The foraging window

This big-bellied Pickwick Lake bass has a large foraging window and appears well fed.

Understand how the food supply affects bass

Bass aren’t picky eaters. What they eat boils down to a simple two-step rule: small enough to swallow, big enough to be worth eating. In other words, the forage needs to be within a certain size range, which I call the “foraging window.”

When is forage small enough? Bass, like all other fish-eating freshwater fish, swallow their forage head first and whole. Research has demonstrated that largemouth bass can swallow prey whose body depth is less than the width of its mouth. Applying that rule, it can eat a shad up to half its body length. A bass can swallow a more slender-bodied prey, like a minnow, that is a little more that one-half the bass’ body length. And bass can swallow deep-bodied prey, like sunfish or crappie, up to one-third their length.

What is big-enough forage? That hasn’t been worked out. I’ve seen some 3- and 4-pound bass eat some awfully small forage, but those small foods were densely on the underside of hydrilla mats. It’s a matter of energetics: if a bass gets more energy from a meal than it expends chasing, capturing, swallowing and digesting it, then it is big enough.

As you can see in the figure, the largest forage a bass can eat increases as it grows. The smallest forage it is willing to chase increases, too. This simple principle is fundamental to bass management.

• Foraging windows and bass recruitment

Each year-class of bass hatches and begins feeding on invertebrates. As they grow, young bass switch to small fish if available. Shad usually spawn about two to four weeks after bass in most Mississippi waters. The young bass have abundant food in the recently spawned shad, but young shad quickly outgrow the maximum size young bass can consume. The shad outgrow the young bass’ foraging window. Continued good growth by young bass depends on other forage, like minnows or sunfish, that fit in their foraging window. A second shad spawn can provide another wave of vulnerable forage, a definite benefit to a bass year class.

The fishery management principle No. 1: a forage base of fish of different sizes is necessary for good bass recruitment.

• Foraging windows and trophy bass

Because bass have rather broad foraging windows, there is a lot of overlap in the size of forage eaten by bass of different sizes. This means widely different sizes of bass can share, and potentially compete for, forage. For example, a 12-inch largemouth can eat a 6-inch shad. A 16-inch largemouth can eat an 8-inch shad, a shad larger than the 12-inch largemouth can eat. But a 6-inch shad is still a pretty good meal for a 16-inch bass. The bigger bass can eat forage that it does not share with the smaller bass, but the smaller forage are shared by smaller and larger bass.

This overlap in food eaten is not a problem as long as there is plenty of forage. But note that a lot of forage is more than just a lot of bodies. The forage needs to be the right size. In the above example, if all shad are consumed before they reach 6 inches, then the 16-inch bass is competing with the 12-inch bass.

This potential competition is why slot limits are used to increase the growth rate of larger bass and produce trophy fish. Let’s look at a 15- to 18-inch protected slot limit. There is a lot of overlap in forage between what 12- to 15-inch bass eat and what 15- to 18-inch bass eat. When food is in short supply, these two groups are competing for food. Growth of both size groups of bass will be slow. By harvesting 12- to 15-inch bass, more food is available to the bass in the protected slot, their growth rate increases, and they grow quickly to 4 to 5 pounds if they are not harvested. Nice fish.

The fishery management principle No. 2: a forage base of fish of different sizes is necessary for good growth of large bass and sustained production of quality bass.

There is a corollary to this management principle: when bass recruitment is strong, manage the forage to manage the bass. While a harvest regulation appears to be directed at the bass and may change the bass population, the final effect on the bass often is a result of changing the forage base. And what the bass eat and what sizes of bass compete are determined by their foraging windows.

About Hal Schramm 182 Articles
Hal Schramm is an avid angler and veteran fisheries biologist.

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