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Stock Assessments Benefit From Rockhopper Trawl Efficiency Study

March 18, 2024

Understanding how efficiently our survey gear captures bottom-dwelling species is a key improvement.

These gear efficiency studies used a twin trawl vessel that conducted simultaneously side-by-side tows using two identical nets with different sweeps; a rockhopper (depicted on the left) and a chain (depicted on the right) to estimate relative catch efficiency. The twin trawl was rigged with two nets with different sweeps; rockhopper on the left and chain on the right. Photo: NOAA Fisheries

A collaborative study conducted aboard a Rhode Island-based commercial trawler from 2015 to 2017 is bearing fruit. It was led by the Northeast Fisheries Science Center and commercial fishermen interested in how catches of the same species varied depending on the type of trawl net used. Since 2019, stock assessment scientists have used the results from this Northeast Trawl Advisory Panel study in multiple stock assessments to ensure sustainable fisheries for several flounder species as well as monkfish and red hake. Specifically, the results help produce more accurate estimates of abundance which can increase the confidence in catch advice for some species.

“Cooperative research is essential to obtain accurate assessments and catch advice in our nation's fisheries,” said Chris Roebuck, former Northeast Trawl Advisory Panel member, commercial fisherman and participant in the experiment.

“This research is an excellent example of collaboration between scientists and fishermen. It provided solid efficiency numbers for each targeted species,” he said. “The information produced was clearly the best available science and has the potential to ultimately influence catch advice for every species evaluated.”

“Any piece of data from a new study used by a stock assessment helps create a more accurate tool,” agreed Terry Alexander, a commercial fisherman and panel member, “This data is especially important for assessments using an index-based model.”

The Northeast Trawl Advisory Panel was instrumental in planning this collaborative study.

More Accurate Catch Indices Developed

There are a range of stock assessment models. Catch indices, a measure of catch per net tow, are used in almost all of them. The indices have more influence on the models when there is little other data on a species.

In the Northeast, we develop indices for species captured in our long-running bottom trawl survey. Over time, the indices provide information on trends in a fish stock’s population. The trawl gear we use is more efficient at catching some species than others because it rolls over the bottom, disturbing less habitat and snagging less often. For example, it is less efficient at catching species that live near the ocean bottom such as flounders.

The results of this study measured how much the catch differed when the research net stayed closer to the bottom to better capture flounders and some groundfish. Stock assessment scientists were then able to adjust the data they obtained using our standard research trawl to account for that difference. This produced more accurate indices for some species.

In the study, two of our research survey trawl nets were fished side by side. Each net had a different sweep—the part of the gear that contacts the bottom while fishing. One net was rigged with our standard rockhopper sweep used in the Bottom Trawl Survey, and the other with a chain sweep commonly used by the fishing industry.

Overhead schematic of the twin trawl study under tow with critical gear numbered and defined in caption.
Diagram of the collaborative study’s gear configuration. 8: One of the two nets is rigged with a rockhopper sweep and 7: the other is rigged with a chain sweep and for both 5: a restrictor rope is used to obtain consistent net spread. The other important components are: 1: the side wires, 2: middle wire, 3: doors, 4: the clump weight, and 6: the acoustic mensuration system. The side where the rockhopper and chain sweep gears were deployed varied throughout the experimental tows.

How Are Data from this Study Improving Stock Assessments?

Both the monkfish and Gulf of Maine winter flounder assessments use index-based models and have benefitted from this study.

The monkfish population appeared to be trending downward in the last assessment based on Bottom Trawl Survey data. “When the catch efficiency estimates from the collaborative study were used in the assessment, we were able to show that relative exploitation rates were quite low, suggesting that fishing may not have been the primary driver of the declining survey trends,” said assessment scientist Jon Deroba. Fishery managers were able to consider this information when deciding on allowable catch alternatives.

Paul Nitschke is an assessment scientist and member of the New England Fishery Management Council’s Groundfish Plan Development Team. “Using these data in the Gulf of Maine winter flounder assessment gave the science team more confidence in the model’s catch efficiency and a much better estimate of catch,” he said.

Alex Hansel is another of our stock assessment scientists who also works on winter flounder. “Using the survey data in the flounder assessment has reduced uncertainty and increased confidence in abundance trends, allowing more accurate estimates of population biomass.”

In addition to monkfish and winter flounder, results from this study have been used in assessments for:

One question that emerged from the successful rockhopper catch efficiency study was how much the net spread impacts the catch. In 2019 we conducted at-sea experiments to investigate how differences in wingspread at different depths might influence the trawl survey catch and the derived indices.

In 2022, we studied using a restrictor rope with the net. The restrictor rope prevents overspreading of the net, but we need to better understand if using the restrictor rope impacts catch efficiency.

“The panel should capitalize on the momentum generated from the success of the efficiency experiments and work toward developing more research questions and studies that continually enhance trawl survey gear efficiency,” said Groundfish Sector manager and panel member Dan Salerno.


Last updated by Northeast Fisheries Science Center on March 19, 2024

Research and Data