The commercial groundfishing industry has long been a cornerstone of New England's coastal communities. For generations, fishermen have harvested species including Atlantic cod, haddock, and flounder, contributing significantly to local livelihoods and the food supply. NOAA Fisheries works to promote science-based management of these species, ensuring both the industry’s economic viability and the long-term health of marine ecosystems.
Why Weight Matters: Bridging the Gap for Accurate Assessments
Commercial fishery quotas and recorded catch are typically based on total fish weight, which is easy to measure in bulk. However, scientists use stock assessment models that require numbers of individual fish to accurately evaluate population trends and dynamics. To bridge the gap between total weight and number of fish, reported commercial catch weights must be translated into fish numbers using length-weight relationships and conversion factors derived from biological sampling data.
The length-weight relationship—a mathematical model to predict a fish’s weight based on its length—is a fundamental tool in fisheries science. Converting length to weight is more complex for species like Atlantic cod, which are typically gutted at sea. A critical step involves converting the fish’s dressed weight back to whole, or live, weight. This conversion is achieved by applying a ‘conversion factor’, which represents the ratio of the gutted weight to the whole weight.
"Conversion factors directly impact our estimates of commercial landings,” said Charles Perretti, a stock assessment analyst with NOAA Fisheries’ Population Dynamics Branch. “So getting those factors right is important. "Many of the existing conversion factors and length-weight relationships used for Northeast species are based on older data, some dating back to the 1960s. Over time, fish growth rates and body shape can change. Fishing practices also evolve along with technology and shifting market preferences. Stock assessment models benefit from up-to-date inputs including conversion factors and length-weight relationships.
Manny Magalhaes, captain of the F/V Hera, explained, “Accurate data is very important, especially when our quotas, and our futures depend on it.”
To address this, NOAA Fisheries launched the groundfish conversion factor project to re-evaluate both gutted to whole weight conversion factors and length-weight relationships for 16 groundfish stocks from 8 species.
On the Water and at the Dock: Industry's Role in Data Collection
The conversion factor project is a collaborative initiative. NOAA Fisheries, the commercial groundfish industry, and the Massachusetts Division of Marine Fisheries are working together to collect the necessary data. Data collection began on October 1, 2024, and will continue for two years. The effort has two key parts:
At-Sea Sampling: NOAA’s Cooperative Research Branch deploys a field scientist on commercial fishing vessels, working alongside fishing crews to collect detailed measurements. The target species are Atlantic cod, haddock, and Atlantic pollock. Scientists measure and weigh fish both before and after gutting. To capture variability among vessels, the scientist also records whether the gills were removed and whether the fish was gutted by hand or by machine.
Portside Sampling: Sampling also occurs at various ports in Massachusetts and Rhode Island in close collaboration with Massachusetts Division of Marine Fisheries and local fish dealers. This approach allows us to collect data from a significant volume of landed fish, providing a comprehensive understanding of regional fish stocks. Field scientists take into account fishing areas and the species caught when they select vessels for sampling, ensuring all stock regions are represented. Portside sampling targets a wider range of species than at-sea sampling, including:
- Atlantic Cod
- haddock
- Atlantic pollock
- American plaice
- witch flounder
- winter flounder
- yellowtail flounder
- summer flounder
Researchers measure individual fish lengths and weights and collect otoliths, or ear bones, which they use to determine a fish’s age.
The project has already made significant progress, with over 1,700 fish sampled at-sea for conversion factor data and more than 3,900 fish sampled portside for length-weight data. We share quarterly progress reports with our fishing industry partners, providing preliminary data observations and soliciting their feedback.
Understanding Seasonal and Regional Changes
This project aims to understand how length-weight relationships and conversion factors may vary seasonally and geographically. Factors impacting these numbers include changes in a fish’s diet because of shifts in prey availability, temperature changes that impact a fish’s metabolism, and a fish’s spawning cycle.
Our collaborative sampling effort captures seasonal fluctuations by sampling consistently throughout the year and recording the sex and maturity stage of the gutted fish.
"Our Bottom Trawl Survey operates in the spring and the fall, but industry members are out there all year, so this cooperative project gives us access to samples we couldn't otherwise obtain," explained Liz Brooks, also a research analyst with NOAA’s Population Dynamics Branch.
By organizing the data into shorter timeframes, such as quarterly periods, seasonal trends become more evident. This detailed approach ensures stock assessments are accurate and dynamic, reflecting the actual conditions of fish populations throughout the year and across areas.
Better Data for Better Management
The conversion factor project highlights the value of cooperative research and industry engagement in the scientific process. Stock assessment scientists will use the data collected for more precise estimation of fish caught and the size of groundfish stocks. This will provide a clearer picture for fisheries managers, support more effective and responsive management decisions, and promote the long-term sustainability of groundfish populations and the industry that relies on them.
“It's great to be able to partner on this initiative,” said Magalhaes. “When scientists and the fishing industry work side-by-side, we all win.”
Getting Involved
We thank all of our partners for supporting this project! We are always interested in collaborating with new partners. If you would like to learn more or are interested in participating in this project, please contact the project lead, Katie Burchard.
