Improving the Bottom Trawl Survey for the Northeast
Highlighting what the Northeast Fisheries Science is contributing to Northeast Trawl Advisory Panel and how we are connecting the advisory panels advice with our science planning, projects, and products.
Our fisheries research surveys are crucial to understanding the overall status of commercially and recreationally important fishery stocks and predicting how the status of these stocks changes over time. At the Northeast Fisheries Science Center (NEFSC) we value the expertise of industry and other stakeholders. We look for ways to collaborate with others to make our science better.
This is why we are active members of the Northeast Trawl Advisory Panel (NTAP), an industry advisory panel set up by the New England and Mid-Atlantic Fishery Management Councils. The panel includes commercial trawl fishermen, academic and industry gear experts, leaders of other regional surveys, state scientists, and the Center staff.
For more on Northeast Trawl Advisory Panel, group meetings, and a full list of members, visit the NTAP site hosted by the Mid-Atlantic Fishery Management Council.
NTAP Fact Sheet (PDF; 2 pages): Download File
Northeast Trawl Advisory Panel Goals
- Understand NOAA/NEFSC trawl survey gear performance and methodology
- Evaluate ways to complement or supplement regional research surveys
- Improve understanding of NOAA/NEFSC trawl survey data quality and results
Northeast Trawl Advisory Panel Bottom Trawl Survey Working Group
A current focus area for the Northeast Trawl Advisory Panel is the Northeast Fisheries Science Center's multispecies bottom trawl survey. A technical working group was formed in March 2018 to look at how the our survey trawl wingspread varies and how that variability affects catch rates, survey indices, and stock assessments in general. This working group was also asked to look at options for joint field research to better understand the potential effects of wingspread on catch rates. We continue to work through this group and within the larger advisory panel to evaluate and improve the survey and the use of the survey data in fisheries stock assessments.
To work effectively with the Northeast Trawl Advisory Panel, our science center has formed an internal team. Team members come from our survey, stock assessment, cooperative research, and leadership groups.
Current NTAP Projects
- What is Acceptable Tolerance for Wingspread?
- What Gear Configuration will Keep Wingspread within that Tolerance?
2019 Experiments and Field Work
2019 Flume Tank Demonstration: In July, a model of the R/V Henry Bigelow bottom trawl survey net was deployed in the flume tank at Memorial University in Newfoundland. The goal was to better understand how the net’s wingspread affects gear performance. NTAP members gathered at the School of Marine Science and Technology in Dartmouth, Massachusetts to watch the deployment over a live video feed from the flume tank. Flume tank operators tested the model’s performance at optimal spread, as well as at narrower- and wider-than-optimal spreads. They measured and recorded wingspread, headrope height, and water flow for each experimental treatment. Tank operators also captured photographs to get paired measurements and visualizations.
2019 Net Spread Experiment: The NEFSC contracted with Salt Pond Fisheries to charter the F/V Karen Elizabeth for 14 days in September 2019 to evaluate catch efficiency of the standard NEFSC survey trawl at various wingspreads and tow depths. Wingspread is how widely the net is open. The objectives were to quantify species and length of fish caught, and to compare catch at various wingspreads in shallow and in deep water. The Karen Elizabeth can fish a twin-trawl rig (towing two trawl nets at once), providing a great platform for gear comparison studies. The experiment was modeled after the chain-sweep catchability study conducted by NTAP from 2015-2017. It was a 24-hour operation with day and night treatments. During the study, net wingspread affected total catch. Catch increased as the swept area increased. “Swept area” is the horizontal area fished by the net when it is open.There was no evidence, however, of a wingspread effect on catch efficiency -- how completely the net captures all the fish in its path -- for any of the target species. These results suggest that incorporating actual observed swept-area in survey biomass estimates is an important next step for this long-running survey. However, we may not need to incorporate efficiency conversions based on the width of the net for particular species or fish lengths.
Door Trials Henry Bigelow: In close consultation with the Northeast Trawl Advisory Panel, the NEFSC has been working on achieving consistent net spread with the survey's trawl gear in shallow water aboard the NOAA Ship Henry Bigelow. The goal is to find a door that achieves an optimal net spread without needing extreme amounts of wire, and which provides consistent wingspread performance over the range of depths sampled during the NEFSC’s biannual bottom trawl survey. In 2019, 8 sea days were set aside in August for door testing. NTAP members had the opportunity to sail and observe door performance. In 2018, scientists completed approximately 24 hours of testing for 66" Thyboron Type IV doors during calibration work leading up to the annual autumn bottom trawl survey.
Using Experimental Results
Understanding stock assessment model accuracy
In 2017, scientists updated assessments for 19 Northeast groundfish stocks were updated. They used the results of the 2015 and 2016 catchability experiments to check the accuracy of the bottom-trawl survey biomass estimates for flatfish species in general, and to develop specific approaches for Georges Bank yellowtail flounder and witch flounder. In 2019, assessment scientists used the experimental results to compare the bottom-trawl survey biomass estimate accuracy for Cape Cod and Gulf of Maine, and Southern New England and Mid-Atlantic yellowtail flounder, Georges Bank winter flounder, and American plaice stocks.
Setting catch advice when models are not enough
Sometimes analytical models do not provide a clear enough picture of stock status to develop catch advice so scientists must find alternative methods. These methods use survey indices and commercial catches to provide catch advice. Every stock assessment includes this type of method as an alternative plan. The catchability estimates have been incorporated into these plans for all windowpane and witch flounder stocks.
Direct input to an assessment model
NTAP’s chainsweep efficiency research conducted on the F/V Karen Elizabeth in 2015-2017 identified significant differences in catch efficiency of flatfish, red hake, and skates between chain-sweep and rock hopper gear. In 2018 assessment scientists used results from this research directly in analytic models to convert catch data from the bottom-trawl survey into a population estimate for summer flounder.