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Twin Trawls


Supported by Natural Resources Canada (PERD) Energy Efficiency Task

This report describes A project which was carried out in 1996 to transfer twin trawl technology from Europe to the shrimp industry in Canada. The objectives of this project were to investigate if the introduction of this trawling technology would achieve the above described objectives of improved energy efficiency, effective conservation characteristics, and enhanced product quality.


The development and implementation of Responsible Fishing technology and practices is necessary for achieving long-term sustainability of our fisheries, as well as protecting our environment as a whole.

For example, the draft Canadian Code of Conduct for Responsible Fishing Operations includes the following guidelines or objectives:

  • Minimize total energy consumption in fishing operations.
  • Minimize the catch of non-targeted species.
  • Maintain the quality of the catch.

In response, the Canadian fishing industry, in cooperative partnerships with the Department of Fisheries and Oceans (DFO) and other government departments such as Natural Resources Canada, has undertaken more than 100 conservation harvesting technology projects in recent years. These have been done from coast to coast using commercial fishing vessels and gear, and were designed to promote responsible fishing. The key to the success of these experiments is that they originated from proposals by fishers, fishing companies and industry associations.

This report describes such a project which was carried out in 1996 to transfer twin trawl from europe to the shrimp industry in Canada. The objectives of this project were to investigate if the introduction of this trawling technology would achieve the above described objectives of improved energy efficiency, effective conservation characteristics, and enhanced product quality.

This project was also undertaken as a cooperative undertaking or partnership of the following participants:

  • The federal Department of Fisheries and Oceans, with funding assistance from Natural Resources Canada.
  • The New Brunswick Department of Fisheries and Aquaculture, together with the Fisheries School at Caraquet, New Brunswick.
  • Captain Bertrand Mallet, whose vessel the "Naomi Karen" was used in the experiments in parallel with normal fishing operations, since no funds were available to charter his vessel.
  • Crimond Enterprises Limited, the company responsible for designing and constructing the fishing gear, supplying the electronic equipment, and supervising the sea trials.

This joint effort of the private and public sectors again demonstrated the cost effectiveness of this new approach to conducting such projects, in contrast to the past when fishing vessels were chartered and the projects completely funded by government.

The results of this very successful project indicate that:

  • The use of twin trawl systems could reduce the fuel consumption of shrimp trawlers in the order of one-third, compared to the present single trawl systems.
  • If all shrimp trawlers in Canada used twin trawl systems, this could result in savings of up to 36 million litres of fuel annually. This is a maximum estimate, since it is unlikely that all of these vessels will use this new system.
  • Savings in energy costs from the use of this technology could be in the order of 11¢ per kg of shrimp landed, and total savings to the shrimp fishing industry could reach about $9.5 million per annum.
  • The increased catch rates of twin trawl system is not a conservation concern since most shrimp fisheries are managed with individual vessel quotas. Rigid selectivity grids can also be used in the twin trawl system, which provides excellent gear selectivity and minimum by-catches.
  • Higher catch rates will result in shorter trips and higher quality of landed shrimp, which is a growing priority in the industry.
  • The twin trawl gear can be installed on shrimp vessels with minimal conversion costs. Learning to operate this new type of gear also does not appear to pose significant problems.


With all other variables such as fish or shrimp densities being the same, the catch during a bottom fishing trip is primarily dependent on the area of the bottom which is swept by the trawl fishing gear, and to a lesser extent, on the volume of water which passes through the net.

Most of the power used by a trawler while towing is absorbed in the trawl. The size of a trawl which can be towed at the required speed (ie: 2 to 4 kts depending on the fishery) is therefore limited by the propulsive power of the vessel's engine. The drag components of a single trawl are approximately as follows: 60% from the netting, 25% from the doors, and 15% from the floats, groundgear and bridles. For a given available horsepower, the design challenge is therefore to achieve as wide a net opening as possible, together with an optimum net opening height which is dependent on the species being targeted and the local fishing conditions.

Simply increasing the size of the trawl net used by a given vessel is usually not an option for achieving a greater swept area and volume. In most cases, the vessel propulsion system will be unable to overcome the resulting increases in the combined drag forces.

An approach for getting around these limitations is to tow several smaller nets, rather than one larger one. The general arrangement of a twin trawl system is illustrated in Exhibit 1.

The principle behind multi-rig trawling is that a greater swept area can be achieved by increasing the footrope spread without adding to the netting surface area. This results in an improved catch potential with minimal increase in drag and fuel consumption.

Towing multiple trawl rigs is not a new idea and has been practiced in various forms in a number of countries. For example, shrimp trawlers in the Southern Hemisphere tow up to four trawls when operating in shallow waters. This method of fishing has recently been successfully adapted in Europe for the catching of prawns, shrimps, and certain groundfish species, under conditions which are very similar to those in the Canadian fisheries.

European twin trawling involves one vessel towing two nets, using two or three wires (ie: warps), and thereby increasing the swept area by a large margin. This usually results in a significant increase in catches of certain species over conventional single trawl systems. For example, Canadian offshore shrimp trawlers fishing single trawls have noted that Scandinavian vessels fishing in the same areas with similar horsepower and twin trawl systems were achieving almost double their catch rates.

Another advantage of twin trawls is that any anomalies in the operation of one of the trawls shows up in their fishing performance. If one of the trawls out-performs the other, then this quickly indicates a problem. In addition, if one trawl is fouled or damaged during fishing, then only half of the catch is lost, compared to the loss of the total catch when towing one trawl. As well, if the vessel has to revert to single trawling for any reason, this can be achieved in the relatively short time required to detach one of the trawls.

Danish flume tank experiments have shown that the towing drag of a certain twin trawl system was 2.6 tons at 1.8 kts. compared with 1.53 tons for the same trawl when used as a single trawl. The drag of the twin system was then less than twice that of the single trawl. This can be construed as being a more fuel efficient system, especially in terms of comparable swept areas.

Reasons for this improved towing efficiency of twin trawl systems include:

  • For the same wingend spread or swept area, the netting surface area and the associated drag forces in a twin trawl system are substantially less that for a single trawl system.
  • With single trawl systems, towing two nets separately would require the use of four otter trawl doors (ie: two for each net). In contrast, the twin trawl system only requires the use of two doors, thereby saving the equivalent and substantial drag of two doors, for the same swept area.

Using the two wire system illustrated in Exhibit 1, it is possible to adapt any trawler to the twin trawl system without the need for radical rigging and construction modifications. Most vessels which start fishing with the two wire technique, however, soon invest in a three winch / three wire system. Independent split winches are preferable, since this allows more precision tuning of the gear. With the three wire system, two wires go directly to the trawl doors, and the third wire goes to the center of the gear. Having a separate third wire allows the operator to adjust the crucial length and tension parameters on the center wire, which is essential since it has nearly twice the tension of the outside door wires.

There has been a considerable increase in the number of European fishermen adopting multi-rig trawling techniques over the past few years. It is only natural that Canadian fishermen should now evaluate and adapt these techniques to the conditions in their own operating depths and bottom conditions.



3.1 Objectives and Conduct of the Experiment
As a result of a proposal from the shrimp fishing industry in New Brunswick, a project was initiated to transfer twin trawl technology from Europe, and to adapt it to shrimp fishing conditions in the Gulf of St. Lawrence. This project was also a cooperative venture of the fishing industry, the gear supply sector, and several government departments as described in Section 1.

The main rationale for introducing this technology was that more efficient harvesting (ie: higher catch rates) would result in economic and environmental benefits such as reduced fuel consumption and lower vessel operating expenses. The resulting shorter sea trips would also result in better quality catches, which is particularly important in this fishery because of the perishable nature of shrimp. As well, since the shrimp fleet is managed with individual vessel quotas, there was no concern that the introduction of this technology would result in increased overall catches or overfishing.

The specific objectives of these trials were as follows.

  • To design and construct equipment necessary for shrimp trawling utilizing the European twin trawling, two-warp system. This system should not involve major alterations and expense for converting existing vessels.
  • To determine the optimum rigging of the trawling equipment by using Scanmar acoustic sensing and underwater video equipment.
  • To conduct comparative fishing trials with traditional shrimp vessels operating with single trawls in the same area, in order to evaluate the relative catching and fuel efficiency of the twin trawl system.
  • To prepare a comprehensive report for industry on the conduct and results of the experimental trials.
  • To incorporate the information from these trials into the Responsible Fishing Program of the School of Fisheries in Caraquet, New Brunswick.

The experimental trials were conducted during 1996 on a typical Gulf of St. Lawrence midshore shrimp vessel, the "Noemi Karen". This vessel was skippered by Captain Bertrand Mallet, from Caraquet, New Brunswick. It has an overall length of 22.4 m. (73 ft.) and a main engine of 634 hp.

The twin trawl gear was designed and constructed by Crimond, which was also responsible for the overall conduct of the experimental trials. The twin trawls were two-bridle Crimond model 938 Labrador shrimp trawls. Each of the twin trawls were constructed 24% smaller than the "Noemi Karen's" normal single trawl (Labrador 1110). Both trawls were fitted with selectivity grids and an additional shrimp grading grid was evaluated.

Only one vessel modification was needed. A center flange was fitted to the net drum to accommodate the two trawls.

Before the actual sea trials, models of the twin trawls were tested in the flume tank at the Marine Institute in St. John's Newfoundland, as illustrated in Exhibit 3


3.2 Experimental Results
The project was conducted over a two week internal during September 1996. Sea trials were conducted on commercial shrimp grounds in average depths of 165 fathoms, and on mainly soft mud bottom conditions. Fishing areas were 8 miles south of Anticosti Island in 4Ss and 30 miles east of Anticosti Island in 4 Sx. All vessels in the fleet reported relatively poor shrimp fishing in both of these areas during the project period.

The twin trawl rigging plan, including the wire tensions and gear dimensions recorded by the monitoring equipment are illustrated in Exhibit 4.
The vessel's existing doors were used (Bison #11++) together with a center weight , or clump, of 454 kg (1000 lb). An increase of 100 to 200 RPM was required to tow the twin rig, compared to the vessel's normal single trawl.

Using two Scanmar systems in parallel, as well as a complete range of sensors, it was possible to determine the simultaneous configuration of the two nets and to determine the correct fishing parameters in just ten tows. The results of these basic gear measurements are illustrated in Exhibit 5.

Unlike European shrimp twin trawls where net openings are normally 10 to 12 feet high, the project trawls consistently showed headline heights of 24 to 25 feet. These are the net heights considered necessary with the single nets used in this fishery, since shrimp in this area rise above the bottom at night and in the summer.

The results of these sea trails were as follows.

  • In general, the two trawls were easy to shoot and haul, but split net drums would have resulted in easier handling of the gear. Turning and maneuvering, however, were slower than with a single trawl. One day when the wind was force 7 to 8 with a heavy swell, the captain expressed the view that the twin trawl gear was fishing better in bad weather than his single trawl.
  • The catches from fourteen valid tows were measured from each trawl, and the weight difference in total shrimp catches was only 0.01%.
  • Comparison of harvesting performance with other shrimp vessels fishing in the same area showed substantial increases in catching efficiency for the new gear technology. Information supplied by the captain on the fuel consumption of single trawl operations during the same fishing period in 1995, indicated a 28.2% reduction in fuel costs for the equivalent shrimp catches using twin trawls.
  • The higher catch rates enabled the shrimp trawler to spend relatively less time fishing and therefore resulted in a shorter trip. Such shorter fishing trips will enhance the dockside quality of the shrimp catch, which is dependent on the storage time on the vessel. Improving the quality of landed shrimp has become a priority with shrimp buyers and processors, as well as the fishermen themselves.


3.3 Conservation Aspects of the Experiment

As previously mentioned, both trawls during the experiments were equipped with rigid selectivity or Nordmore grids to minimize the bycatch of finfish. These consist of closely spaced bars which are made of metal or plastic, as shown in Exhibit 6. The grids are attached in the trawl nets at an angle of about 500, ahead of the codend. They direct fish towards an opening at the top of the nets, where they escape unharmed, while allowing the shrimp to pass into the codend.

The basic Nordmore Grid was designed primarily to reduce the bycatch of finfish. It has not been as effective in reducing catches of the smaller and less valuable "industrial" shrimp. As result, significant gear development work is ongoing in Canada and abroad, aimed at improving shrimp size selectivity.

Most of the recent experimental work on shrimp size selectivity, has focused on the use of multiple grids in the trawl. These systems show great promise, and they are being currently being used successfully in our offshore Northern Shrimp fishery. The basic configuration of the multiple grid system is shown in Exhibit 7.

In this system, the Nordmore Grid first sorts out the groundfish bycatch, which exits the net though the outlet at the top of the net. After the shrimp passes though this grid, they are guided by a net panel towards the bottom of a Size Sorting Grid (which has much smaller bar spacings than the first grid). The smaller shrimp pass through this second grid and exit though an outlet at the bottom of the net. The larger shrimp reach the top of the size sorting grid and pass into the codend.

During the second phase of this experimental project, this type of shrimp size grader grid with 10 mm bar spacings was placed behind the by-catch grid on one of the nets. During all 5 tows using this arrangement, the net with the additional grid caught less shrimp. It was then determined that the grid spacings were too small for the larger shrimp found in the area. This part of the experiment was therefore discontinued because time constraints prevented the replacement of the grid with one having larger bar spacings. Further trials to refine this multiple grid system in twin trawl gear are therefore planned.



4.1 Energy Savings on Vessels Using Twin Trawls

The 1996 experiment in the Gulf of St. Lawrence indicated that:

  • The trawl door spread was increased from 47.2 to 80.2 m, a 70% increase.
  • The trawl wingend spread was increased from 15.8 m (1 trawl) to 29.6 m (2 trawls), an 87% increase.
  • The mouth opening of single trawl (106 m2) increased to 193 m2 for the twin trawls, an 82% increase.

From experience over a range of species and fishing conditions, the catch rate of bottom trawls appears to be about 70% dependent on the area of the sea bottom which is swept, and about 30% dependent on the volume of water which passes through the net. There is some uncertainty, however, as to whether the effective swept area is dependent on the trawl door spread or the trawl wingend spread, and this can vary depending on the species being fished.

During the experiments, the twin trawl system increased the door spread by 70% and the wingend spread by 87% compared to the previous single trawl system. Although shrimp catch rates appear to be more dependent on wingend spreads, a conservative value in-between these two values, or an effective increase of about 78% in swept area will be assumed in the following calculations.

Since about 70% of the increase in catch rate is proportional to the increase in effective swept area (ie: 78%) and about 30% is due in the increase in swept volume (ie: 82%), it will be assumed that the use of twin trawls can potentially increase the catch rates of shrimp trawlers by about 80%.

The fuel consumption per kg of shrimp caught by the various fleet sectors is estimated in following Exhibit 8.

These estimates are based on an average diesel engine fuel consumption of 0.22 litres / bhp / hour. Since the average weight of diesel fuel is 0.84 kg / litre, an average fuel consumption of 0.18 kg / bhp / hour will be assumed.

A weighted average fuel consumption figure for the overall Canadian shrimp fishing industry is therefore in the order of 0.8 kg. fuel / kg. shrimp caught.

With 80% higher catch rates, fuel consumption on trawlers using twin trawls would therefore decrease to about 0.8 / 1.8 = 0.44 kg of fuel per kg of shrimp harvested. In other words the use of twin trawls could result in energy savings in the order of 0.36 kg of fuel / kg of shrimp.

The total catches in all of the Canadian otter trawl shrimp fisheries over the next few years could therefore be approximately as follows:

Atlantic Northern 59,000 mt
Gulf of St. Lawrence 18,000
Scotian Fundy 3,000
Pacific 5,000
Total CANADA 85,000 mt

If all of these shrimp vessels used twin trawls, the annual potential savings in fuel per year could therefore be in the order of:

  • 85,000 mt (shrimp catch) x 0.36 (mt fuel saving / mt shrimp) = 30,600 mt of diesel fuel.
  • In other words, an annual saving of 36.4 million litres of diesel fuel could be possible if all shrimp trawlers converted to twin trawling.
  • At an average cost of 26¢ / litre, this would result in annual cost savings for the Canadian shrimp fishing fleet of up to $9.5 million, or 11¢ / kg shrimp landed.

As mentioned, this is an upper bound estimate of the potential savings if all of the shrimp vessels in Canada operated twin trawls. This is unlikely for a variety of reasons. For example, catch rates on the offshore shrimp trawlers are currently so high with single trawls that catches are being limited by their onboard processing capacity. Nevertheless, this estimate illustrates the energy saving and economic potential of this technology.

When considering the overall percentage fuel saving to the shrimp trawling industry, one should note that a significant portion of the fuel used during fishing trips is consumed "steaming" back and forth from the landing port to the fishing grounds, as well as searching for good fishing areas. This can vary significantly, depending on the particular fisheries and the related distances involved, however it could add from 10 to 30% or more, to the total engine operating time during a fishing trip. Energy consumption during those parts of a fishing trip would therefore not be affected by the installation of twin trawl systems.

It was estimated above that the use of twin trawls could result in fuel savings of (0.36 / 0.8)x100 = 45% during actual fishing operations. If it is assumed that 25% of an average shrimp fishing trip is taken up in "steaming", then this implies overall fuel savings per trip in the order of 0.75x45% = 34%.

One can therefore conclude that overall fuel savings on shrimp trawlers using twin trawls could be in the order of one-third, as compared to single trawl operations This is in the same order of magnitude as the approximate fuel savings per kg of shrimp (ie: 28%) estimated during the experiment, from data provided by Captain Mallet on fishing trips using single trawls during 1995.

Andrew Duthie
Chief Responsible Fishing Operations
PP&C, Fisheries Management
Fisheries and Oceans Canada
200 Kent Street, Station 1412
Ottawa, Ontario K1A 0E6
Tel: (613) 990-0157
Fax: (613) 990-9691
Prepared by: 
George Richard, Tavel Ltd. and
David Tait, Crimond Enterprises Ltd.
© Minister of Public Works & Government Services
Cat. No. Fs 23-300/2-1997E
ISBN 0-662-25813-4
Issued also in French under title:
Techniques de pêches à la crevette au double chalut


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