Forced temperature changes are commonplace for bass. Not a bass alive can avoid some degree of thermal fluctuation, regular or irregular, at some point in its life. Summer cold fronts and the like are more of a nuisance than a hardship. But there is at least one season when things get much more serious: winter.
Beginning in late August in the northern latitudes, later in southern latitudes, the air and water temperatures begin to steadily cool. Bass therefore become more mobile in the fall as they seek to evade winter's approach. Ultimately, however, finding warmer water becomes a losing proposition. Without a constant source of warmth, such as the heated effluent of a power plant, searching bass must eventually face the inevitable and suffer a drop in body temperature.
For every drop of 18°F (10°C) in body temperature, a bass' metabolic rate is reduced to one-third its previous rate. Liver and digestive processes are hit especially hard. A meal of shad that at 70°F took only two or three days to digest, at 60°F may need four to five days to complete. The bass is beginning to struggle physiologically.
Still, at 60°F there may be few outward signs of the dynamic overhaul taking place. To anglers the bass still appears active, agile, and quite responsive to live prey and lures. But change is only
a matter of time. With each degree drop in temperature, the bass' nervous system, already taxed, begins to falter. Around 60°F, the outward behavioral signs of cooling may be marginal. From 60 to
50°F, however, they become readily observable as the animal grows increasingly more sluggish. Dropping from 50 to 40 and the upper 30°F, full-fledged winter, the behavioral changes are major.
Below 45°F, a bass' willingness to move of its own accord falls off sharply. In many natural lakes they begin congregating in preparation for migration into deeper water where they will spend the winter.
At temperatures below 40°F, bass essentially enter a state of dormancy. Largemouths can occasionally be found in huge aggregates located near well-oxygenated water and little current. At any given
time a large portion of the bass population will be resting with their pelvic fins and tails in contact with the bottom, wholly inactive, with gill rates measuring about one breath per minute. A few active individuals swim several feet above, breathing more rapidly. These fish are capable of short bursts of active swimming and short bouts of prey (and lure) pursuit. Cold bass can't swim very fast, long, or far before tiring. Once it feeds, a bass at these super-cold temperatures may not feed again for another month.