September 22, 2025 — 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.