Demersal Trawl - Nephrops trawl

A low demersal trawl towed on the seabed, with the mouth held open by a pair of otter boards (trawl doors) designed to target nephrops.

Seabed impact

As with all trawls there will be a certain degree of seabed impact with the trawl doors. When used properly the weight of any trawl door on the seabed will be much reduced compared to its weight on land.

In most demersal trawl fisheries the weight of the trawl door on the seabed will be approximately 20-25% of its weight in air. This is due to several factors. One is about an 8-10% reduction due to the weight of steel and other materials in water. The other major factor is the tension of the gear behind the trawl door and the uplift from the warp towing the trawl door.

There will be very little impact on the seabed from the sweeps and bridles or the ground gear of the trawl as this is generally very light gear when in use. It generally has light weight grass rope footrope fitted to enable the fishing line (bottom of the net) to just skim over the seabed.

Discards

Nephrops and prawn trawls can be susceptible to by-catch of small round fish and flat fish below the minimum conservation size due to the small mesh sizes that have to be used in these trawls to retain nephrops. This can be minimised by using relevant selective devices or by avoiding known areas where immature fish are known to congregate.

In certain areas there may be problems with by-catch of larger fish that the vessels do not have quota for. As before this can be managed to a certain extent by the design of gear used, used of relevant discard reduction devices and spatial avoidance.

This trawl is specifically designed to catch Nephrops also known as Langoustine and Dublin Bay Prawn, but are commonly referred to as 'prawns' by those in the UK industry, hence the name prawn net. It is a long winged low net with lightweight ground gear for towing over the soft muddy areas where Nephrops are found.

Sweeps and bridles are used between the trawl and the trawl doors with this net as the fishermen usually want to catch some of the other bottom-dwelling fish to boost their landings. These sweeps and bridles can range from 35 metres right up to 140 metres in length on each side of the net. The sweeps etc are made up of combination rope (a wire and rope mix) to withstand the abrasion of being hauled across the seabed to herd fish into the mouth of the trawl. Nephrops do not react well to this herding by the sweeps, with the effective herding area being only a few metres ahead of the wing ends of the trawl. In this rig the sweeps have two main roles, firstly to herd in some of the bottom living fish such as flats and monkfish that are a valuable bycatch in a prawn net. The second being to help with tuning the opening of the net in relation to the opening of the trawl doors in relation to the type of seabed and the speed that the gear is being towed at.

If any of these criteria is changed it can be very difficult to get the gear back to a situation where it is fishing to its optimum efficiency again. For this reason the skipper tends to try to avoid changing the dimensions of trawl doors, sweeps or the trawl if he can.

The amount of bycatch very much depends on the area that is being worked with the gear. In many places the fisherman has refined his knowledge of the fishing grounds and is able to target areas where knows there will be very little unwanted bycatch. This is often done on a seasonal or time of day basis. Often the skipper will move grounds to avoid high volumes of bycatch as this usually reduces the amount of nephrops retained in the trawl. It is well accepted that the target catch of nephrops will maintain its quality better if there is no bycatch in the net. Therefore it is in the interest of the skipper to minimise v bycatch to ensure minimal loss of nephrops and to prevent damage to the catch.

Generally a traditional prawn net will have a headline height (the height of the trawl) in the region of 1 to 1.2 metres. The net is designed to be very low to target the nephrops on the seabed with minimal round fish bycatch (haddock, whiting, cod etc) that usually swim higher off the seabed. The mesh size in the wings and body of the trawl is small, often 80mm but the exact size will be determined by the minimum mesh size permitted by regulation in the particular sea area. This mesh size will be carried throughout the trawl right down to the cod-end to try to retain as many nephrops as possible. However this small mesh size can cause problems with bycatch of immature fish if they are present on the prawn grounds.

As most nephrops grounds are in areas of soft mud the ground gear on the trawl is very light, often just bights of grass rope with leads on. This is designed so that the relatively fragile bottom of the net known as the fishing line skims a few inches off the seabed with the leaded bights of grass rope trailing on the seabed encouraging nephrops and any fish to rise of the bottom and fall back into the trawl. In areas where the seabed is firmer many of the vessel tend towards more protection for the bottom of the trawl and fit a rubber leg footrope to their prawn trawl.

To fish in areas of seabed interspersed with patches of harder ground the gear is sometimes fitted with a disc footrope to save damage on the seabed. Usually by this stage the actual net will need some redesign to prevent damage. Traditional nephrops trawls are designed with the lower netting panel slightly slacker than the top panel, if the net is towed over harder ground this slacker netting is very prone to damage on the seabed. For this reason the net needs to be designed differently to help keep the netting clear of the seabed if it is to be worked over harder ground.

This is where the vessel would move to a nephrops hopper trawl or disc trawl. To minimise unwanted bycatch in most areas the fishermen have to fit a square mesh panel to help release the small fish. They also have to have a section of larger mesh in the top panel of the trawl again to help with releasing unwanted bycatch. There are various other options available, many of which are already being used by skippers in the UK, to the skippers to help reduce bycatch such as larger square mesh panels, larger diamond mesh section in the top panels of the trawl and the use of coverless trawls. Although the use of rigid grids has been adopted in other European nephrops fisheries they have never been a favourite of UK fishermen or legislators.

In some areas over time the traditional prawn net design has evolved to have longer wings to make the net more efficient for targeting bottom fish such as monkfish, megrims, plaice etc. In other areas where there is opportunity for a more mixed fishery the gear has evolved to be higher standing to enable it to catch a higher proportion of round fish such as cod, whiting, haddock etc.

Prawn nets are towed by vessels ranging from under 10m fleet up to vessels around 24 metres but usually the larger vessel are targeting a more mixed fishery. They are also towed in twin rig set up by many vessels. Access our twin rig nephrops trawl gear database record for further information.