Dry weather, elevated salinity and warm water have combined to create especially favorable conditions for sea nettles across much of Maryland’s Chesapeake Bay this summer, helping explain the heavy concentrations being reported by boaters, anglers and swimmers.
The Maryland Department of Natural Resources said in its Aug. 19-25 fishing conditions forecast that Bay salinities are running higher than normal throughout Maryland waters because of continued dry conditions. Surface temperatures in the main Bay and near river mouths are holding in the low 80s.
Those conditions are closely aligned with the habitat preferred by bay nettles, the stinging jellyfish common to the Chesapeake. DNR described the current combination of high salinity and warm water as “perfect conditions for sea nettles” and said the animals are concentrated from the Chesapeake Bay Bridge south to the mouth of the Potomac River. DNR also reported nettles extending upstream in the Potomac to near the U.S. 301 bridge.
The setup began months earlier.
NOAA’s Chesapeake Bay Interpretive Buoy System reported in July that precipitation across the Chesapeake region was well below average during spring 2026. In at least two monitored areas, spring rainfall was the lowest recorded since 2007. The lack of rainfall reduced freshwater flow into the Bay, allowing salinity to rise.
NOAA found salinity several practical salinity units above average at monitoring stations near Annapolis, Gooses Reef, the Potomac River and York Spit during the spring. At the Potomac buoy, salinity on May 20 was 52.8% above the historical average.
The connection matters because the Chesapeake Bay is an estuary, where freshwater flowing from rivers mixes with saltwater moving in from the Atlantic Ocean. During wetter periods, increased river flow pushes salinity down. During dry periods, less freshwater reaches the Bay and saltier water can move farther into areas that would normally be fresher.
The bay nettle, scientifically known as Chrysaora chesapeakei, is particularly sensitive to that balance. NOAA’s National Centers for Coastal Ocean Science says the species is most abundant in middle-Bay tributaries where salinity ranges from about 10 to 20 parts per thousand. Research used in NOAA forecasting has found favorable conditions when water temperatures are roughly 79 to 86 degrees and salinity is about 10 to 16 practical salinity units.
That places much of the Bay within or near the environmental range nettles favor this summer.
Conditions can be especially favorable along portions of the Eastern Shore. According to the Chesapeake Bay Program, water on the eastern side of the Bay generally tends to be saltier than water on the western side. Most freshwater enters from northern and western tributaries, including the Susquehanna and Potomac rivers, while the Bay’s circulation also helps direct saltier water toward the Eastern Shore.
The result is that dry years can expand suitable nettle habitat into tributaries and shoreline areas where swimmers and boaters may notice them more frequently.
The opposite pattern has been documented during wet years. In 2018, unusually high river flows lowered Bay salinity and bay nettles became significantly less abundant, according to the Chesapeake Bay Program. That year demonstrated how strongly rainfall and freshwater flow can affect where nettles appear.
Not every translucent animal seen in the water is a stinging jellyfish.
The Chesapeake also supports large numbers of comb jellies, including sea walnuts and pink comb jellies. Comb jellies are not true jellyfish and do not have stinging tentacles. The Chesapeake Bay Program says they are much more abundant in the Bay than true jellyfish, and sea walnuts can gather in large swarms near the surface.
Moon jellyfish can also occur in the lower Chesapeake during summer. Sea nettles, however, are the primary stinging jellyfish associated with warm-season conditions in much of the Bay.
Bay nettles are native to the Chesapeake and are part of its food web. They feed on fish, shrimp and comb jellies. Their populations can also affect other organisms because comb jellies feed heavily on plankton and fish larvae and can consume oyster larvae.
Scientists do not maintain a Bay-wide count of individual sea nettles, so current monitoring does not establish that 2026 has produced a record number of jellyfish. NOAA also cautions that its sea nettle forecast estimates the probability of encountering nettles rather than their density, which can vary substantially from year to year and by location.
The environmental evidence, however, points to a summer highly favorable to them.
NOAA’s 2026 monitoring shows reduced spring rainfall and elevated salinity, while DNR’s August observations show warm water and higher-than-normal salinity persisting across Maryland waters. Together, those conditions have expanded the areas where bay nettles are likely to thrive.
DNR’s latest forecast places the strongest concentrations between the Bay Bridge and the mouth of the Potomac, with nettles also reaching well into the river. NOAA maintains a Chesapeake Bay sea nettle probability forecast that uses water temperature and salinity to estimate where encounters are most likely.
