Twin Rig Trawl - Nephrops

This is a fishing method of towing two otter trawls side by side to target nephrops. The nets are usually smaller than an equivalent single net but are designed to sweep a wider area of seabed using less fuel. 

Possible bycatch issues due to small mesh used in nephrops trawls. This can be minimised by use of selective devices. Square mesh panels and areas of larger diamond mesh are mandatory in most nephrops fisheries. There will be very little seabed impact from the net, sweeps and bridles as they are designed to just skim over the soft mud.

The trawl doors and clump weight will ‘lose’ about 75% of their weight when working on the seabed. they may well penetrate the soft mud seabed a few  centimetres but this will soon return to its original state.

Further details of the gear can be found below. 

The nephrops twin rig developed initially in North East Scotland as a follow on from the Danish twin rig set up for shrimp and nephrops. To start with the boats adopted a two warp system, a larger version of what was already in use in the twin and triple sole rig in South East England. It fairly quickly progressed to the more versatile three warp system.

The two warp system enables vessels fitted with a standard two barrel trawl winch to tow two trawls simultaneously using their existing winch and minimum alterations to the deck layout. The trawls used are smaller versions of that used by the vessels if they were towing a single trawl. The nets, sweeps and bridles would all be decreased in size by approximately one third compared to what the vessel would use as a single trawl to achieve a sensible size for the vessel to operate as twin rig.

A typical rig for a two warp twin rig nephrops vessel working clean ground would be two 30m prawn (nephrops) trawls with a total of 100 metres of sweep and bridles. This gear would be spread using a pair of trawl doors attached to the sweeps from the outer wing ends of the trawls, the sweeps from the inner wing ends of both nets would lead to a clump weight. The purpose of this clump weight is to keep the inner wing ends and sweeps on the seabed. In two warp system it is usually to use several links of heavy chain as a clump weight.

All nephrops trawls are made with small mesh netting to retain the relatively small nephrops, this can result in considerable bycatch of juvenile fish in some areas. This unwanted bycatch can be reduced by using various selective devices such as square mesh panels and larger diamond mesh to release many of the small fish. In most nephrops fisheries use of these selectivity measures is mandatory. Most skippers will use their experience and local knowledge to avoid areas known to have an abundance of immature fish. A build up of fish in the prawn trawl will cause the net to lift off the seabed and not catch any nephrops therefore the skipper wants to avoid capture of fish if possible.

Prawn trawls are designed for towing over soft mud where the nephrops live in burrows. They are rigged so that the fishing line skims over the seabed with any ground gear such as a grass rope or chain bights or rubber leg footrope is in contact with the seabed but not digging in at all. If the gear starts to dig in the net will fill up with mud and damage the net or the net will dig in so much that the boat will stop, referred to as ‘coming fast’. Either of these can cause a lot of damage to the gear or loss of the gear altogether. For this reason the skipper ensures that his gear will just skim over the seabed maybe disturbing the top 20mm of the sediment. He does this by balancing the weight of the footrope with the number of floats on the headline in conjunction with the speed of towing. Similarly the size and weight of the sweeps and bridles between the net and the trawl doors is chosen so that they also just skim the seabed to heard any ground fish into the trawl.

The weight of the trawl doors and clump weight between the nets is made so that they too maintain contact with the seabed but do not dig in and cause the gear to come fast. They may well penetrate the soft mud several inches but the seabed should return to its normal state reasonable soon due to wave action and water movement close to the seabed. The doors and clump weight look very heavy in air (on the quayside) but when they are working on the seabed the weight of them acting on the seabed will be in the region of 25% of the weight in air. This is due partly to the loss of some weight when steel is in the water (5-8%), but mostly due to the tension in the sweeps and bridles behind the door and clump and the warp ahead of the trawl door and clump having a lifting component that takes much of the weight of the doors and clump weight off the seabed.