Bottom Trawl Restrictor Rope Study

Project Overview

Offshore wind energy development has the potential to play an important role in U.S. efforts to combat the climate crisis and build a clean energy economy. 

The vast majority of U.S. offshore wind development so far has occurred in the Atlantic Outer Continental Shelf. This new use of marine waters requires substantial scientific exploration, regulatory review, and monitoring. This will ensure that new projects are compatible with other ocean uses, including commercial, recreational, and tribal fishing.

Under offshore wind energy development lease agreements with the Bureau of Ocean Energy Management, offshore wind companies are required to conduct fisheries surveys. These surveys evaluate how operations and development in their lease areas will impact fishery species. With a number of different companies leasing offshore areas, that could mean different surveys happening in different ways, at different times, with different gear or gear configurations. To ensure the data they're collecting are comparable and can be combined with existing regional survey data, we need a consistent sampling program. This would mean each company uses the same standardized gear configuration during their bottom trawl surveys.

The Northeast Trawl Advisory Panel has identified the use of a restrictor rope as a possible way to standardize trawl gear configuration, since it controls how wide a net spreads underwater. In the past, we haven’t used restrictor ropes in scientific bottom trawl surveys. This study is evaluating the use of a restrictor rope on industry-based bottom trawl surveys to determine if and how it may impact catch. If the restrictor rope does not have a significant impact on catch, it could be a way to standardize surveys conducted by offshore wind companies.

Project Goal and Research Questions

We want to compare bottom trawl survey gear that use and don't use a restrictor rope, To do that, we designed a collaborative study to address the following questions: 

• Does using a restrictor rope affect the catch composition when added to a trawl configuration?
• Does using a restrictor rope affect the catch rate of different species when added to a trawl configuration?
• Does using a restrictor rope affect the size distribution of catch when added to a trawl configuration?

Study Methods

In the spring and fall of 2022, we worked alongside members of the panel, scientists from the Virginia Institute of Marine Science, and the captain and crew of the F/V Darana R to conduct restrictor rope experiments. 

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Vessel and Gear

The Darana R is a commercial fishing vessel operated by Captain Jimmy Ruhle, son and first mate Robert Ruhle, and crew. This vessel is also used by the Virginia Institute of Marine Science to conduct the Mid-Atlantic Northeast Area Monitoring and Assessment Program. The program is an integrated, cooperative state/federal fisheries-independent data collection program. The Darana R is 90 feet long and used the program’s 4-seam, 3-bridle net for this study. For the restrictor rope, we used a 7/16 inch, high-strength, low-stretch, single-braid polyester Tenex rope. Viking hooks attached the restrictor rope to each of the trawl doors. 

Location

We conducted our experiments in Rhode Island Sound and Block Island Sound off the coast of Rhode Island. We chose this location for several reasons, including the consistent depth and bottom type, so that we would have consistent trawl performance during our experiments. This is important so that we can effectively evaluate the effect of the restrictor rope on catch. We used a stratified random sampling design to choose tow-specific locations.

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Experimental Design

To address our research questions, we conducted a series of paired tows, each lasting 20 minutes. Each pair of tows comprised two tow types: control tow (no restrictor rope) and a treatment tow (with restrictor rope). At each location within our study area, we conducted control and treatment tows within 400 meters of each other. To reduce potential biases from tow-type order, we switched the order between pairs. We conducted 71 paired tows over 2 weeks in 2022, one week in the spring and one week in the fall. 

Data Collection

For each tow, we collected:

• Tow metrics: GPS location, direction, tow speed, tow depth
• Gear metrics: door width, net width and height, warp length 
• Catch: fish species, counts, lengths, total weight by species
• Hydrographic: water temperature, salinity, dissolved oxygen
• Atmospheric: air temperature, wind speed and direction, barometric pressure, sea state, general weather

Data Analyses and Results

Analyses are underway to determine if the restrictor rope affected the amount, type, and size of fish caught. In the winter of 2022 and spring of 2023, scientists from Virginia Institute of Marine Science and our science center presented their preliminary research findings to the panel. The panel made recommendations to include more variables in the analysis and to include clear interpretations of results in the final paper. This study will be published and help inform discussions about future restrictor rope studies.

Project leads

• Anna Mercer, chief, Science Center Cooperative Research Branch and NTAP panel member
• Captain Jimmy Ruhle
• First mate Bobby Ruhle 
• Crew of the Darana R
• James Gartland, Virginia Institute of Marine Science associate research scientist and study analyst
• Dustin Gregg, Virginia Institute of Marine Science marine scientist, chief of trawl operations
• Andy Jones, Science Center fisheries biologist and study analyst

Collaborators/Partners

• Members of the Northeast Trawl Advisory Panel (PDF, 1 page)