Evaluating the Impact on Energy & Fishing Efficiency from the Use if Alternate Netting Materials

Introduction
Harvesting fish from the sea requires more energy than the production of any other primary food product. Recent increases in fossil fuel prices and the growing concern over greenhouse gas emissions have highlighted the need to investigate ways of improving energy efficiency in the fishing industry.

Shrimp trawling is conducted on both coasts of Canada, and it is the most fuel intensive sector in the fishing industry. This is primarily due to the small mesh size and higher towing resistance of shrimp trawls, combined with the smaller catch rates relative to other trawl fisheries. For example, near-shore shrimp trawlers in the New Brunswick shrimp fleet presently consume from 40 to 60 litres per hour, and average fuel costs have increased as much as 60% to 70% over the past 12 months.

As a result, a project was undertaken on the Shrimp Trawler MV “Dominic Francis” operating from Caraquet New Brunswick and fishing in the Gulf of St. Lawrence.

This was to evaluate the impact on energy and fishing efficiency from the use of alternate netting materials in shrimp trawls in place of the traditional polyethylene material.

 

Project Results

• The current price paid for fuel in this area is 48.9 cents per liter. Therefore, the use of Composite netting will save this boat owner close to $4,100 per year in fuel alone.
• Estimated fuel savings from the use of such trawls could amount to 20.0 litres per metric ton of landed shrimp. Since current shrimp landings on both coasts of Canada are currently about 120,000 mt per annum, this suggests potential fuel savings in the order of 120,000mt x 20.0 liters/mt = 2,400,000 liters per annum if all shrimp trawlers used Composite trawls.
• The decrease in the towing resistance of the Composite trawl netting was actually greater than is implied by the measured gear tension. Previous tests have indicated that on average, about 25% of the drag after the doors is due to the sweeps, foot-rope and floats, and about 75% is caused by the netting. Since only the netting was changed in the two trawls, and since the gear tension measured after the doors decreased by 8.4%, this suggests that the drag force of the Composite net itself was reduced by about 8.4/0.75 = 11%.
• It should also be noted that significant fuel savings could also be achieved by incorporating Composite twine in groundfish trawls.
• The lower resistance and wider opening characteristics of Composite nets suggests that the use of smaller and lighter doors might be possible with such nets. Such sea bottom impacts would therefore be reduced with the use of smaller doors.

Materials
Spectra is a synthetic fibre which has a tensile strength similar to steel cable. As a result, Spectra twine in a trawl can be 50% thinner than Polyethylene twine of the same strength.

These strong Spectra fibers can be used to make low-diameter twines for use in trawl netting. This will therefore result in more efficient trawls which can be towed with less power and energy consumption.

In addition, Spectra netting has a high wet knot strength, in contrast to knots in Polyethylene netting which lose about 20% of their dry knot strength.

 

Project Objectives
The long-term, or general objectives of this program was:

To develop trawls with at least 10% less drag or towing resistance compared to traditionally constructed trawls of the same size, and

To raise the profile of energy conservation in the fishing industry by showing how the energy efficiency of fishing operations can be improved with new technologies.

The objective was to evaluate the impact on energy and fishing efficiency from the use of alternate netting materials in shrimp trawls in the net, and to the diameter of the twines and ropes used in its construction.

 

The specific objectives of this project were

• To compare the towing resistance, fuel consumption, trawl geometry, and fishing performance of:
• a standard polyethylene shrimp trawl, and
• a composite trawl of the same shape and size, but constructed from Spectra, nylon and polyethylene.
• To make vessel owners and operations aware of the potential fuel savings from the use of lower drag trawls. This would be achieved through a variety of approaches including the distribution of reports and videos, flume tank demonstrations and inclusion of project results in Responsible Fishing training programs.

 

Project Participants
This was a joint project with the following participants:

• The Department of Fisheries and Oceans, Responsible Fishing Operations, (Andrew Duthie) supported by PERD which was responsible for the overall co-ordination of the project.
• The technical staff at Crimond Enterprises of Dartmouth, Nova Scotia, was responsible for constructing the trawls, conducting the experiments at sea, analyzing the results, as well as preparation of the project report and video.
• Captain Daniel Gionet, the owner and operator of the MV “Dominic Francis”, (Captain Daniel Gionet) the shrimp trawler used for these experiments.
• · The Marine Institute of St. Johns, Newfoundland, (John Foster) supported by the Canadian Center for Fisheries Innovation, who were responsible for developing the experimental protocol, construction of a model trawl, and flume tank trials.
• Croquet School of Fisheries. The school curriculum contains courses to qualify fishermen of all grades to o9btain certificates of competency. It was to first fisheries college in the world to establish a course in Responsible Fishing Practices and has considerable expertise in the management of selectivity and other related fishing projects. The school was responsible for project management.

 

Project Details
The vessel used in this project was the MV “Dominic Francis”, a modern shrimp trawler which operates from the port of Caraquet, New Brunswick. The propulsion system consists of an 855-hp engine driving a 1.7m diameter variable pitch propeller with a nozzle.

The experiment was conducted on fishing grounds off Anticosti Island in the Gulf of St Lawrence. The tests occurred during two commercial fishing trips during August and September 2000, which provided 17 days at sea with the equivalent of 15 fishing days. The main difference between the two trips was that catch rates decreased significantly during the last trip.

The identical design used in constructing both the Polyethylene and Composite shrimp trawls was a three bridle Labrador type #1362, which has a 1360 x 60 cm mesh size. The same Bison #13 trawl doors and identical ancillary equipment were also used with both types of trawls. A plan of this trawl design is included as Exhibit 1.

 

Exhibit 1 Labrador type #1362 Shrimp Trawl
Preliminary estimates indicated that the Composite trawl had approximately 40% less twine surface area. In addition, a scale model of this trawl was constructed and tested in the flume tank at the Marine Institute.

During each tow, fuel consumption, gear geometry and gear tension were monitored with electronic sensors and all such data was stored electronically for later analysis. Environmental conditions and shrimp catches were also recorded. The specific types of data collected during each tow is illustrated in Exhibit 2.

 

Exhibit 2 Data Collected During Tows

Date	Mean Sounding
Haul #	Mean Headline Height
Bottom Type	Mean Wingend Spread
Wind Speed and Direction	Mean Door Spread
Wave Height	Swept Area of Trawl
Light Conditions (Surface)	Swept Volume of Trawl
Start Towing Time	Swept Area of Gear
End Towing Time	Swept Volume of Gear
Tow Direction	Distance Towed from Start to End
Tide Direction	Fuel Consumption
Distance Towed	Engine Load
Speed over Ground	Shrimp Catch
Mean Log Speed

During the experiment, 25 valid tows of approximately 4 hours duration were made with each type of trawl. Prior to this, two tows of one-hour duration were conducted with both trawls to check that their geometry was similar, and to make any necessary adjustments.

Generally, an attempt was made to provide each trawl with equal fishing time, and the same number of tows with and against the tide to minimize any experimental bias.

 

Results
The average performance of these two trawls during the 50 fishing tows is shown in a tabular format in Exhibit 3, and graphically in Exhibit 4.

Trawl Netting Material	Poly Twine Only	Composite	% change
Gear Tension (kg)	3690	3380	- 8.4
Door Spread (m)	41.8	46.0	+ 10.0
Wing Spread (m)	16.75	17.17	+ 2.5
Headline Height (m)	9.861	9.865	+ 0.04
Swept Area (m3)	323,000	334,000	+ 3.4
Fuel - Liters / Hour	61.0	54.6	- 10.50
Shrimp Catch / hr (kg)	458	485	+ 5.9
Catch / liter of fuel	7.51	8.88	+ 18.2
Liters of fuel/ kg shrimp	0.1332	0.1126	- 15.5

Exhibit 4 Graphs of Results
The decrease in the towing resistance of the Composite trawl netting was actually greater than is implied by the measured gear tension. Previous tests have indicated that on average, about 25% of the drag after the doors is due to the sweeps, foot-rope and floats, and about 75% is caused by the netting. Since only the netting was changed in the two trawls, and since the gear tension measured after the doors decreased by 8.4%, this suggests that the drag force of the Composite net itself was reduced by about 8.4/0.75 = 11%.

The use of Spectra/Nylon twine in this trawl therefore resulted in lower towing resistance, as well as larger door and wing spreads compared to the similar trawl constructed of only polyethylene twine. The larger shrimp catch rates were probably the result of these increases in door and wing spreads, and the resulting larger swept areas.

This improved performance in both towing resistance and higher shrimp catch rates, resulted in fuel savings of 0.0206 liters/kg of shrimp during fishing operations. In other words, the use of Spectra/Nylon twine in the shrimp trawl resulted in average fuel savings while fishing of 15.5% per kilogram of shrimp.

These tests therefore suggest that a trawl constructed with Spectra/Nylon twine:

• Could be towed by a vessel with less horsepower, or
• Might be operated with smaller and lighter doors and footgear, therefore resulting in less sea bottom impact, or
• May allow an existing trawler to tow a larger net, and thereby increase catch rates even more than is indicated in these experiments

 

Benefits to the individual vessel owner
The shrimp fishery in which this experiment was conducted is managed on the basis of individual boat quotas, and the trawler involved had a quota of 407.8 metric tons for the year 2000. Based on these results, it is projected that the use of Composite netting could result in annual fuel savings for this vessel of about 8,400 liters (i.e.: 407,800 kgs shrimp x 0.0206 liters/kg saved).

The current price paid for fuel in this area is 48.9 cents per liter. Therefore, the use of Composite netting will save this boat owner close to $4,100 per year in fuel alone. Fuel savings may actually be larger than this since the resulting higher catch rates might possibly result in fewer fishing trips per year.

A Composite trawl of this type could cost about $6,000 more than one constructed of polyethylene. The “payback period” for this additional investment would therefore be slightly more than one fishing season, just from the $4,100 or more in fuel savings! If we add the savings from the other hourly operating expenses (eg: lube oil, equipment wear, etc.) for such a vessel, then the payback period could be in the order of only one fishing season. After this, the benefit would be additional profits.

 

Benefits to the environment as a whole
In addition to the economic benefits to the individual boat owner, the reduction in fuel consumption from the use of Composite trawls will also result in reduced emissions of greenhouse gases, which is now a global priority.

As calculated above, estimated fuel savings from the use of such trawls could amount to 20.0 litres per metric ton of landed shrimp. Since current shrimp landings on both coasts of Canada are currently about 120,000 mt per annum, this suggests potential fuel savings in the order of 120,000mt x 20.0 liters/mt = 2,400,000 liters per annum if all shrimp trawlers used Composite trawls.

It should also be noted that significant fuel savings could also be achieved by incorporating Composite twine in groundfish trawls. In the 1980’s, for example, ground-fish trawler landings in Canada were over 500,000mt per year (i.e.: five times current shrimp landings). This suggests that the use of Composite in these fisheries as well, could result in potential fuel savings, which could be orders of magnitude greater than in just the shrimp trawl fisheries.

Finally, the lower resistance and wider opening characteristics of Composite nets suggests that the use of smaller and lighter doors might be possible with such nets. The most noticeable impact of trawls on the sea bottom are the “tracks” caused by the bottom edge of doors. Such sea bottom impacts would therefore be reduced with the use of smaller doors.