Chesapeake Bay 2023 Fall Oyster Survey Records Outstanding Spatfall

Strong oyster reproduction observed in Maryland waters

DNR Secretary Josh Kurtz joins Fishing and Boating Services staff analyze to oyster spat during the fall survey. Photo by Robert Bussell, Maryland DNR.

The Maryland Department of Natural Resources’ Fall Oyster Survey recorded a remarkable year for juvenile oysters in Maryland waters, finding both prolific numbers and a widespread distribution throughout many regions of the Chesapeake Bay.

The survey’s spatfall intensity index, a measure of reproductive success and potential population growth for oysters, was 86.8 spat, or juvenile oysters, per bushel, nearly four times the 39-year median of 23.6 spat per bushel and the fifth highest in that timeframe.

“We have not recorded this extent of oyster spat recruitment in the fall survey in a generation,” said Department of Natural Resources Secretary Josh Kurtz. “Both the quantity and the wide distribution of spat throughout the Bay, including several areas where our biologists have rarely observed spat in nearly 40 years of results, are outstanding. We plan to build on these natural spatsets by continuing oyster restoration efforts and promoting aquaculture to bolster the overall oyster population.”

This survey, which measures the population status of oysters in the Bay, marks the fourth consecutive year of above-median results for juvenile oysters, a promising sign for restoration efforts for the bivalve, which has faced precipitous population declines over the decades.

Aside from the spatfall intensity index, the department looks to see how widely young oysters are distributed throughout Maryland’s oyster habitat. For example, the 1997 survey produced the highest index in the history of the survey at 277 spat per bushel, but the spatset of that year was concentrated in particular areas of the Chesapeake Bay: the eastern portion of Eastern Bay, the Miles River, the northeast portion of the lower Choptank River, and in parts of the Little Choptank and St. Marys rivers. 

By comparison, the 2023 survey found spat in areas where they are rarely observed, including in the upper reaches of Bay tributaries that are typically too brackish for strong oyster reproduction. The distribution of the 2023 spatset far exceeded prior spatsets.

The Potomac River and two of its tributaries, the Wicomico River and Breton Bay, in addition to the Patuxent River and the Tred Avon River, received a once-in-a-generation spatset. A supplemental survey in the Potomac found numerous spat well above the Route 301 bridge, an area where devastating freshwater deluges of 2018-19 had all but wiped out the oysters there. Eastern Bay, where oysters have been in decline for over a decade, also saw a significant increase in spatset. Many of the areas that typically receive high spatsets did so again in 2023: the lower Eastern Shore, the lower Western Shore, Broad Creek, and others.

This year, the department recorded spat at 50 out of 53 key bars, the primary sampling sites for the spat survey. Spat have not been recorded at this many key bars since 1985.

Maryland Spatfall Intensity Indices (columns) are the annual averages of spat counts on 53 key bars for the most recent 17 years. The annual medians (orange dashed line) are measures of the distribution of spat – the larger the median, the more widely distributed the spatset.

Environmental conditions, such as adequate water salinity, play a role in successful oyster recruitment. This past year, salinity measurements have been above their long-term averages due to below-average rainfall in the watershed, providing ideal conditions for oyster recruitment. However, in some years, spatset may be lower than expected despite adequate salinity conditions, emphasizing that other forces influence recruitment.

While many factors played a part in the 2023 spatset, Maryland Department of Natural Resources scientists believe the considerable numbers and distribution recorded — in conjunction with the three previous years of above-average recruitment — are encouraging signs for ongoing management efforts.

Eastern oysters, the species found in the Bay and parts of the Atlantic coast, are critical to the Chesapeake ecosystem. They are an important food source and the reefs they form provide habitats for fish and crustaceans. Each mature oysters can filter gallons of water daily and consume algae, which removes excess nutrients and helps clarify the water.

Oyster harvesting is an economic engine for Maryland watermen, second only to crabbing in terms of dockside value. Over time, overharvesting, habitat loss, pollution and oyster diseases have decimated the population of oysters in the Bay, leading to intensive management practices aimed at bolstering the population.

The department works in consultation with the Oyster Advisory Commission to manage oysters in Maryland’s portion of Chesapeake Bay in an ongoing effort to increase the population and habitat as well as to facilitate long-term, sustainable oyster harvest. Following a three-pronged approach to restoration, the department protects 50% of the most productive oyster bars in sanctuaries, creates new opportunities for oyster aquaculture, and utilizes a data-driven management strategy for public fishery areas.

Since 1939, the department and its predecessor agencies have monitored the state’s oyster population with an annual field survey — one of the longest-running programs of this kind in the world. Locations monitored include natural oyster bars, oyster seed production areas, seed and shell planting sites, and sanctuaries. 

Oysters reproduce by external fertilization, releasing their eggs and sperm into the water column when triggered by both temperature and the presence of gametes from other oysters. The more closely situated adult oysters are to each other, the greater the likelihood of successful fertilization. 

Oyster spatfall intensity and distribution in Maryland, 2023. Intensity ranges represent regional averages.

Once the eggs are fertilized, they develop into free-swimming larvae and are subject to wind, tides, currents, and a host of predators, such that only a minute fraction survive to settlement. At that time, using a tiny foot, they seek out hard substrates, namely the shells of other oysters, on which to attach and remain for the rest of their lives. After attachment, the juvenile oysters are known as spat. By the following year, they have reached sexual maturity and are considered young adult oysters.

Despite the hostile environment for oyster larvae and spat, some make it through, sometimes in spectacular numbers. The spatfall intensity index in 2020 (109 spat per bushel), 2021 (43.9 spat per bushel), and 2022 (32.1 spat per bushel) were all well above the 39-year average. A robust number of mature oysters from these three previous year classes is another influence that may have driven the intensity and the distribution of the spatfall up this year.

“Oysters are unique in that they are environmental engineers, meaning they create their own essential shell habitat as they grow,” said Department of Natural Resources Shellfish Division Director Christopher Judy. “This large influx of young oysters will help build a foundation for more oysters in the future.”

The Fall Oyster Survey Report is being presented to the Oyster Advisory Commission at its Jan. 9 meeting. Results will include preliminary findings related to the five indices intended to assess the status of Maryland’s oyster populations: spatfall intensity, oyster disease, total observed mortality, biomass (number and weight of oysters), and cultch (a measure of habitat).

Old oyster shells pulled from the Bay shows signs of new spat. Photo by Robert Bussell, Maryland DNR.