From my experiences fishing in bass tournaments, doing research on survival of tournament-caught fish, and working with tournament organizations, summer is more of an excuse than a reason for high fish mortality. Now, please give me an opportunity to justify that bold statement. I'll focus on largemouth bass and crappie, fish that Mississippi anglers may want to keep alive in a livewell.
First off, whenever fish are caught on hooks, some are going to die. A common and valid number for hooking mortality is about 5 percent, and that shouldn't change seasonally.
A lot of complex physiological changes happen when fish are caught, handled and retained in livewells, but the two things that anglers need to be concerned about are temperature and dissolved oxygen.
High temperatures can kill largemouth bass and crappie, but water temperatures up to 90 to 92 degrees are not, by themselves, a problem. You've caught fish in 90-degree water, right? They were healthy and active enough to bite, right? Temperature in a livewell can become a problem on a hot, sunny day if you don't exchange or cool the water. The sun's heat on the boat deck and livewell lid will eventually warm the livewell water to lethal temperatures. But generally, temperature is not what kills fish held in livewells
Insufficient oxygen is the main reason fish held in livewells die. Biologists know from research on walleye, a fish that may require slightly more oxygen than bass, that 8 hours in a livewell at 2 parts per million (ppm) dissolved oxygen is lethal. The fish may not be dead at weigh in, but they will die in the next 24 hours.
The problem with oxygen is you can't see it and, unlike temperature which can be measured easily, measuring oxygen requires expensive meters that anglers don't have (and really don't need).
Here's a primer on oxygen in water. Water holds a certain amount of oxygen. This is called the saturation concentration, and it is the amount of oxygen in your livewell water if you have run the aerators and no fish are in the livewell to use the oxygen. The saturation concentration decreases with temperature. Warm water holds less oxygen.
A safe level of oxygen for survival of bass and crappie is 5 ppm. That is a mere fraction (0.0025 percent if you do the math) of the amount of oxygen in the air we breathe, but fish are marvelous machines.
A quick look at the table indicates even at 90 degrees, the livewell water will hold enough oxygen to keep fish alive. But fish in a livewell are using oxygen. The warmer the water or the more pounds of fish, the faster the oxygen is consumed. Livewell aerators replace the oxygen and restore the livewell water to saturation, but a heavy catch of fish in a warm livewell can quickly lower the oxygen concentration to lethal levels.
The simple fix is to run your aerators continually, put the livewell on recirculate and cool the water. The cool water holds more oxygen, and fish in cool water consume less oxygen.
A simple, cheap and no-hassle way to cool livewell water is to put a frozen, water-filled, half gallon milk jug into the livewell. Use the frozen jug, not ice cubes. The ice cubes cool the water quickly, which could shock the fish, and the cooling effect doesn't last. The milk jug cools the water slowly and, because the ice block melts more slowly than small ice cubes, will continue to cool the water for a couple hours, even on a hot day.
How much should you cool the water?
If the lake water is 75-80, cool the livewell to 75. If the lake water is 81-85, cool the livewell to 78. If the lake water is 86 or warmer, cool the livewell to 80.
Properly cooling livewell water is not just another "bright idea;" it has been tested, and it improves fish survival. Properly cooling livewell water will help you weigh in live fish, but more importantly these fish are more likely to survive to grow to a larger size. Isn't that what live release is about?
Try it. Look at the fish and see how colorful they are. Feel how slimy and slick the fish are from the cooled livewell. Then feel the dry, sandpapery skin of a bass or crappie from a livewell that is 90 or warmer.