New Study Offers Improvements to Estimating Shellfish Populations

HATTIESBURG, MS / ACCESSWIRE / March 15, 2022 / 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.

Ocean Quahog. Source: www.scemfis.org

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.

About SCEMFIS

SCEMFIS utilizes academic and fisheries resources to address urgent scientific problems limiting sustainable fisheries. SCEMFIS develops methods, analytical and survey tools, datasets, and analytical approaches to improve sustainability of fisheries and reduce uncertainty in biomass estimates. SCEMFIS university partners, University of Southern Mississippi (lead institution), and Virginia Institute of Marine Science, College of William and Mary, are the academic sites. Collaborating scientists who provide specific expertise in finfish, shellfish, and marine mammal research, come from a wide range of academic institutions including Old Dominion University, Rutgers University, University of Massachusetts-Dartmouth, University of Maryland, and University of Rhode Island.

The need for the diverse services that SCEMFIS can provide to industry continues to grow, which has prompted a steady increase in the number of fishing industry partners. These services include immediate access to science expertise for stock assessment issues, rapid response to research priorities, and representation on stock assessment working groups. Targeted research leads to improvements in data collection, survey design, analytical tools, assessment models, and other needs to reduce uncertainty in stock status and improve reference point goals.

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SOURCE: Science Center for Marine Fisheries