



Consistently fishing a trawl survey net is essential to creating a consistent data time series.
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.
To quantify species and length of fish caught, and to compare catch at various trawl net wingspreads in shallow and in deep water.
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.
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.
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.
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.
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)
You Can Learn A Lot by Towing Two Nets at Once
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2019 Net Spread Study Targets Flatfish, Reveals Subtle Differences
Contact Phil Politis
Return to Improving the Bottom Trawl Survey for the Northeast.