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Fishing Trawlers: The basics

17th December 2019

Also known as draggers, a trawler is a commercial fishing vessel which is designed to pull (or drag) a trawl net behind it to catch fish. The net is known as the trawl and can be deployed at a specified depth to target different species of fish depending on the zone of the trawl. A trawler can operate a single net or several depending on the rig of the ship. Trawls can also be dragged by more than one vessel.

Trawling is generally a method used by commercial fishermen but can also be used by research vessels to collect large samples to survey an area of the ocean.

A brief history

Taking its name from the Dutch word for a fishing vessel that can tow a net, the British developed an early form of trawler during the 17th century called the Dogger. However it was in Brixham where the need to develop a vessel capable of operating further afield meant that a new design of fishing boat was required. The overfished areas of South Devon forced the crews of Brixham to adopt a sleeker and faster vessel on order to make longer trips at sea but being robust enough to tow in deep waters and harvest greater yields. The design was a huge success and led to a boom in the fishing industry which instigated one of the industries largest migrations as fishermen flocked to Northern port communities so they could access more fishing ground. By the middle of the 19th century, towns like Hull, Yarmouth and Scarborough were thriving and Grimsby became the largest fishing port in the world. The Brixham trawler opened the doors, not just for the British but, for the whole of Europe to expand their fishing grounds.

As technology in propulsion advanced so too did the capabilities of the trawler to further extend its reach into deeper waters and steam powered vessels allowed fishermen to operate with more nets and thus catch more fish. Because they were capable of returning from sea more quickly their yields would be fresher than sailing trawlers and they were able to command higher prices. Steam eventually gave way to diesel engines and trawling operations continued to develop alongside modern technology and the advent of mechanised hauling made further efficiencies with the method.

Modern trawlers still vary greatly in terms of their sophistication but also their architecture and fishing methods.

Trawling methods

Depending on where the trawl net is deployed in the water trawling can be classed as either midwater or bottom trawling.

Known as pelagic trawling, trawls deployed midwater is used to target fish species that are found in the benthic zone such as mackerel, tuna, shrimp and anchovies. The trawl nets used midwater are typically larger than bottom trawls and have much larger openings in the mesh and rarely have chaffing or ground gear.

Bottom trawling is the term used for trawls that are towed along the bottom of the benthic zone and/or the demersal zone. This method targets semi-pelagic fish such as squid, cod and halibut as well as groundfish.

The trawl doors are shaped differently for both methods and provide the horizontal spread required to keep the nets open. Essentially acting like wings, trawl doors vary in size and are specialized pieces of equipment to improve the success of a trawl.

Types of trawling vessels

The Food and Agriculture Organisation of the United Nations classifies trawlers into a variety of types depending on the gear they use and the fish they target but which fall into one (or more) of the following categories:

Outrigger trawlers

These vessels use booms that are known as outriggers which extend to the sides of the boat whilst in operation. Usually attached to the foot of the mast the booms can operate with a single or twin otter trawl and can be deployed forward or aft. Outrigger trawlers with a mid-ship working deck and aft rig are more commonly beam trawlers. Those vessels which operate with both a forward superstructure and working deck to the aft are often used to target shrimp and have a towing winch located at the bottom of the forward rig to feed bollards on the cap rail with drums.

Otter trawlers

Otter trawlers come in a range of sizes from small canoes to supertrawlers. They can operate using a single (or multiple) parallel trawls that are deployed horizontally and kept apart using otter boards (or trawl doors). It is most common for the trawl rig to forward but they can also be midship or aft. Towed across the ocean floor or midwater an otter trawler will most commonly have two gallows at the stern through which towing warps are run through blocks to a winch. Some vessels use outrigger vessels to tow the trawls to each side.

Trawler/purse seiner

Some vessels are adapted so they can use both trawling and purse seining depending on the waters they are fishing and the target species. Able to convert quite quickly these vessels are largely classified as trawlers instead of purse seiners.

Beam trawlers

A type of outrigger, the beam trawler operates with reinforced booms at each side of the vessel. The warps run through blocks at each end of the boom to more easily stow the equipment when not in use. Beam trawling is used in the North Sea in flatfish fisheries and are usually quite high-powered vessels that can tow at speeds of up to 8 knots though they are generally restricted to a maximum enginer power of 2000hp. This is for safety reasons which, along with safety release systems and winch brakes can help the vessel avoid capsizing in the event that the trawls snags on the seabed.

Stern trawlers

As their name suggests, stern trawlers operate with trawls that are launched and hauled from the stern. Able to operate independently or in pairs the architecture of the rig is forward facing with a working deck to the aft. Larger vessels are designed with a ramp.

Side trawlers

Unlike stern trawlers a side trawler deploys its trawl from the side using bocks that are suspended from twin gallows. Up until the mid twentieth century, most trawlers were side trawlers but are gradually being replaced with stern vessels. The superstructure is most commonly towards the stern of the boat, fish hold mid-ship with a winch forward of the rig.

Pair trawlers

Trawlers that tow a single trawl in a pair formation are known as pair trawlers. The rig does not use otter boards to keep the trawl apart but instead deploy the trawl horizontally between the vessels by keeping a regular distance. They can operate bottom or midwater trawls and use a forward or midship superstructure with an aft working deck.

Wet fish trawlers

Used to classify trawlers where the catch is kept in a fresh condition using ice and cold water, wet fish trawlers spend less time at sea to ensure their hauls are returned to land in optimum condition.

Freezer trawlers

Most modern day trawlers are freezer types and maintain the condition of the fish by freezing them. This allows the vessel to fish for longer periods.

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