Recent studies of postrelease mortality of black bass released after summer tournaments in Alabama, Texas, and Oklahoma have shown that total mortality often exceeds 25 percent. Although bass clubs and tournament groups have long been informed about increased mortalities associated with water above 75°F, many continue to hold live-release events in summer. Some observers think these death rates are unacceptably high, and concerned sportsmen are seeking ways to reduce postrelease mortalities with improved livewell, handling, weigh-in, and scheduling procedures.

The Problem

One contributor to higher summer and possibly winter postrelease mortality is that bass are taken from greater depths in these seasons, when some fish move deeper to find stable and milder water temperatures and prey. Bass taken from deep water in summer often experience a large temperature change and develop overinflated gas bladders. Bass and other fish that lack a duct between the gas bladder and the esophagus often suffer decompression problems when pulled from deep water. For largemouth bass, that depth may be only 15 feet, while walleyes and perch can experience loss of equilibrium and be unable to return after being caught from depths of 25 feet or more.

Although floating bass can eventually regain bladder control, many floaters are injured by over-exposure to sunlight and are killed by predators, increasing posttournament mortalities. Moreover, a loss of buoyancy and swimming control is in itself stressful, increasing susceptibility to disease.

Based on release experiments in the 1980s, California biologists suggested that tournament directors relieve excessive gas bladder pressure with hypodermic needles prior to release, treating any bass with equilibrium or buoyancy problems. A more recent study of the effect of deflation on yellow perch bladders also suggests that the technique works. But some biologists have been concerned that the deflation process, called “fizzing,” has been inadequately studied, and if done improperly may harm more bass than it helps. As a result, the technique is not widely used.

In a 1983 experiment by Feathers and Knable1, groups of 6- to 10-inch juvenile and 11- to 15-inch young adult largemouth bass were placed in a pressure chamber and acclimated to pressures equivalent to 30, 60, and 90 feet. When the bass were rapidly depressurized to simulate capture, they reported mortality rates of 28, 42, and 48 percent.

Depressurizing from these depths caused obvious damage. Bass were unable to submerge. They floated for at least 24 hours before gas bladder volume was naturally reduced.

The stomachs of some, but not all, of the bass were pushed out of their mouths by expanded gas bladders. Hemorrhaging was visible around fins. These injuries were most evident in fish depressurized from 60 and 90 feet. Internal examinations showed little damage other than ruptured body-cavity linings in fish from simulated 30-foot depths, but bass depressurized from greater depths suffered severely damaged viscera. Mortality was caused by hemorrhaging and gas-bubble formation (bends) in the gills, aorta, heart, and brain. Fortunately, largemouth bass are seldom found and hooked this deep.