Where are the Fish?

By: Patrick O’Rouke, GA DNR Fisheries Biologist

Fisheries Technician Chris Looney drops a dissolved oxygen probe in Lake Lanier to measure a reservoir profile near Buford Dam.

Georgia’s lakes and reservoirs can get very warm on the surface during the summer, which makes for comfortable swimming, boating and skiing for us humans. However, that’s not so good for some fish. Coolwater species like striped bass and walleye and coldwater species like trout can’t tolerate the warm surface temperatures that reach over 80 degrees (F).

Fortunately the water temperature in a lake can vary greatly depending on the depth. The density of water changes with temperature, and colder water sinks in the water column while warmer water floats. So when the surface of your favorite lake starts to warm in the spring, the warm water stays on top and the deepest water, which receives little to no sunlight, continues to stay cold from the winter – even in the dog days of summer.  This water stays completely isolated and doesn’t receive any new sources of dissolved oxygen, which is slowly used up by the decomposition of organic matter once the short transition zone – the thermocline – is established.

Fisheries staff monitor these temperature gradients and dissolved oxygen levels on many lakes and reservoirs in Georgia.  For example, early in the summer staff take a boat out once per month on Lake Lanier and slowly drop a probe, one meter at a time, to measure temperature and oxygen all the way to the bottom of the lake.

The data collected can be used to share the likely depth of sportfish species with interested anglers. Staff also keep an eye on conditions to make sure there is not a threat of large fish kills. In some years, fish like stripers can become highly stressed or even die if cool water with enough oxygen isn’t available before surface temperatures cool in the fall. Additionally, it helps know what kind of water quality the rivers below dams will experience. In the case of Buford Dam below Lake Lanier, we can tell what sort of water quality to expect in the coming fall.

On the morning of Aug. 4, fisheries technician Chris Looney collected data for one of these reservoir profiles on Lake Lanier.  He checked five sites around the lake, including one at Brown’s Bridge (mid-lake). Here’s a graph of temperature and oxygen at that location:

To help you understand what this means, here is a color-coded graph our staff has developed to assess potential striped bass habitat. The top of the graph represents the surface of the lake. Blue means the water temperature and oxygen levels are sufficient for stripers. Orange means the temperature is marginal (stripers could be there, but probably won’t if there is any water available in the blue category), red means it is too hot, and gray means there isn’t enough oxygen for them to survive:

As you can see, stripers will likely be holding somewhere between 30-90 feet deep where the habitat is best for them.  If the water is below 76 degrees or so, and has more than 2.0 mg/L of dissolved oxygen, stripers can comfortably live.  If there is cool water with even more oxygen, they’ll probably go for the better oxygen assuming they can find food.  If you can find the preferred habitat and find their food, you’re 90% of the way to catching just about any species of fish.

Our staff will continue to measure these profiles through the fall until the lake “turns over”, meaning that the surface temperature has cooled enough to match the bottom and water at all depths is able to mix again.  Based on our profiles so far this year, Lake Lanier’s water quality looks good and we do not expect any stressful conditions for striped bass this summer or fall.

If you have questions about reservoir profiles on a particular lake, please contact your regional fisheries office or send us your question via email. Our biologists and technicians are happy to talk with you to help you better understand your favorite lake so you can get out and Go Fish Georgia!