Striped bass trapped in a collapsing layer of oxygenated water continued to die at Thurmond Lake during the weekend, according to biologists monitoring the phenomenon.

"The total collected is about 211 fish, and that is cumulative," said Jamie Sykes, a fisheries biologist for the Army Corps of Engineers' Savannah District. "Roughly 60 to 70 percent of them have been in the 5- to 10-pound range."

During the weekend, dead and dying stripers were collected behind the dam and in the Savannah River tailrace on the dam's lower side.

Fish kills aren't uncommon this time of year, when changes in the depth of oxygenated water can force fish closer to the turbine intakes.

What is unusual this year is that the fish killed include large numbers of adult striped bass. Such incidents usually involve small baitfish such as blueback herring.

Herring are being drawn into the turbines, too, which is called entrainment. However, their numbers aren't substantial enough to trigger a shutdown of the dam's hydropower turbines.

"The most herring we've had in one day was 2,000, which was on Saturday," Mr. Sykes said. "It's been a light year for herring entrainment compared to previous years."

If herring entrainment reaches 5,000 fish per hour, it would halt hydropower production. Mr. Sykes said fish kills involving striped bass are so rare that there is no set number that would affect power generation.

"We don't have a particular criteria for the bigger fish because we've never entrained the bigger fish before in these numbers," he said. "This type of thing has occurred at Lake Murray, Greenwood, even Lake Norman in North Carolina, but it has not occurred on any of the Savannah River lakes."

Stripers tend to seek out cooler, oxygenated water. This year, because heavy rains refilled the drought-stricken lake rapidly, the oxygenated layers are deeper than usual. When the limited oxygen in those layers dissipates, the fish trapped in the "bubble" are asphyxiated and die.

That bubble, about 90 feet beneath the surface, has levels of oxygen that are about 2 parts per million. Outside the bubble, oxygen at those depths can be as low as 0.3 part per million.

Monitoring of the fish kills will continue throughout the week, Mr. Sykes said.

Reach Rob Pavey at (706) 868-1222 or rob.pavey@augustachronicle.com.