At the end of each meteorological season, scientists analyze data from the previous three months. They define whether conditions were average—or whether they differed from average. NCBO scientists discuss how these differences might affect some of the species that live in the Bay.
The NCBO Summer 2025 Seasonal Summary noted that low dissolved oxygen likely reduced the area of the Bay that was possible for blue crabs and striped bass to live in. Also, salinity was lower than average for much of the summer. That may have made both oyster reproduction in the upper Bay and production of oysters at hatcheries more difficult.
Here are some of the concepts discussed in the Summer 2025 Seasonal Summary. For more information, graphs, and discussion, you can access the full scientific document.
Water Temperature
Overall, for June through August, surface water temperatures were slightly above average, especially in southern portions of the Bay. Outside the Bay, coastal waters from southern Virginia to Delaware were also a little bit warmer than average.
The Seasonal Summaries include observations from four NOAA CBIBS buoys that give a little more detail on how warm the water was—and when temperatures changed. For example, at the CBIBS Gooses Reef buoy, water temperatures were below average in early June and then were above average from mid-June through early August. At all four buoys locations this year, water temperatures rose quickly after mid June, peaking by late June in the low to mid 80°Fs. This is different from last year, when water temperatures rose more gradually in June and did not exceed 80°F until early July. CBIBS is maintained in partnership with the Maryland Department of Natural Resources and the Chesapeake Bay National Estuarine Research Reserve in Virginia.
Precipitation and Salinity
Following this spring’s higher-than-average precipitation, summer rainfall was average for tidewater Virginia and slightly below average for southern Maryland. Salinity was lower than average for much of the summer. At the Annapolis and Gooses Reef buoys, salinity increased to slightly above average toward the end of the season.
Dissolved Oxygen
Hypoxia monitoring buoys tracked dissolved oxygen (DO) levels at various water depths throughout the summer. These buoys are managed by the NOAA Chesapeake Bay Office in partnership with the U.S. Environmental Protection Agency. Water from the surface down to 5 meters below the surface stayed above the biological threshold of 4 mg/L throughout the season. Fish and crabs can survive in waters with this much DO. But in waters at 11 m below the surface, DO levels were below the biological threshold for most of the summer.
What Could These Conditions Mean for Living Resources?
Each summer, the Maryland Department of Natural Resources prohibits fishing for striped bass for a time (in 2025, the closure was July 16-31) because poor conditions mean that the fish are more vulnerable to dying after being caught and released. But this year, waters that warmed earlier than usual, combined with low dissolved oxygen levels in deeper waters, may have posed challenges for striped bass earlier than the closure dates.
Low dissolved oxygen levels in deeper waters may have spurred blue crabs to spend more time in shallower waters. Recreational crabbers reported a high proportion of smaller blue crabs in shallower waters this summer. Low DO also can harm the clams and worms that blue crabs rely on for food and the underwater grasses juvenile crabs need for protection from predators.
Lower-than-average salinity may have hurt oyster reproduction in the upper Bay, as well as operations at oyster hatcheries. Low DO could have hurt oysters as well, because they can’t swim to better areas as can fish and crabs.
Juvenile summer flounder do well in waters where the water temperature is less than 78.6°F. But areas of the Bay were over 80°F for much of the summer. These temperatures can be bad for juvenile summer flounder growth rates.