As rain-free skies finally cleared sufficiently to let the sun shine for more than two or three days in a row this month, our average air temperatures have increased, and our local lakes’ surface temperatures have likewise increased.

As the water near the surface heats up, deeper water, insulated by the water above it and shielded from the sun, remains cool. Warmer water is lighter than cooler water so the water begins to separate into a warm upper layer (called the epilimnion) and a cool bottom layer (called the hypolimnion). There is a separating layer of water between these called the thermocline. The thermocline is typically several feet thick, and within this horizontal band, the greatest change of temperature takes place, with the water getting colder as the depth increases.

For a number of reasons, the warm upper layer is oxygen rich, and the cool lower layer is oxygen poor. Local residents who chance to drive across Belton Dam in the summer months can hardly help but notice the strong, sulfuric “rotten eggs” odor, especially near the south end of the dam where the water is released into the channel downstream into Miller Springs Park.

This smell is a result of anaerobic decomposition of organic matter down below the thermocline in the deeper, cool water that is released through the dam. Fish life cannot survive in such anaerobic conditions.

Due to this lack of oxygen at depth, all fish are forced, based on their need for oxygen, to stay in the warmer, oxygen-rich upper layer. Anglers fishing below the thermocline are simply wasting their time.

Thermocline now developing

As I fished Belton Lake this week, a stratification of the water column was apparent on sonar. Around 47-52 feet deep, a separation of these warm and cool water layers was beginning. With the sensitivity turned up high on my sonar unit, I was able to detect a horizontal band which showed up as blue and black speckling on the screen, with an otherwise clear screen above and below this band.

Many fish, and especially the pelagic, open-water species like hybrid striped bass, white bass, blue catfish and threadfin shad, no longer relate to the lake’s bottom as they have since last November when last summer’s thermocline broke down. Rather, these fish begin to suspend at and above the thermocline.

Impact on anglers

There are many ramifications for anglers concerning the annual formation of the thermocline. First, live bait fisherman must appreciate the depth at which the thermocline is set up. Dropping baits below this depth for any length of time will guarantee that baits will perish very quickly without sufficient oxygen to breathe.

Vertical presentations like “slabbing” lose their effectiveness, and the effectiveness of horizontal presentations, like trolling and downrigging, increase. This happens because fish orient in a horizontal band above the thermocline and roam in search of forage there.

Fishing in the low-light and cooler times of the day just before and after sunrise and sunset can provide fast action as gamefish make brief near-shore feeding runs during these times.

Local anglers cash in

Such was the case for Chad Zuckero and his sons, 10-year-old Josh and 6-year-old Blaine, of Salado. As the light levels began to increase just after sunrise on their recent visit to Belton Lake, we encountered a great number of white bass and hybrid stripers that moved very quickly up into fairly shallow water. We used downriggers to present five baits on two rods by using a tandem rig with two lures on one rod and a three-armed umbrella rig to present three lures on the second rod.

For about 35 minutes, each pass over a well-defined 40-yard area gave up at least one — if not multiple — fish. Had I encountered these same fish the month before in cooler water, we would have used slabs or bladebaits to tempt these fish, but the warming water we now have to deal with demands a horizontal tactic for consistent success.

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