Largemouth bass may try to eat prey almost as large as themselves, or they have to settle for larger forms of tiny animal plankton when larger prey aren't available. Nevertheless, bass size sets practical upper and lower limits on prey size.
The lower limit is set by several factors. Prey must be large enough to be visible and worth chasing. Largemouth bass rarely waste more energy chasing food than they get from digesting it.
Prey also must be large enough to be held by the gill rakers -- thin, fork-like spines blocking the areas between the gills. Water flows out, while food items remain in the bass's throat. Prey smaller than the spaces between rakers usually escape. The space between rakers gets larger as bass grow, so the minimum size of food gets larger.
Upper limit of prey size is determined by the gape of the mouth and throat, and the ability of bass to catch and hold strong, struggling prey. Crayfish may be too tough or painful to eat, even if they fit into a bass's mouth. Large gizzard shad may be strong and swift enough to escape unless they're taken headfirst. Catfish and bullheads erect spines and wriggle strongly to escape. Such preyfish may be low-percentage targets.
Maximum length of prey varies with the shape of the prey and other features, like spiny fins. Bass take longer thin preyfish in longer sizes than thicker sunfish. Studies show that bass occasionally eat fish that are about 60 percent of their own length. Stomach samples show, however, that bass typically eat prey between 10 percent and 50 percent of their own length. Twenty percent to 45 percent are the most typical sizes. Preyfish this size are the best balance between the energy used to capture and the energy gained from eating.
Each prey species has special abilities and tactics to avoid being eaten. The dash speed of fish increases with size. Bass need a proportional size advantage to run down prey. They usually attack only injured or sick larger prey.
The shaded area shows the actual size of preyfish bass typically eat. Although bass may eat preyfish as small as about 10 percent of their own length, or as large as 60 to 70 percent, most preyfish fall between 20 and 50 percent.
Larger bass tend to eat larger prey. Most of the food of young adult bass falls between 20 percent and 40 percent of their own length, while older and larger bass tend to select preyfish in the 30 to 50 percent range, depending on prey shape.