by Tori Hamby

MOORESVILLE – A natural process that causes Lake Norman to stratify into high and low oxygen layers is responsible for the striped bass fish kills, N.C. Wildlife Commission officials say.

Brian McRae, Piedmont Regions fishery supervisor for the commission, shared a report with Lake Norman anglers on Oct. 16, which detailed the reasons behind the striped bass fish kills that have plagued Lake Norman for years. The Commission has reported striped bass kills of varying sizes each summer since 2009, as well as in 2004.

As the summer progresses, McRae said, the lake divides into three layers – an upper layer with hot water, high oxygen levels and a food source; a middle layer with cool water, no oxygen and little food; and a lower layer with cold water, high oxygen levels and a food source.

Striped bass must decide whether to swim to the upper or bottom water layers for food, McRae said.

Alewif – a fish known to anglers as river herring – tend to swim at the bottom lake and serve as a major food source for striped bass, McRae said. The non-native fish were most likely introduced to Lake Norman in the 1990s as a result of unauthorized stocking by anglers hoping to increase the size of striped bass in the lake.

“The striped bass have a decision to make,” McRae said. “They can either stay in the upper layer … or they can go down deep where it’s cold and has lots of oxygen and food in the form of river herring.”

As the summer continues, however, the middle of the lake widens, pushing in on the lower oxygen-rich layer, he said. This process makes it difficult for striped bass to swim back to the top of the lake for oxygen, resulting in fish kills.

The thickness of the oxygen-depleted middle layer, however, varies from year to year, which determines whether or not a fish kill will take place.

More research is needed to understand the process, but McRae said the commission speculates that heavy spring rains push leaves and detritus into the lake. As organic material filters down to the bottom of the lake, it uses up oxygen.

Heavy rains could cause the middle layer to widen, resulting in more severe fish kills.

McRae said there are a few options to deal with the problem: continue stocking the lake with 162,500 striped bass annually knowing that fish kills are a likely possibility; switch to stocking the lake with hybrid bass without knowing how the fish will affect the lake environment or if they will survive; or install oxygen injection system that pumps liquid oxygen into a lake near the dam of a reservoir.

Several anglers who spoke during a comment session said they wish to see the commission stock the lake with hybrid striped bass, despite not knowing the affects the fish could have.

Robert Curry, chief of inland fisheries, said the commission would continue to research possible solutions.