A record-breaking number of juvenile oysters, known as spat, settled in the St. Mary’s River during 2025, according to annual monitoring conducted by the St. Mary’s River Watershed Association.
The SMRWA’s yearly study, which began with a pilot in 2018, documented an average of 4,427 spat per wire mesh cage across deployment sites. This figure surpassed the previous high of 398 spat per cage recorded in 2022 by more than eleven times. The exceptional spatfall signals strong natural reproduction for the river’s oyster population, which plays a key role in filtering water, creating habitat for marine life, and sustaining a local commercial and recreational fishery in Southern Maryland.
SMRWA deploys four wire mesh traps filled with oyster shells at each of 12 sites throughout the river in May. The traps remain submerged until late October to mimic natural oyster reefs on the river bottom. Free-swimming oyster larvae seek hard surfaces, preferably oyster shells, to attach and metamorphose into spat, remaining fixed in place for life. Researchers count recently settled spat on the shells to gauge reproduction success.
In 2025, the volume of spat overwhelmed initial counting efforts. Volunteers, including 29 community members and high school students from the Forrest Career and Technology Center in Leonardtown, assisted in identifying and tallying the juveniles. The group managed to count spat in half the cages, a process that required three times the usual effort due to the unprecedented numbers.
The Maryland Department of Natural Resources funded the study. Gina Hunt, associate director of DNR’s Fishing and Boating Services, highlighted the broader context. “These spat results are great news for the oysters in the St. Mary’s River,” Hunt stated. “Throughout the Chesapeake Bay, we’ve seen overall oyster population increases, several years of good spatsets, low mortality, and successful reproduction at Maryland’s large-scale restoration sanctuaries, including the St. Mary’s Sanctuary.”
Environmental conditions likely contributed to the massive spawn. Emma Green Ewing, executive director of SMRWA, pointed to weather patterns. “Oysters are triggered to spawn by warming temperatures,” Green Ewing said. “The weather conditions may have been just right for a massive spawn.” A wet May combined with warm June and July temperatures aligned with optimal spawning cues.
Restoration efforts also factored into the success. Maryland completed large-scale oyster restoration in the Upper St. Mary’s River Shellfish Sanctuary in 2022. SMRWA has since planted additional oysters and installed reef balls at multiple sanctuary locations. The sanctuary, closed to harvest, supports a healthy, robust oyster population that produces larvae capable of dispersing downstream to benefit public fishery areas.
For the first time in the study’s history, the site with the highest spat count occurred in the public fishery zone rather than within the sanctuary. This shift suggests improved river-wide health and potential spillover effects from sanctuary restoration.
The high 2025 spatfall promises long-term benefits. Green Ewing noted the reproductive advantage of maturing oysters. “As oysters get older, their reproduction rate increases,” she said. “We’re hopeful that the oysters that spawned this summer will produce more larvae and drive a rapid increase in the St. Marys’ River oyster population.”
The findings align with regional trends of oyster recovery in the Chesapeake Bay, where sanctuaries and habitat enhancements have supported sustained reproduction and population growth. In Southern Maryland’s St. Mary’s River, these results reinforce the value of ongoing collaborative restoration between state agencies and local watershed groups.
