use.
Outrigger Trawlers
These trawlers use booms or outriggers to drag a trawl. An outrigger is a protruding structure on a boat. In a fishing operation, outriggers are fastened to the mast extending over the sides of a trawler. Each of the sides can be deployed a single or twin trawl. Warp winches are installed on the deck to haul a catch.
Beam Trawlers
They are a kind of outrigger trawlers with a midship working deck and a superstructure aft.
Beam trawlers use powerful outrigger booms on each side. These outriggers are controlled from a mast. Each side of a trawler tows a beam trawl.
The warps go through blocks at the end of the boom. The arrangement allows easier handling and stowing of large beams.
Beam trawling is common in the North Sea, where it is used in flatfish fisheries. Beam trawlers are equipped with a multibeam echo sounder for finding fish and a piece of equipment for hauling the net and stowing it aboard.
Beam trawlers are installed with winch brakes to avoid capsizing if the trawl snags on the seafloor. They are also installed with safety release systems in the boom stays. For additional safety, its engine power is restricted to 1472 kW.
Otter Trawlers
These trawlers use one or more parallel trawls set apart horizontally using otter boards. Otter trawls vary in size from sailing canoes to super trawlers. The trawls can be dragged along the bottom or in mid-water.
Otter trawlers have two gallows with towing blocks on the stern quarters. A stern gantry in these boats is for operating the otter boards. Its winch regulates these towing warps. Large and medium trawlers have a stern ramp used for hauling the trawl into the deck.
Otter trawlers empty the cod end using fish pumps.
Side Trawlers
As depicted in their name, this trawler has trawls deployed on the side. Its trawl warps pass through the blocks overhanging from an advancing gallow and another aft gallow.
The superstructure is in the direction of the stern while the horizontal trawl winch progresses the superstructure.
Side trawlers were the most popular boats used for deep sea fishing in North Atlantic fisheries before being replaced by stern trawlers.
Stern Trawlers
Its trawls are deployed and retrieved from the stern. Larger stern trawlers have a ramp. For small stern trawlers, they are designed without a ramp.
Stern trawlers are designed in such a way that they can operate in most weather conditions. Besides they can work best in either bottom or midwater trawling. The stern has gallows or a gantry for running otter boards.
In the deck houses or below the deck is where fish processing takes place. A wet stern trawler stores fish in ice or refrigerated seawater. Frozen blocks or boxes store fish in a freeze stern trawler, while a factory stern trawler processes the catch.
Pair Trawlers
Pair trawlers operate together while towing a single trawl. They keep a space from each other to allow the trawl to be horizontally open.
Pair trawlers do not use otter boards. They operate both bottom and mid-water trawls.
These trawlers can have fish pumps to empty the cod end.
Freezer Trawlers
Freezer trawlers are the majority of trawlers operating on the high seas. They are equipped with facilities that preserve fish by freezing. These trawlers can remain at sea for extended periods.
Freezer trawlers vary in size, but all have the same general arrangement as a side or stern trawlers.
Trawler/purse seiners
Its design includes deck equipment and an appropriate combination winch that can be rearranged in both methods. To control the warp and pursuing lines, the trawl gallows, blocks, rollers, and pursue davits need to be readjusted to reduce the time required to convert from one arrangement to the other.
Naval Trawlers
Naval trawlers were created during the World Wars. Some of the countries converted and armed existing trawlers or built new vessels to stand trawler designs. The trawlers were equipped with a small naval gun and mainly used to patrol and escort other ships.
How Does A Fish Trawler Work?
Fish trawls are towed as single nets over the back end of a boat. Some trawlers may incline the boat’s back to retrieve and set the net.
Fish trawls are equipped with elongated metal cables that connect the trawl boards with the net. The metal cables are called sweeps. They allow the boards to spread much further than their overall width. The sweeps herd fish till they are tired and drop back into the net where they are finally caught. To enable these nets to move over the seabed, they are equipped with rollers or bobbins on the ground gear. They are also vital in reducing snagging and minimizing bottom contact.
How Does A Prawn Trawler Work?
Prawn trawlers can be towed in multiples with extended arms or booms. The arms/booms extend out of each side of the boat allowing the nets to open fully.
Nets used by prawn trawlers do not have long sweeps as fish trawls. These nets have a smaller mesh and use ground chains for weight to skim the seabed and encourage prawns on the seafloor up into the trawl mouth.
How Is Seafood Caught: Bottom Trawling?
It is a type of fishing process that involves pulling a net along the seafloor.
According to science, bottom trawling is divided into benthic and demersal trawling.
Benthic trawling involves dragging a net at the bottom of the ocean, while demersal trawling involves dragging a net just above the benthic zone.
Anglers commonly use bottom trawls to catch shrimp and bottom trawling fish like sole, skate, cod, and flounder. However, the nets also catch various ocean life, usually thrown back dead or dying, in addition to these target fish.
Dragging heavy gear across the seabed can also damage sensitive seafloor habitats. Reducing the harmful effects of bottom trolling requires modifying the fishing gear or limiting the areas where trawling is allowed to protect susceptible bottom-dwelling organisms and habitats.
Trawling Environmental Effects
Recently, many nations heavily regulate trawling. However, it remains the target of many protests by environmentalists. Here are the concerns related to trawling:
Lack of Selectivity
Trawling is non-selective. The practice sweeps both undesirable and marketable fish of illegal and legal size. Other species are killed accidentally as well as the trawling process damages their habitats.
However, several methods have been implemented to minimize this. Among them is the fitting of square mesh panels or catch reduction grids to parts of the trawl. It allows certain species to escape while retaining others.
Environmental Damages
Bottom trawling, which involves dragging heavy fishing gear over the seabed, can cause large-scale destruction on the ocean bottom, including damages to habitats, coral shattering, and seaweed removal.
Bottom trawling also stirs up bottom sediments and loading of suspended solids into the water column. It results in threatening a variety of species and directly killing coral reefs by breaking them up and burying them in sediments.
Over and above that, trawling can also indirectly kill corals by wounding the coral tissue. It leads to the reefs being vulnerable to infection.
Conclusion
Trawling operation has high adaptability, flexibility, and production efficiency. However, it can unselectively damage fishery resources.
Trawling nets are not suitable for large-scale commercial operation due to their poor selectivity on fish species, impact on ecological diversity, and high energy consumption.
However, trawling has been highly regulated to prevent any damages to either the environment or ocean life. The industry has put in place safety performance records with greater federal involvement for effective and cohesive programs.
