2019 Northeast Trawl Advisory Panel Wingspread Research

Overview

Consistently fishing a trawl survey net is essential to creating a consistent data time series. Optimal width is 13 meters, but it can vary with depth, which affects net efficiency. Net efficiency is a measure of how effectively a net captures animals in its path. This study investigated how differences in wingspread at different depths might influence our index time series.

Research Questions

• Does wingspread (horizontal net opening size) affect the catch of flatfish species?
• Do you catch more fish with a wider net?
• Does wingspread influence catch efficiency for flatfish species?
  • Do fewer flatfish escape under the gear with an optimal spread? 
  • Is tension just right at 13 meters?

Project Goal

To quantify species and length of fish caught, and to compare catch at various trawl net wingspreads in shallow and in deep water.

Project Description

The Northeast Fisheries Science Center 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. The Karen Elizabeth can fish a twin-trawl rig (towing two trawl nets at once), providing a great platform for gear comparison studies. 

Study Methods

During the experiment, the wingspread of one net was maintained at 13 meters for every tow as the control for the experiment. The wingspread of the other net was the “treatment” for the experiment, and was varied between 9 and 16 meters between tows.

The experiment consisted of 170 paired tows over 14 days (two seven-day trips) that targeted four species: winter flounder, witch flounder, windowpane flounder, and American plaice. The first seven-day trip was in the Gulf of Maine, targeting stations in deeper water. The second seven-day trip was in Southern New England targeting shallower stations. These stations occur at different depths, and  were targeted to ensure that sufficient numbers of the target species were captured.

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An exploratory data analysis was performed after the first seven-day trip to provide rapid feedback to the captain and field team as they planned the second trip. Subsequently, a length-based hierarchical generalized additive model was used to analyze the data.

Results

• The results of this experiment suggest that varying wingspread in the bottom-trawl survey net does not have a significant impact on how efficiently the net captured the flatfishes we targeted. 
• Flatfishes are the most likely species to be impacted by the changes in wingspread because they stay close to the ocean’s bottom  and can swim under the ground gear, avoiding the net.
• There was an impact of wingspread on the total catch (not efficiency). Specifically, an increase in catch was observed with an increase in wingspread. This suggests that it may be important to use the swept-area measured on a given survey tow rather than an average or ideal width.

Using Experimental Results

Northeast Trawl Advisory Panel recommended that observed area swept be incorporated into calculations of the catch per unit effort based on NEFSC bottom trawl survey data.

This information was available to assessment scientists in the spring of 2021, for consideration in  stock assessments. Starting in 2022, the Population Dynamics Branch’s Oracle-based system called StockEff (formerly ADIOS) will use a measured (observed) "swept area" instead of a standard (average) tow area for expanding catch whenever this data is available. Stock assessment leads use StockEff to generate inputs for assessment models and track decisions about how assessment data is compiled.

Collaborators/Partners

Northeast Trawl Advisory Panel

The panel includes New England and Mid-Atlantic Fishery Management Council members, commercial trawl fishermen, academic and industry gear experts, leaders of other regional surveys, state scientists, and the Center staff assigned to work with this important panel.

Salt Pond Fisheries - Christopher Roebuck (owner)

Contact Information

Project/Principal Investigators 

• Terry Alexander, NEFMC member
• Vincent Balzano, NEFMC member
• Tony DiLernia, MAFMC member
• Wendy Gabriel, NEFSC scientist
• James Gartland, MAFMC scientist
• William Gerencer, NEFMC stakeholder
• Vito Giacalone, NEFMC stakeholder
• David Goethel, NEFMC stakeholder
• Dustin Gregg, MAFMC scientist
• Pingguo He, NEFMC scientist
• Michael Luisi, MAFMC member
• Anna Mercer, NEFSC scientist
• Timothy Miller, NEFSC scientist
• Frank Mirarchi, NEFMC stakeholder
• Christopher Parkins, ASMFC representative
• Michael Pol, NEFMC scientist
• Philip Politis, NEFSC scientist
• Christopher Roebuck, MAFMC stakeholder
• Robert Ruhle, ASMFC representative
• Michael Sissenwine, NEFMC scientist

Project Team

• NTAP Panel 
• Salt Pond Fisheries, Captain/Owner Chris Roebuck and crew
• Field Staff
  • Dominique St. Amand, NEFSC scientist
  • Jack Wilson, NEFSC scientist
  • Tyler Pavlowich, NEFSC scientist
  • Calvin Alexander, NEFSC scientist
  • Chris Parkins, NTAP member/Rhode Island Department of Environmental Management
  • Paul Kostovick, NEFMC scientist
  • Giovanni Gianesin, NEFMC scientist
  • Jill Price, NEFMC scientist 
• Analysts
  • Andy Jones, NEFSC scientist
  • Tim Miller, NEFSC scientist 
  • David Richardson, NEFSC scientist

Publications/Reports

Experimental assessment of the effect of net wing spread on relative catch efficiency of four flatfishes by a four seam bottom trawl

You Can Learn A Lot by Towing Two Nets at Once

Wingspread Study Hits Silver Dollar Jackpot

2019 Net Spread Study Targets Flatfish, Reveals Subtle Differences

Data

Contact Phil Politis

Return to Improving the Bottom Trawl Survey for the Northeast.