By Dr. Hal Schramm

Big baits catch big bass. But lots of oversize bass are also caught on small baits. What size prey do bass prefer, and how can anglers use that information to catch more, and maybe bigger, bass?

How Bass Eat
Have you ever wondered how you can hook a bass with a big worm when the hook is 9 or 10 inches from the tail? Six decades ago, J.M. Lawrence earned his doctorate in fisheries at Iowa State University studying feeding behavior of largemouth bass. Lawrence first observed that bass ingested fish headfirst. When a bass caught a forage fish sideways or tail first, the fish was spit out and quickly turned around before swallowing.

Smart bass. Forage fish swim better forward and are less likely to escape in reverse. Also, headfirst down the throat is more streamlined and “disarms” a baitfish’s anatomical defense mechanisms (fin spines). Apparently, that applies to giant worms, but I don’t think it applies to crayfish. I’ve caught a lot of bass with crayfish claws visible in their gullet but never with a crayfish tail visible. Tail first is definitely a safer wgraphay to eat a crayfish.

Having established that bass ingest forage fish headfirst, Lawrence learned that bass could and would consume forage fish whose body depth was slightly greater than the bass’ mouth width. Technically speaking, Lawrence’s research established that the maximum size of forage that a bass can swallow was determined by the distance between the cleithra bones, a pair of bones associated with the pectoral girdle that forms the back edge of the gill cavity. These bones limit what can enter the gullet. For all practical purposes, a bass can swallow a forage fish whose body depth is less than the mouth width.

By measuring the mouth widths for a range of bass sizes and by measuring the body depth and corresponding body length for several prey fishes, Lawrence was able to determine the size of prey bass can eat.

Two things are evident from the table. First, the cavernous maw of a largemouth bass allows them to eat large forage fish. Largemouth bass grow substantially larger than the fish Lawrence measured. From the trends in Lawrence’s data, a 10-pound largemouth can eat the biggest gizzard shad and golden shiner swimming.

The second important inference is that bass can eat a much longer shallow-bodied prey fish, like a golden shiner, than a deep-bodied prey, like a bluegill. Indeed, bass eat a lot of bass, a relatively slender-bodied fish. A 20-inch largemouth can easily swallow a 10-inch bass.

As a rule of thumb, a largemouth can eat a shad (or a shiner, trout, blueback herring, or a swimbait) up to one half its length and a sunfish up to one third its length.
shad
What Bass Eat
Optimal foraging theory is a well-established principle in ecology that predicts that a predator will eat the food item that gives it the greatest net energy return. Thus, the theory predicts that a bass will get the greatest energy gain from the largest forage fish it can consume.

Although optimal foraging is a nice theory, does it really apply to bass? Big bass get caught on small drop shot worms and tiny crankbaits. What sizes of forage fish do largemouth prefer? Four studies of food consumed by largemouth in the wild and three different laboratory studies, all of which allowed bass to select forage from a range of sizes, found that bass chose forage at the lower end of the sizes available. This clearly does not match the prediction of optimal foraging theory.

The largest forage may not be the most energy-efficient meal if it is harder to find, harder to catch, and harder to ingest—all of which require energy. So, peak energy gain may be achieved by eating a food item smaller than the maximum size the bass can ingest.

The selection of smaller forage in the wild could be attributed to the greater availability of small than large forage, but this would not apply to the laboratory food selection studies where different sizes of prey were equally available. In short, optimal foraging theory, although a very useful concept, does not fully account for the size of prey bass eat.

A slightly twisted version of optimal foraging theory may apply. A small shad does not provide as much energy as a larger one, however, if the amount of energy provided by that shad is greater than the energy the bass expended to catch it, there’s still a net gain.