Benchmark stock assessment completed this year estimates more than 12 billion oysters are in Maryland’s waters of the Chesapeake Bay
Oysters sit in a bucket after being measured by Department of Natural Resources staff as part of the annual fall survey, a major data source for the stock assessment that found an increase in Maryland oysters. Photo by Joe Zimmermann, Maryland DNR
The population of Maryland’s oysters has grown significantly in the past 20 years, according to the results of the latest benchmark stock assessment for the species.
The Maryland Department of Natural Resources and the University of Maryland Center for Environmental Science released the summary results of the stock assessment on Monday.
“Good news for oysters is good news for the Chesapeake Bay,” DNR Secretary Josh Kurtz said. “This stock assessment shows that oysters have made important progress during the past two decades. That’s a testament both to our continued investment in oyster restoration and our careful management of the oyster fishery. These findings will help guide management decisions during the next several years.”
The stock assessment analyzed the status of the population of eastern oysters, the keystone species that also holds important economic and cultural value for the Chesapeake Bay region, from 2005 to 2024. After a low point in 2005, shortly after oysters had been decimated by disease, the mollusks have increased in the following two decades.
The stock assessment’s summary results give insight into multiple aspects of the oyster population and oyster fishery, with the full report scheduled to be released by DNR in June. As overall oyster abundance increased in Maryland’s portion of the Chesapeake Bay, researchers found positive population growth at many oyster restoration sanctuaries as well as decreased fishing pressure in harvest areas compared to the previous stock assessment completed in 2018.
Overall oyster abundance has increased
The stock assessment estimated that more than 12 billion oysters live in Maryland’s waters of the Chesapeake Bay in 2024, including about 7.6 billion adult oysters and over 5 billion spat, or juvenile oysters. The assessment estimates there were only 2.4 billion adult oysters in 2005.
Mike Wilberg, a professor at the University of Maryland Center for Environmental Science who led the assessment, said the increase is likely driven by three main factors.
“The first one is that we have had some good spat sets,” he said. “The second one is that natural mortality rates, or particularly disease, hasn’t been as bad as it was in the 1980s and 1990s. And then the last one is that the department has maintained restrictions on harvesting that have allowed the oysters that are in some of these areas to continue to survive and reproduce and provide habitat and do all the things that we want them to do.”
The 2025 stock assessment estimated Maryland’s overall oyster population over time, showing a progressive increase from 2005 to 2024. DNR/James Point Du Jour
The 2025 stock assessment is the second major stock assessment conducted on the eastern oyster in Maryland. The state’s Sustainable Oyster Population and Fishery Act of 2016 requires DNR and UMCES to conduct a benchmark stock assessment every six years, as well as update assessments in intervening years. Benchmark stock assessments take account of available data from multiple sources to arrive at mathematical models for the status of fishery.
A major source of data is DNR’s fall oyster survey, which monitors the oyster population at sites across Maryland’s waters of the Chesapeake Bay every year. In 2005, the survey began collecting additional data in order to acquire a measure of oyster density that can be expanded to estimate the number of oysters in larger areas of known oyster habitat throughout Maryland’s portion of the Bay. That change in methodology is why the 2025 stock assessment focused on the years after 2005, Wilberg said.
The 2025 stock assessment also considered data from Maryland’s Bay bottom survey, sonar surveys, plantings, patent tong surveys, harvest reports, peer-reviewed studies, and even the state’s “Yates survey” that mapped oyster bars starting in 1906.
The latest stock assessment found a considerable increase in oyster abundance from the 2018 stock assessment, because of both continued growth of the population and improved methods to analyze the population, such as the additional dredge data, Wilberg said.
An independent peer review panel evaluated the 2025 stock assessment and approved of the methodology and findings.
The stock assessment analyzed oyster populations by specific oyster harvest reporting regions. In each region, it analyzed two important measures: oyster abundance and fishery pressure. In terms of abundance, there are three important benchmarks: 1) a long-term abundance target that represents the goal for oyster populations in the years ahead; 2) a cautionary level that indicates the need for vigilance in these areas; and 3) a lower limit below which oyster populations are depleted.
Results of the assessment indicate that one area was above the long-term abundance target, 24 areas were between the long-term target and cautionary levels, three were between cautionary levels and the lower limit, and seven were below the lower limit.
“The oyster population has been increasing across most of the Bay,” Wilberg said. “There are only a few areas where we have had decreases over the last five years. That’s a good sign from my perspective.”
Wilberg also noted that, while most areas are not above long-term abundance targets, those goals “really are long-term.”
“It may take decades or longer for the population to get back up to those levels,” he said. “This is where we want the population to get to, but we’re not there yet.”
Chris Judy, director of DNR’s Shellfish Division, said the progress on oyster abundance is encouraging, while ongoing and planned restoration work should help long-term efforts.
“Maryland has many large-scale projects underway and more to come,” Judy said. “Most recently we developed and initiated the Eastern Bay project to restore oysters in that region for the three key oysters sectors together: sanctuaries, fishery, and aquaculture. With such commitments now and in the future, we look forward to more progress.”
Decreased fishing pressure
Stock assessments help to determine fishing mortality reference points that help guide future management of a fishery. These reference points show the fraction of the population that can be harvested in an area while still achieving sustainability goals for the abundance of a species.
Looking at the 2023-2024 harvest data, the assessment found that 29 areas were below the target fishing mortality, meaning these areas were considered to have low fishing pressure. Two areas were between the target and threshold mortality reference points, and four were above the limit, meaning that oyster populations in those areas may not grow over time and could begin to decline.
While some areas are still seeing high fishing pressure, the results are an improvement over the 2018 stock assessment, which found that 19 areas were above the fishing mortality limit.
“This indicates that the fishery is in a better condition than it was when we did our last benchmark in 2018,” Wilberg said. “Overall things are looking better, and a lot of the credit for that goes to DNR for changing aspects of management in response to the last stock assessment. I think those changes have been beneficial in helping to better achieve the targets.”
The stock assessment assessed fishing pressure on oysters across the Bay, finding 29 out of 35 areas (in green) with low fishing pressure, two areas between target and limit (in yellow), and four areas above the limit (in red). DNR
The previous stock assessment quantified the abundance measure and harvest rate measure for oyster management, for each major harvest region, Judy said. With those figures as a framework and guide, the department has adjusted harvest rules in response.
Continued growth at restoration sanctuaries
The stock assessment does not directly analyze the large-scale oyster restoration sanctuaries, as these are monitored in a separate initiative, but it does provide insight into the status of these areas that have been a major focus for efforts to restore the oyster population in the Chesapeake Bay.
Large-scale restoration sanctuaries make up substantial portions of several harvest reporting regions, so the growth of oyster populations in those areas can largely be attributed to growth in the sanctuaries, Wilberg said.
At two sanctuaries, the stock assessment found a fivefold increase in oyster population since oyster restoration efforts began after 2010. In the Little Choptank sanctuary, an estimated 100 million adult oysters pre-restoration increased to 500 million, while a population of 40 million in the Harris Creek sanctuary in 2010 grew to 200 million after restoration was completed. In the Tred Avon River sanctuary, the population increased by about four times, from 40 million to 175 million.
“The three major restoration sanctuaries that are pretty much finished have all been really strong successes,” Wilberg said. DNR and partner organizations have heavily planted oyster spat at these sanctuaries to boost their populations.
The effects of the two more recently completed large-scale sanctuaries, St. Marys River and Manokin River, on oyster abundance will likely take more time to become apparent.
These results come with caveats—the data does not define how much of the increase is due to direct restoration efforts (intensive spat plantings) compared to favorable environmental conditions, and the stock assessment can’t determine how much population growth in sanctuaries is contributing to a Bay-wide increase in spat sets. But Wilberg said it does indicate that “overall they’ve been very successful in bringing oyster abundance back up” in the large-scale sanctuaries.
Eastern oysters declined to a fraction of their population in the Chesapeake Bay since the 1800s, due to historical overharvesting, disease-related mortality, habitat loss, and water degradation. About 75% of the oysters in the Bay died of disease mortality between 1999 and 2002.
“From such low points as this, oysters have made significant progress,” Judy said.
DNR will propose its recommended harvest rules for the upcoming oyster season based on the results of the stock assessment, the fall survey data, and other considerations at the Oyster Advisory Commission meeting June 9.
