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Summertime Dissolved Oxygen Levels—What Do They Mean for Fish…and Fishermen?

CBIBS buoys track dissolved oxygen—“DO”—levels. Having adequate DO is essential for all species that live in the Bay; like humans, the Bay’s living resources—like fish and crabs—need oxygen. Different species can tolerate different levels of oxygen, but scientists generally agree that the Bay’s aquatic species can survive with at least 5 milligrams of oxygen per liter. 

Water is considered hypoxic when DO levels are between 0.3 and 2 mg/l; anoxic with very little or no oxygen, 0 to 0.2 mg/l. Large areas of hypoxic and/or anoxic water are often called “dead zones”; in most recent summers, a significant dead zone has formed in the deep area of the mid-Chesapeake Bay. NOAA-supported forecast models indicated the Chesapeake may see a slightly smaller than average dead zone in summer 2013.

Temperature limits the amount of oxygen water can hold—for example, one liter of water can hold less oxygen when the water temperature is 80 degrees than can a liter of water at 60 degrees. So in general, summertime Chesapeake waters can hold less oxygen than can winter waters.

But in the summer, Bay species face a one-two punch: In addition to high water temperatures, summertime is also when conditions are right for algae to grow…and then die and decompose. The decomposition process uses oxygen, which further diminishes available DO in the Bay. 

In addition to temperature and algae, development of dead zones can be affected by human activities that contribute excess nutrients (like nitrates and phosphates) that fuel the growth of algae. Weather, including wind speed and direction and precipitation, can also change oxygen levels. For example, a high-wind event can help “stir up” the water, mixing oxygen through the water column, while a large rainfall—while initially mixing up the water—can deliver excess nutrients into the Bay via runoff.

DO levels can affect where Bay species spend their time. While certain species can survive in lower DO areas, they generally prefer higher DO. This means that when they encounter a low-DO area, fish and crabs—unlike oysters, which cannot escape a hypoxic or anoxic episode—often swim to areas with higher DO.

Fishermen around the Bay have noticed that some areas with less-than-ideal water quality are developing. Recreational and commercial anglers seeking summertime striped bass in the Chesapeake Bay are finding higher concentrations of rockfish in the northern portions of the middle Chesapeake Bay, where oxygen levels may be higher. 

According to local charter captains, in the lower portion of the middle Bay near Point Lookout (at the mouth of the Potomac River), fishing this year has been challenging. Highly experienced captains have been doing well, but as a whole, anglers in these regions are finding fewer striped bass. Reports suggest the upper parts of the middle Bay (near Gooses Reef) are more productive for striped bass as both the commercial and recreational striped bass fisherman have been successful there. 

Several full-time charter captains who usually keep their boats in the lower portions of the middle Bay, near Solomons Island, are moving their bases of operations further north in hopes of shorter runs to the fishing grounds, helping them save money on boat fuel.

In summer, Chesapeake fishermen generally focus their efforts in the top ten feet or so of water where the fish they seek—as well as the fish on which those fish feed—spend much of their time. This is because—while this varies in time and geography—there is generally more DO in waters near the surface than in deeper waters. Phytoplankton—algae—are plants that live and bloom near the surface. Because they are plants, they create oxygen through photosynthesis. But when they die, they sink to the bottom, and the process of decomposition they go through consumes oxygen. The difference in DO levels between surface and deeper waters encourages fish to spend time closer to the surface—often referred to as “vertical habitat squeeze” by scientists.

All CBIBS buoys measure DO just under the Bay’s surface; the Gooses Reef buoy also has a sensor that tracks DO at the bottom under the buoy. 

To explore DO levels in the middle Bay, scientists recently used the “Data Download” feature from the CBIBS website to pull and analyze data from the surface at three CBIBS locations—Gooses Reef, Annapolis (the buoy nearest to Gooses Reef to the north), and Potomac/Point Lookout (nearest buoy to the south)—from June 1 through July 16. 

A wide variety of factors play into fish habitat preference, and subtle differences in DO levels may have encouraged fish to spend more time near the Gooses Reef CBIBS buoy: While DO at Gooses Reef was above 7 mg/l 87% of the time, it was below 7 mg/l 44% of the time at the Annapolis buoy and 43% of the time at the Potomac buoy. These differences are relatively small and serve as a reminder that many variables—DO, salinity, water temperature, and much more—affect fish habitat choices.

Low DO affects more than the species that live in the water—it can dramatically affect the economy. Scientists have estimated that if dissolved oxygen levels in the entire Chesapeake Bay were less than 3 mg/l, there would be a loss to the economy of more than $145 million.

Dead zones and DO levels can vary widely from year to year, and habitat preferences of fish depend on more than just DO levels. What this year holds for this Chesapeake Bay summer—based largely on weather events we’ll experience over the next few months—will determine the extent of this year’s dead zone. And CBIBS buoys will be tracking DO levels and other water-quality parameters at all ten buoys, every hour, so scientists and fishermen alike can monitor what’s going on in the Bay.