Oysters measured during the 2024 fall survey. Photo by Robert Bussell, Maryland DNR.
Maryland 2025 - Oyster populations in the Chesapeake Bay are faring well, according to preliminary results of the Maryland Department of Natural Resources’ annual Fall Oyster Survey.
After a remarkable year for oyster reproduction in 2023, the survey’s 2024 spatfall intensity index, a measure of reproductive success and potential population growth for oysters, was above the 39-year median for the fifth consecutive year.
During the 2024 fall survey, the 2023 year class oysters, or spat, were now “smalls” and were abundant in many areas, boding well for both the fishery in the near term and for the Bay’s oyster sanctuaries. The once-in-a-generation spatset on the upper bars of the Potomac River appears to have survived the high freshwater flows of the winter and spring. Little mortality was observed, although their growth was stunted by lower salinities. Also, disease levels in Maryland have largely receded due to a wet start to 2024.
“The 2024 Fall Survey confirms Maryland’s oyster population is doing well, with the key indicators showing encouraging results,” said Christopher Judy, Director of the Department of Natural Resources Shellfish Division. “Certain low salinity regions still need to improve, but overall the findings are promising for the near-term future of this vital species.“
The 2024 fall survey was conducted from Oct. 8 to Nov. 25 throughout the oyster-growing waters of the Maryland portion of Chesapeake Bay and its tributaries, including the Potomac River. Beginning 85 years ago, the survey is one of the longest-running continuous oyster monitoring programs in the world.
Biologists collected 364 samples from almost 300 oyster bars. They monitored locations that included natural oyster bars, oyster seed production areas, seed and shell planting sites, harvest areas, and sanctuaries. Of these, 95 oyster bars within 38 sanctuaries were sampled during the survey.
The fall survey tracks the five indices intended to assess the status of Maryland’s oyster populations: spatfall intensity (reproductive success), oyster disease, total observed mortality, biomass (number and weight of oysters), and cultch (a measure of habitat). The spatfall intensity and cultch indexes were derived from 53 long-term monitoring sites. To monitor oyster disease levels, 43 sentinel disease sites are sampled along with nine supplemental stations to provide comprehensive results across the Bay. The sentinel disease sites also form the basis for the biomass and observed mortality indexes.
More details on these indices are below. A summary of the Fall Oyster Survey results is being presented to the Oyster Advisory Commission at its March 17 meeting and will include preliminary findings for 2024.
Comparison to 2023
A brief review of the 2023 Fall Oyster Survey provides context for the 2024 results, as 2023 was an unusual year for the Maryland oyster population. The dominant environmental condition of 2023 was drought, which raised salinities throughout the Bay, in some cases to record high levels. Elevated salinities are associated with successful oyster reproduction, as evidenced by the widespread and high spat counts observed in the 2023 survey.
However, elevated salinities can also foster the parasitic disease MSX, which can cause dieoff in oyster reefs. In 2023, MSX expanded beyond its previous confines and was found as far north as Hacketts Point, a short distance south of the Bay Bridge. The disease also spread throughout many of the Bay’s tributaries. Dermo, another parasitic disease affecting oysters, had also increased, rising above the long-term average for the first time in six years.
Although the number of dead oysters recorded in the 2023 survey rose somewhat, there was a risk of a further increase in mortality due to the extent of the diseases if they persisted in 2024.
Relief came in the form of consistent rains, which began in December and lasted through May of 2024. Fishery managers conducted a spring survey to evaluate the effect of the elevated freshwater input on the oysters and oyster diseases. Salinities had dropped dramatically, by as much as half in some areas, including the Choptank River. Consequently, MSX mostly retreated back to the lower Bay.
In the 2024 survey, both indexes of Dermo – prevalence and intensity – dropped substantially in values and distribution. Even more encouraging news was the retreat of MSX. Of 26 samples that have been completed, 17 had no sign of MSX (0% prevalence), and the highest prevalence was 7%. Although drought conditions once again returned during the latter half of 2024, MSX has not shown signs of spreading. Scientists believe the colder water temperatures through the winter will have further inhibited the disease. As a consequence, observed mortalities declined somewhat from last year to the low to moderate range. Despite the recent uptick in diseases, observed mortalities have remained below the 40-year average for the 20th year in a row.
By mid-2024, the fresh water coming into the Bay had decreased, and salinities began to climb again, supporting another healthy spatset. Although not as strong as the 2023 recruitment event, the 2024 spat index was above the long-term median and was widespread in some areas (Figure 1). A small number of spat were even observed in the Bay north of the Bay Bridge – a rare occurrence. An experimental crushed concrete planting in Pocomoke Sound produced the highest natural spatset of the survey. Unfortunately, following the exceptional previous year, recruitment in the Potomac River reverted back to its previous pattern of modest spatsets in the lower third of the river and nothing above that.
Recent Maryland annual spatfall indexes, 2007-2024, including median values.
Biomass and Cultch Indices for 2024
The oyster biomass index is a relative measure of how the oyster population is doing over time. It accounts for recruitment, individual growth, natural mortality, and harvesting in a single metric. The biomass index has been trending upward since 2011, following an eight-year recovery period after the devastating millennial epizootics. The 2024 Biomass Index was the third highest of the 32-year record. This was a 5% increase from the 2023 index and represents a gain of 62% over the long-term average.
The cultch index is a relative measure of oyster habitat. The three-year rolling average for the 2024 Cultch Index was the second highest in the 20-year time series and a 28% increase from the previous year. Strong regional differences in cultch mean volumes were evident. The areas with the lowest standardized cultch averages included the mainstem of the bay, followed by the combined Chester River/Eastern Bay region. The highest cultch indexes were in areas with more favorable recruitment and consequent additions to cultch, specifically the Tangier Sound and Choptank River tributaries, as well as the Patuxent River.
The preliminary results of the 2024 Fall Oyster Survey as reported to the Oyster Advisory Committee will be available on the Maryland DNR website.
