No one argues that summer heat in the South climbs well above the comfort level – for humans and animals.
The beaming August sun makes the cooling waters of the Savannah District’s three reservoirs inviting for a cool dip or swift boat ride. But while the surface water feels cool to humans, some fish find it most uncomfortable – particularly striped bass and blueback herring.
During the summers of 2013 and 2018 Hartwell Lake officials discovered a large number of striped bass died during the hottest part of the season. Members of the public noticed the large sport fish floating on the surface.
Since these were the first instances of fish kills in the history of the reservoir, their deaths caught the attention of Corps officials, representatives of the two states’ Department of Natural Resources and the public.
“Two things really impact striped bass in Hartwell,” James Sykes, fisheries biologist for the Savannah District’s reservoirs, said. “One is temperature and the other is dissolved oxygen.”
While humans find most open water below body temperature “cool” fish don’t. Their body temperature matches the water temperature – and they must have it cooler.
As summer heat drags on, the surface of the reservoirs warms up during the day but doesn’t cool enough at night. By August the surface level is much warmer than the lower levels, Sykes explained.
In addition, the upper warm layer, exposed to the atmosphere, has plenty of dissolved oxygen, but the cool deep layer experiences an ever shrinking amount of oxygen as the summer progresses.
So fish, particularly striped bass and blueback herring progressively dive deeper. There they reach another obstacle – little oxygen at deep depth. It just doesn’t exist at greater depths naturally.
The fish get caught between low-oxygen below and declining oxygen and high heat above, according to Anthony Rabern, also a fisheries biologist who works for the Georgia Department of Natural Resources. Rabern has spent years studying reservoirs and lakes in northeast Georgia, including Savannah’s Hartwell reservoir.
As the water warms and the deeper layer continues to lose oxygen, the zone of cool, oxygenated water in the reservoir gets smaller and smaller. In 2018, it finally disappeared altogether, Rabern discovered. Georgia DNR has an ongoing study of water quality in the fisheries of Hartwell Lake. Rabern heads the data-gathering effort.
Rabern and colleague Tony Anderson spend much of their time on Lake Hartwell tracking fish movements. They use tracking devices surgically implanted into 39 striped bass that send a signal that sensors attached to buoys in the reservoir to detect and record the fish as they move about the 55,900 acres of Hartwell Lake.
The shrinking oxygen habitat for fish commonly occurs in Southeastern U.S. reservoirs, according to Rabern.
“We call it ‘the bubble’ or ‘the summer squeeze,’” Sykes said. In most years the bubble is of sufficient size to support striped bass and blueback herring through the summer, but in 2018, according to Rabern’s data, ‘the bubble’ finally disappeared altogether.
Then the dead and dying fish floated to the surface near the dam grabbing the attention of anglers and the passing public.
Biologists like Rabern and Sykes can detect when the bubble begins to shrink and know the fish will soon suffer distress.
“It’s like a train,” Sykes said. “You can see it coming, but there’s not a lot you can do about it.”
Not all the bass died. Most managed to find safe haven somewhere else in Hartwell Lake. In order to find cool water with oxygen, some fish travel many miles.
That includes one tracked striper nicknamed “The Wanderer,” a 12-pound specimen, which has passed receivers more than 4,000 times since its capture, tagging and release early this year.
Some “pings” have happened more than 28 miles from The Wanderer’s release location. The study aims to find where the other locations in the reservoir that support fish in the summer may be.
Besides fish movement, Sykes and Park Ranger Jess Fleming provide water quality data in the joint effort. The South Carolina Department of Natural Resources also contributes to the three-way partnership. They all want to determine the reasons behind the possible decline in heavy-weight bass populations.
“Until about five years ago, folks would pull in 40 to 50-pound stripers from time-to-time,” Sykes said. “Those are very rare now.” Stripers grow about two pounds per year, Rabern explained.
Unusual rainfall or high water release rates in 2013 and 2018 may have factored into the loss of the bubble those years, Sykes speculated. In both years Lake Hartwell received well above average rainfall which led to higher discharge rates, according to project records.
The root cause that led to the fish mortality in those years may remain a mystery for a while. However, Georgia and South Carolina continually stock Hartwell Lake and other reservoirs and lakes in the region with striped bass and other sport fish to replenish the population.
Many of the stocked fish carry tags easily visible to anglers. Those tags include a message for reporting the catch, which also carries a small reward. The DNRs use the information about the catches to aid in their management of the fish population.
As the summer heats up, fishing will likely remain a popular pastime for those seeking relaxation and who want the challenge of finding and catching those elusive fish. It may take extra skill to find them, but an old American proverb says, “The worst day fishing beats the best day working.”
~ Billy Birdwell, Corporate Communications Office