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  • MLCA

Assessing Vertical Line Use & Functional Breaking Strength in the Maine Lobster Fishery

first published in Landings, February, 2020

In the summer of 2018, the Department of Marine Resources (DMR) and partners at FB Environmental and the University of Maine received a three-year grant from NOAA to assess the use of vertical lines throughout the Gulf of Maine region. The results have been instrumental in the development of the DMR proposal to reduce the risk of right whale entanglement in Maine fixed gear. Over the next year, DMR will continue to collect data to develop a suite of options to fulfill the weak point requirements in the whale plan. This work will be done in collaboration with fishermen and industry stakeholders.

The most prevalent gear in the Maine lobster fishery is 3/8" rope, reported by 47% of respondents.

Volunteer Gear Survey The first phase of the project was to establish a baseline of the rope used for vertical lines in the region’s lobster fisheries. In the summer of 2018, DMR conducted a vertical line survey of lobstermen throughout the Northeast region. More than 800 responses were received through online, paper, and phone-based options. The survey results showed that rope diameters vary, including by distance from shore. While there is a range of rope diameters used in Maine, survey responses show that the most prevalent diameter of rope used in the lobster fishery is 3/8”, with 7/16” as the second-most common diameter. The survey results also indicated that as the fishery moves farther offshore, the use of line greater than 3/8” in diameter increases.

Functional Breaking Strength The second part of the project involved collecting vertical lines donated by lobstermen throughout the region with the goal of documenting the spectrum of functional breaking strengths for vertical lines used in the fishery. These data serve as a basis to determine which configurations of vertical lines already break at or below 1700 lbs., and to better understand the conservation benefit gained by requiring weak points in vertical lines.

Whole vertical lines were collected from fishermen, along with information about where the line was fished, rope types, rope diameters, age of rope, gear configuration, etc. A total of 215 samples were broken on the Tinius Olsen tensile-testing machine housed at the DMR lab in Boothbay Harbor. These lines included a wide range of knots and splices used to connect ropes together, as well as clear or unmodified pieces of rope.

The proportion of ropes greater than 3/8" increased as gear moved offshore.

The ages of the rope generally ranged from two to six seasons fished but went up to as many as twelve seasons. The break strength tests determined that vertical lines currently deployed in the fishery will break at less than 1,000 pounds to just over 4,000 pounds. The majority of ropes broken ranged from 11/32” to 7/16” diameter ropes, reflecting the distribution of rope diameters within the fishery. Knots and splices reduce the breaking strength of rope, which is of importance since less than 5% of lines fished in the fishery are used without a modifying link of some kind.

When a line of two different diameters knotted or spliced together was tested, the smaller diameter broke 100% of the time and the knot or splice always stayed with the larger end if it didn’t unravel. Of all the rope diameters tested, 5/16” is the only size that consistently broke below the threshold of 1700 lbs., especially with the addition of a knot. While many of the larger diameter ropes broke below 1700 lbs. with some regularity when fished with a knot or splice, the type of knot or splice impacted the results. In order for larger diameter rope to be considered as a weak point, specific knotting and splicing techniques would need to be identified to ensure the rope would consistently break at or below 1700 lbs.

Statistical analyses suggest that rope diameter, modifications to the rope such as splices and knots, and age or seasons fished all have a significant effect on the breaking strength of the vertical line. It should be noted that the relationship between the diameter of rope and the breaking strength stands regardless of material used since all types of ropes used in the lobster fishery are represented in the dataset.

Weak Point Workshop In July 2019, DMR held a small workshop to start testing various configurations of vertical line connections that might meet the requirements of an approved weak point. Several fishermen came with ideas that they wanted to test.

Most lobstermen use knots and splices, or a combination of both, to rig vertical lines. Less than 5% of respondents do not modify vertical lines ("clear").

More than twenty configurations were initially tested. A subset that showed promise of breaking below 1700 lbs. was chosen for further testing by DMR. Those test results formed the basis for initial discussions with the industry to-date, including certain attachment types in 3/8” rope , a knotted or spliced-in piece of 5/16” rope, and the development of a manufactured weak point, such as a dog bone, that can be inserted into the line. The manufactured weak point was brought forward by industry members interested in a solution that, unlike rope, will not degrade in strength over time.

Future Work DMR is continuing to work with collaborators and the fishing industry to find and document options for weak points that can be integrated into existing vertical lines. The next steps in this work will include testing any ideas brought forward by the industry to either understand the breaking strength of their existing gear or to experiment with ways (such as different types of knots and splices) to reduce the strength to below 1700 lbs.

In conjunction with developing a suite of options that achieve a 1700-pound weak point, DMR will continue to test the weight of hauling loads to account for fisherman safety while fishing with weak points. Secondly, DMR is securing funding to work with manufacturers to develop an in-line option for a 1700lb weak point that can be field tested by the industry ahead of the implementation of any new regulations.

Lobstermen are encouraged to share ideas for weak points by contacting Erin Summers at DMR through email at or by phone at (207) 633-9556. DMR will test these weak point ideas to ensure a variety of options are available.


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