March 14, 2022 — The following was released by the Science Center for Marine Fisheries:
A new study, jointly conducted and funded by the Science Center for Marine Fisheries (SCEMFIS) and the National Marine Fisheries Service (NMFS), finds ways to reduce a source of uncertainty in the Atlantic surfclam and ocean quahog population estimates: defining the efficiency of the survey gear. The study, published in Fishery Bulletin, is part of the Center’s long-running efforts to improve our understanding of shellfish.
The dredge efficiency issue, which dates to the early 1990s, is about how effective clam research survey vessels are at catching surfclams and ocean quahogs. The efficiency of a clam dredge—which is a measure of the proportion of Atlantic surfclams or ocean quahogs on the ocean floor that a dredge can catch—is one of the key variables in stock assessment population estimates. The efficiency of the dredge is a primary factor affecting the biomass estimate of the stock and therefore an important criterion determining how the assessment evaluates the status of the stock relative to management goals.
One way researchers measure efficiency is through field depletion experiments, where a dredge is run multiple times in a single area to measure the percentage of available shellfish it caught with each tow. These experiments are used in both the Atlantic surfclam and ocean quahog assessments to help estimate their dredge efficiencies. There is no way to know what true dredge efficiency is, but conducting many depletion experiments allows scientists to make a reasonable estimate.
The first depletion experiment was conducted in 1997 as a joint effort between NMFS, academic scientists, and the clam fishery. This team carried out many such studies over the following 15 years, making this one of the longest running successful collaborations of this type. This recent study addresses the remaining uncertainty surrounding dredge efficiency by looking at data from this large set of depletion experiments conducted over two decades and examines the quality of these experiments and the efficiency estimates they produced. Specifically, the study took a close look at simulated depletion experiments, where scientists know what the true dredge efficiency is, to identify common characteristics of studies that do a good job estimating dredge efficiency compared to those that do a poor job.
By focusing on the “best” experiments and identifying field experiments that potentially produced inaccurate efficiency estimates, scientists can refine the set of depletion studies used to help estimate efficiency in stock assessments. This refined set of depletion studies has particular influence on non-model-based biomass estimates, which play an important role in verifying the model-based results and are commonly used in management. Once identified, characteristics of good and bad depletion experiments can also inform methodology used in future experiments.
“Identifying field depletion experiments with accurate efficiency estimates helps confirm present stock assessment models’ gear efficiency estimates,” said Leanne Poussard, of the University of Southern Mississippi’s Gulf Coast Research Laboratory and the lead author of the study.
“Ms. Poussard’s work has rekindled interest in the cooperative depletion experiment dataset and produced important guidance for anyone undertaking similar experiments in the future”, said Dr. Dan Hennen of NMFS.