Thermal Shock

Rick Bramwell

In all of my outdoor wanderings, I’ve never seen nature pull such a stunt as we experienced last week. On Wednesday, the heat index reached 105 degrees, accompanied by a temperature of 95 degrees.

A massive cold front brought temperatures down to the fifties on Tuesday night. Thursday, we never saw 70 degrees. With the cold front came two inches of rain. Water temperatures on Geist, a very shallow reservoir, dropped 16 degrees quickly. This possibly replicates a fall turnover when the surface water becomes colder than the water in the thermocline (the middle layer with the most oxygen) and sinks.

James Lasswell and I fished Geist on Saturday. The main lake surface temperature was 73 degrees. Before the front, I’m guessing the surface temperature was about 90 degrees. We had down-scope, side-scan, and Live Scope. The fish lit up the screens but wouldn’t bite. Even the small bluegill and yellow bass that typically drive you nuts were not interested in our offerings. Our fish fry would have been cornbread and beans. I had my theory, but wanted an explanation from an expert. I called Kyle Haley, a fisheries biologist from Jones Lake Management.

“In that shallow of a lake, you could have had a partial turnover or mixing event. This could have dissipated the oxygen enough to make the fish lethargic,” he said.

Haley had another plausible explanation and new terminology, “Thermal Shock.” His clarification, “When you purchase tropical fish for your aquarium, you put the sealed bag in your tank to allow the water in the bag to slowly equilibrate to the temperature in your tank. This prevents thermal shock.”

This prompted me to do an online search for thermal shock. I found one mention on YouTube. Rodman Pool is a damned stretch of river in Florida. A sign on a boat ramp at the end of a long canal reads: Attention Tournament Anglers. After the weigh-in, please return your fish to the main lake.

The video explained that there was a 10-degree temperature difference. Bass released at the ramp will die from temperature shock before they can migrate back to the main lake.

Through Wednesday, nighttime temperatures dropped into the low 60s. I thought the bite would be on Tuesday morning. The day began with light rain and little wind. These are conditions I consider perfect. The fishing had not improved. Kaden Kovacs and I tried main lake flats, docks, points, and bays. I think we caught three bluegills, a redear, a small yellow perch, and a short bass.

At the ramp, I spoke with two men who were leaving. They had four good readers, about 10 small bluegills, and a 12-pound catfish. They were sitting against the bank, casting out with bobbers set three feet deep over eight feet of water using live bait. I ran this by the Jones’ biologists. His reasoning, “Fish are opportunistic. The live bait sitting still was a much easier prey than my paddletail plastic grub quickly swimming by.” Kyle Haley thinks the fish just need a little more time to adjust.

My next trip will be to a local pond. A place to heal my bruised ego. Another bad day on Geist could label me a one-hit wonder. Jones Lake Management provides comprehensive lake and pond management services, including fish stocking, fountain installation, and aeration systems. For years, Jones has managed a 10-acre lake for a friend of mine here in Central Indiana. The lake has three aerators and is managed explicitly for walleye, smallmouth bass, yellow perch, wipers, and redeared sunfish. I get to fish his lake with barbless hooks. I’m amazed that the experts at Jones can pull off such a feat. They can make a lake or pond a more pristine body of water.