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Design of a proposed research survey bottom trawl to conduct
standardized resource surveys on a newly designed research vessel.

Trawl Parameters
 Maintain a consistent bottom contact over a speed range of 3.0 to 3.8 knots.

 Maintain a headrope height of 4.5 - 5.5 meters.

 Utilize interchangeable sweeps, one for good bottom and one for rough bottom
trawl schematic
Comments Requested

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For complete trawl specifications: Click Here   (850k PDF File Get PDF Viewer Here)

R/V Henry B. Bigelow - Currently Under Construction
Frequently Asked Questions

1. Why is the Center changing the gear that it uses to conduct trawl surveys?

We are getting an entirely new research vessel, the Henry B. Bigelow, one that is larger and more powerful that any we have used in the past. The Bigelow will eventually replace the Albatross IV, which has conducted the resource surveys since 1963. In addition, we are tracking many more species now than when the survey started in the early 1960s, and we hope to broaden our scope with a different net design.

2. Why is the Center asking for comments?

We want to add stakeholder expertise to our own experience of more than 40 years of research surveys in order to make the best possible decisions about the new design. We want to ensure that the gear and trawling procedures adopted will provide consistent information for a variety of species. To do so, we have been working closely with the Mid-Atlantic / New England Trawl Survey Advisory Panel on aspects of gear design, gear testing, and trawling procedures to be used in future surveys.

3. What is the Trawl Survey Advisory Panel?

The Trawl Survey Advisory Panel includes regional fishery management council members, industry gear experts, academic scientists, and Northeast Fisheries Science Center scientists. The group has been working since April of 2003, providing advice regarding trawl surveys.

4. Who is on the Trawl Survey Advisory Panel?

The Mid-Atlantic and New England Fishery Management Councils jointly appoint members. The Mid-Atlantic Council currently manages panel functioning and support. Panel membership in 2004 includes: Jim Ruhle, Bud Fernandez, Hank Lackner, Rodney Avila, Phil Ruhle, Jim Lovgren, Jim Odlin, Chris Glass, Joe DeAlteris, Eric Powell, Bill DuPaul, Mark Terceiro and Russ Brown.

5. How were decisions made concerning the initial net design presented on this website?

The Trawl Survey Advisory Panel determined the performance characteristics of a bottom trawl system (vessel, trawl warps, doors, bridles, ground cable, and trawl net) to be targeted for a multispecies bottom trawl survey. The committee used a variety of sources, including information gathered during a stakeholder workshop on trawl performance held in January 2003 at Woods Hole, MA, and a scoping session with industry gear experts from both the U.S. East and West Coasts.

6. Which aspects of the new design are you particularly interested in getting fishing professionals to look at and comment on?

Since research vessel surveys inherently involve towing relatively small trawls behind relatively large vessels, we are particularly concerned about balancing the fishing system (tow point width, trawl warps, doors, ground cable, bridles, and net) with the large size and horsepower of the research vessel. There are some key features in the vessel design including infinitely adjustable horsepower that help to offset the size and power of the vessel. We are seeking advice from experts in the commercial fisheries industry to provide recommendations about components of the fishing system to improve the consistency of performance.

7. Do you really expect to develop a net design that will maximize catchability for all of the species that are currently assessed by the Northeast Fisheries Science Center?

No, but we believe we can design a net that improves our catchability across a wider variety of species. The net that we are currently using provides excellent standardized abundance and biomass time series data for some species, and very low or relatively variable catchability for others.

8. Who actually generated the net design under consideration?

Three gear companies (Trawl Works, Superior Trawl and Reidar's Manufacturing) who participated in the scoping session worked jointly on a design to provide an initial net for testing. The Panel reviewed initial designs and selected this one for testing.

9. How will you evaluate the initial trawl system design?

We hope to evaluate the trawl system design through a series of field tests, flume tank work, and with computer modeling.

10. When and where will it be tested?

Initial field-testing will be conducted on the R/V Delaware II in October 2004. The Delaware II has the same warp diameter and construction as that recommended for the R/V Henry B. Bigelow. Flume tank work is planned at the Marine Institute in St. Johns, Newfoundland during the early part of 2005, and there are plans for additional field-testing during the Spring and Autumn of 2005.

11. When will the design be finalized?

The Center hopes to have a trawl system design finalized by December 2005 so that it will be available for testing on the Henry B. Bigelow when it arrives. Currently, the vessel is expected in Spring 2006.

12. What aspects of the new net design are expected to improve the performance of the trawl survey?

Improvements in bottom contact and headrope height are two promising areas.

13. What are the improvements for bottom contact?

The current bottom trawl survey tows a relatively light net at a relatively high speed, and bottom contact consistency is an issue, particularly under marginal weather conditions. The new net design incorporates significantly heavier ground gear, and significantly reduces gaps where fish can escape through the footgear. In addition, we are planning to upgrade to a more modern door design that will deliver stable performance over a variety of depth and bottom types.

14.What are the improvements for headrope height?

The Yankee 36 trawl currently used has a relatively low headrope height, potentially resulting in size and species selectivity when fish are vertically distributed off the bottom. The new net design has a target headrope height that is approximately 2.5 times higher than the current Yankee 36 trawl.

15. Will a single design be used to conduct all surveys over all types of habitats?

Currently, our working model involves a single net design with two interchangeable sweeps. The basic design of the net is a four-seam, box-net design with three bridles. We intend to determine the best lengths of ground cables and bridles through experiments.

16. Aren't the ground cables and bridles too short in the initial design?

They will be short in comparison to those typically used in a commercial fishery operation where the design is intended to maximize catch, by fishing the largest gear possible from the vessel and achieving wide door-spreads in order to herd as many fish as possible into the trawl. In contrast, a research vessel survey tow is trying to maximize catch of those fish that are actually encountered between the wing tips, while minimizing catch that occurs as a result of herding fish encountered outside of the wing tips.

17. Why is the net so small? The research vessel Henry B. Bigelow is capable of towing a much larger net.

Basically, we are designing the smallest gear that can be fished in a consistent manner and still obtain a representative sample of the fish and invertebrates that are encountered. We are not trying to catch the most fish that the vessel's power would allow. Utilizing a larger net that generates larger catches does not necessarily generate improvements in data quality as long as representative samples are obtained.

18. Don't you want larger catches?

No, we want consistently comparable catches, from which we derive a whole series of measurements and samples collected from fish and invertebrates captured on each tow including lengths, weights, age samples (scales, otoliths, spines, vertebrate), and other data and samples.

19. What happens when you get a large catch?

The time required to sort and process the catch increases, which adds to the time required to conduct the work-either increasing the cost or decreasing the amount of geographic area we can cover. Also, large catches must be subsampled to estimate what has actually been caught during each tow. The combined effects potentially increase error associated with both sampling (reducing the number of stations) and subsampling.

There are other reasons to take only the fish we need to get a good sample. First, we do not want to waste fish. Second, whatever is caught by the research effort is no longer available for commercial and recreational harvest. Finally, research vessel and cooperative research project catches may eventually be counted against quotas and TACs for species where these are used as a management tool.

20. Can you compare survey landings from an entirely new vessel and net to those in the past?

Yes, as long as a series of experiments are conducted to calibrate the catchability of the nets occurring before and after the change.

21. How important is it to keep the trawl survey data series comparable over time?

Very important; it is one of the longest of its kind in the world and quite an asset to scientists in the Northeast.

22. Have you been able to keep the date set consistent over time with the existing net?

Yes, but it has proven increasingly complex and expensive to do so as components of the gear are no longer manufactured and substitutions must be made. Standardizing catch rates across time is critical, essentially meaning that if the abundance of fish 20 years ago is approximately the same as the abundance of fish today, the catch rates in the survey should be approximately equal.

23. How many catchability comparisons have there been in the resource trawl survey gear since 1963?

The Northeast Fisheries Science Center has conducted three calibration experiments: an experiment to compare the catchability of the Yankee 36 to a Yankee 41 trawl that was used for a period of years in the Spring multispecies bottom trawl survey, an experiment to compare the catchability of Oval BMV doors used previous to 1985 and Portuguese Polyvalent doors utilized beginning in 1985, and a long-term study comparing the catchability between our two primary research vessels, the Albatross IV and Delaware II.

24. How can I assist with designing and testing the net design to be used for future resource surveys?

The Northeast Fisheries Science Center and the Trawl Survey Advisory Panel want to hear from those who share our interest in optimizing performance of survey fishing gear. You may provide comments on the current net design or the process that we are using by the following methods:

Russell Brown
Net Design Comments
Northeast Fisheries Science Center
166 Water Street, Woods Hole, MA 02543
Fax: 508-495-2258
Please use "Net Comments" as the subject of your email





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