The benefIts of semi-pelagic doors

The benefits of semi-pelagic doors

Fuel is always a major cost for fishermen, particularly those operating towed gear.

While fishermen can’t control the price of fuel, there are opportunities to reduce the amount used and the cost to the business.

Here we look at innovative methods to reduce the drag of fishing gear and cut fuel consumption.

“In calm weather, between 80% and 90% of a fishing boat’s fuel consumption is used to overcome the drag of the fishing gear in the water,” says Mike Montgomerie, Fishing Gear Technologist at Seafish. “Think about how easy it is to manoeuvre around the harbour. Only 10-20% of the engine’s available power is actually required to move the boat forwards through the water.”

Using thinner materials for warps, sweeps and the net itself can drastically reduce the twine surface area and weight of the trawl. But, there is another major source of drag, traditionally seen as unavoidable: trawl doors touching the seabed.

When set up correctly, the effective weight of the trawl being towed is reduced to only 20-25% of its weight on land. But, at several hundred kilogrammes, doors still create a significant amount of drag when trawled along the seabed, burning a lot of fuel while towing.

Some trawl fishermen have borrowed techniques from other types of fishing to reduce fuel consumption by lifting the trawl doors off the seabed but keeping the ground gear in contact with the seabed to maintain their catch of bottom-dwelling fish and shellfish. These are known as semi-pelagic trawl doors.

“Semi-pelagic trawl doors have a greater aspect ratio than traditional demersal doors; this means they are taller and thinner than the square or oval shaped doors often used by UK trawlers,” says Mike. “This different shape means the doors are able to work effectively when in contact with the seabed or when lifted off the seabed.”

“Hydrodynamic steel foils are used to help stabilise the doors when they are not touching the bottom meaning that they stay upright even without the support of the sea floor,” Mike went on to explain.

Mike Montgomerie, Seafish Gear Technologist

Generally, semi-pelagic doors are more hydrodynamically efficient than traditional demersal doors because their shape allows them to generate more spreading force in the water. As a result, semi-pelagic doors can be smaller and lighter than their traditional counterparts but still open the net mouth in the same way.

This technique has been effectively used by Alaskan pollack fishermen in North America and shrimp fishermen in Greenland. Some fishermen in Northern Ireland have, for many years, been using semi-pelagic trawling, with pelagic trawls and doors, but rigged so that the gear skims the seabed to target cod and haddock.

In addition to the financial benefits of reduced fuel consumption, there are associated environmental benefits such as reduced impact on the seabed and reduced CO 2 emissions. Despite these benefits, many have been reluctant to fish with trawl doors off the seabed due to the belief that the action of the doors on the seabed helps herd fish into the trawl mouth.

To overcome this reluctance, researchers in Denmark, working on North Sea fishing boats, have trialled alternative trawl setups and shown that catches can be maintained even with trawl doors off the sea bed. Adding weights to the sweeps of semi-pelagic doors creates the herding effect of trawl doors but with less drag.

When using semi-pelagic doors to target bottom-dwelling species such as cod and haddock, there is a fine line between effective fishing and losing bottom contact with the net, leading to lost fish.

Modern acoustic sensors on some boats are used to give the skipper accurate information on where and how the gear is fishing. Specific sensors tell a skipper exactly how high off the seabed the gear is, as well as the angle and pitch of the doors. This means that the skipper can adjust the gear with a greater level of accuracy than if he is fishing blind, and ensures he gets the most effective use of the gear at all times.

In 2013, researchers from SINTEF in Denmark conducted trials in the North Sea. The aim was to help Danish fishermen reduce fuel consumption by entirely eliminating seabed contact of their trawl doors. To compensate for the lifted doors, the team added in-line chain weights to the sweeps to keep the gear itself on the seabed.

The fishermen involved in the trials adjusted their gear warps so the chain weights touched the seabed, but the doors were “flying” between three and five metres from the seabed. The weights were placed behind the trawl doors at 45 metres from the wingtips of the trawl; this meant that the sweeps continued to herd fish into the mouth of the net despite the doors being off the seabed.

The trial team found that this method of trawling used 15% less fuel than the same trawls using traditional doors on the seabed. Interestingly, these figures match the fuel savings predicted by manufacturers after trialling semi-pelagic doors in the Hirtshals flume tank in Denmark.

For many fishermen working with towed gears, the idea of fishing with their trawl doors off the seabed may seem like a significant departure from their usual methods. But, faced with high fuel prices and mounting pressure to minimise the environmental impact of fishing, UK trawl fishermen are taking inspiration from fishermen further afield to see what tools and techniques could be applied to their fishing. Semi-pelagic doors are just one example of technology being adapted to suit another sector of the fishing industry but this type of innovation is something we may see more of as the UK sector continues to evolve.

More information on a wide range of commercial fishing gears, including selectivity devices and methods of improving fishing efficiency, are available on the Seafish Gear Database.

Testing gear in the SINTEF flume tank

Flume Tank Testing – Semi-Pelagic Doors

One of the core functions of the SINTEF flume tank in Hirtshals is commercial testing of trawls and trawl doors.

Most available selectivity devices and new developments have had a round in the flume tank before they go to sea for final testing, including semi-pelagic trawl doors.

The tank allows designers to see how their trawl doors will behave underwater. Engineers create highly accurate scale models of the doors which are fitted to model nets and monitored in the tank.

These tests allow door manufacturers to test the hydrodynamics, spreading force and drag of their trawl door prototypes and make adjustments before undergoing more expensive and time-consuming sea trials with full-size gear.

For more information on the Trawl Gear Technology training courses at the flume tank please contact mike.montgomerie@seafish.co.uk or your local Producer Organisation office.