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Reservoir water temperature and dissolved oxygen during the summer

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Reservoir water temperature and dissolved oxygen during the summer

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Bobby Peoples

Outdoor Columnist

Over the last few weeks, I addressed an angler’s ability to catch various lake species during the hot summer season. The question becomes “why is it difficult to catch those fish during the late summer season?” There are reasons for that difficulty.

Water temperature and dissolved oxygen levels impact fishing success during hot weather. Lake species react to water temperature and oxygen levels during summer, so let us explore the water temperature and dissolved oxygen levels in more detail.

When we humans get hot from all the heat of summer, we go to a cool air-conditioned spot and look for a cold drink. Normally, fish in reservoirs also seek a cool spot but unfortunately the fish in Lake Oconee and Lake Sinclair have a hard if not impossible time when seeking out a good cool spot during late summer.

Some of the inability to find a cool spot, results from the fact that these reservoirs do not have multiple layers of water like normal lakes and that is due to the pump-back operation at the Wallace Dam. The water in a normal lake or reservoir during the summer will separate into distinct layers.

In those normal reservoirs, the water column stratifies and breaks into different temperatures at different depths and so does the oxygen content in the water. A layer of water called thermocline develops between a layer called epilimnion and a layer called hypolimnion. The thermocline layer contains cooler and highly oxygenated water where fish can thrive during the heat of summer.

The upper layer above the thermocline or the epilimnion also contains good oxygen levels but is the hottest water in the lake. The lower layer or the hypolimnion below the thermocline is cooler but has little to no oxygen.

Unfortunately, lakes Oconee and Sinclair see little to no stratification during the heat of summer and no thermocline develops. The water temperature and oxygen levels are basically the same from top to bottom but there is slightly more oxygen is at or near the lake’s surface.

Dissolved oxygen is a measure representing the amount of oxygen in the water. Most dissolved oxygen comes from the atmosphere, but it can be depleted by high water temperatures that deplete the ability of the water to hold oxygen. It can also be increased by rain and wind.

Most fish species can also adapt to low dissolved oxygen levels, but when dissolved oxygen in the water drops to 5 mg/l or below, the fish are under stress. Fish can survive and adapt to levels as low as 2-3 mg/l for short periods, but below that level the fish will begin to die.

As the summer weather takes hold on Lake Oconee and Sinclair, the water temperatures rise, and dissolved oxygen levels steadily drop. The pumpback operation at the Wallace Dam does not allow either lake to stratify like other reservoirs.

The water in both lakes is continually mixed from top to bottom. Before Plant Branch was closed it also created additional problems in Lake Sinclair due to the hot water discharges. Thankfully that impact has been eliminated.

In normal lakes, the thermocline is easy to find and that is where most of the fish in the lake will reside for the majority of the daytime during summer. Without a thermocline, the fish can be found just about anywhere in the water column and that makes fishing tough.

In fact, the water with the most oxygen during the summer months on both Oconee and Sinclair is at or near the surface. Does that mean anglers should fish shallow water? Not necessarily. The bright sunlight can become a problem for the fish.

This current summer has been extremely hot, and, if the water temperature and dissolved oxygen levels were measured now, it would indicate that fish are very stressed and few areas of either lake at any depths have good combinations of acceptable water temperature and dissolved oxygen.

Angling during late summer can be a real challenge. So where are the fish and where is an angler most likely going to have the best success during late summer?

First anglers have to consider other factors that might impact where the fish are located. Weather conditions like clouds (fish will move shallow due to less sunlight) and wind (creates higher oxygen levels in shallow water) which change the location of the fish on any given day.

Angling success can be better at night because the cooler temperatures cool the shallow water by several degrees. That results in the fish being more likely to move into shallow areas to feed at night.

Other than the option to fish at night, my recommendation is to start the day two hours before daylight fishing dock lights, then fish shallow shady areas until the sun is out brightly. Then move to deeper water for the remainder of the day. That plan works for me but remember the other factors that impact the location of the fish during the summer.

A combination of water current, clouds and wind can improve summer fishing even on lakes that do not stratify. Good fishing and see you next week.