Stomach Contents of Largemouth Bass

Fish Facts
Bass stomachs were pumped, to collect what they've eaten.
Bass stomachs were pumped, to collect what they've eaten.

For years, I've had the pleasure to study lakes and ponds and their inhabitants. Here, around my home base in north-central Texas, Largemouth bass are king of the waterways. Consequently, that's been the fish I've studied the most. If you study bass, by attrition, you have to know their food chain.

During these 40-plus years, I've found a variety of things in any given bass's stomach. By far, the number one item is fish. Maybe that's why we call them L.A.R.G.E.M.O.U.T.H. bass. Big mouth, swallow whole.

I've seen huge bass, languishing at the surface in the middle of a lake, with a fish just slightly smaller than they are, stuck in its throat—both of them mortally wounded. Believe it or not, that's a fairly common sight. I've seen everything imaginable, from ten-inch bass sailing out of the water chasing late summer dragonflies, to all kinds of amphibians, lizards, snakes, and even a barn swallow—yes, a bird.

Bass are opportunistic predators, if there is such a thing.

While my stories are just that, stories, anecdotal tales from decades of "been there, done that," I have plenty of respect for those researchers who zero in on the specifics of their scientific studies.

I firmly believe that habitat influences the food chain. I also completely believe habitat directly influences fish behavior.

A recent study of an east Texas lake resulted in a master's thesis. That study lasted for two years, and its primary focus was to determine the impact of habitat on Largemouth bass growth rates. This 125-acre lake, built in 1952, is being managed as a trophy bass fishery, but like every other pond and lake in these United States, it has been limited by declining habitat over its life. Temporary habitat improvements have been attempted, primarily adding cedar tree brush piles, but those deteriorated within two or three years.

This particular study involved estimating growth rates of bass, figuring out what these predators eat, and then doing a significant habitat improvement project using Mossback Fish Habitat "Fish Cities" to see if there would be a difference in growth rates of Largemouth bass.

The study revolved around three fundamental hypotheses. One, Largemouth bass would select positively for areas with newly added structure, especially since that lake had very little habitat left in it. A second concept was bass would decrease the size of seasonal use areas and move less. A prior study, one which led to a doctoral thesis, monitored bass movements for 18 months. So, this hypothesis suggested adding substantial volumes of good habitat would lead to couch potato bass. The third idea was bass would shift to foraging on more energetic prey, primarily because these fish cities would attract a smorgasbord of baitfish. Bass could hunt close to home. Because of that factor, bass become efficient, and waste less energy chasing food, resulting in better growth rates.

More than 1,200 bass were injected with PIT tags to track individual growth rates.
More than 1,200 bass were injected with PIT tags to track individual growth rates.

As with all excellent scientific studies, this one entailed collecting substantial volumes of data, and then evaluating it with a variety of proven formulas and calculations. Basically, this study evaluated fish growth rates before adding habitat, and then thoroughly evaluated growth rates after. Along the way, many bass stomachs were pumped and contents collected, identified as best as could be, and then quantified. Fish were captured via electrofishing, and using a pulse lavage method, scientists did what most people won't—look through what a fish is digesting at that moment. In the study, it was documented that during the spring season, bass around ten inches and larger ate mostly fish and crawfish in that lake. Almost 57% of their diets consisted of fish, and another 33% was crawfish. According to the study, a large amount of stomach contents were unidentifiable fish parts, but two species were readily identifiable, Gizzard Shad and Largemouth bass. Invertebrates accounted for about 7% of collected diets. During the second spring, scientists noticed an increase in Lepomis spp. More sunfish were identifiable. Summer diets were quite different. There was greater species variety in summer diets. Percentages changed during summer, with more fish, and fewer crawfish. But, in the fall samplings, fish was the primary food of choice for bass. As we might expect, diets were different between the size classes of bass. Big bass eat rather big meals. With supplemental stockings of threadfin shad and tilapia each spring, and occasionally some bluegills, prey abundance was higher than what a lake might normally produce. Fall diets showed some tilapia, but a much higher proportion of sunfish species, including Largemouth bass. Yes, bass eat bass. Winter month diets were mostly fish. Crayfish were unimportant, primarily because they weren't there, which is part of the normal life cycle of that creature.

One important point made in this study was an increase in bluegill consumption after the addition of new habitat. Bluegills congregate there, bass eat them. There was a measurable increase of bluegill in bass diets.

The bottom line of this aspect of the study is that bass growth rates actually improved with the addition of habitat. The presumption is that adequate habitat increased the opportunity for bass to efficiently capture its food. The study also infers adequate habitat enhancements will actually increase the productivity of a body of water. That's pretty profound—if biologists can quantify how much is enough to influence productivity.

Gizzard shad were often identified as a food source.
Gizzard shad were often identified as a food source.

Fish movements were surprising to the researchers. 40-plus bass received surgically implanted tracking devices. And, those were some of the biggest bass in the lake; they had to be large in order to receive a device as big as a man's thumb. Fish movement behavior didn't measurably change after the 21 fish cities were installed in the lake—but the only fish monitored were those tagged fish, which had grown up when that lake was a virtual desert for bass. I think their movement behaviors were conditioned as youngsters. Trackers did show those bass stopped in for a meal, so while the bass movement behavior remained relatively the same, their feeding habits were influenced. Attracting most of a lake's sunfish into structure will do that for bass.

I truly love seeing how these studies come together and how they lead to new questions. From the outside, we'd think a research study such as this leads to conclusions that can give some definitive ideas that can directly impact the way managers do what they do with waters under their care. This study did offer some of those watershed moments—forgive the pun. Addition of habitat influences a fishery, especially in a lake pretty well devoid of such. This study helps quantify the value. Toss in the fact that Mossback Fish Habitat, used in this study, is pretty much permanent, where all things organic are not. One question arising from the study is how much habitat is enough to affect productivity? That's a big question still to be answered. We know from experience that adding fish attractors does just that, it attracts fish. This study is attempting to show how habitat can actually influence a lake's ability to produce more fish. And, in this case, bigger fish.

One of the assumptions in this study was that bass whose stomachs were typically full of other bass, shad, sunfish, and crawfish during the season automatically discounted the value of other foodstuffs. This assumption, made by the professor in charge of the study, indicated that tilapia were of little to no value, and if you look strictly at the stomach contents, that's probably a valid assumption. But, I disagree. A lake, especially one intensely managed, is a working art of biology. In school, we are taught there exists a variety of levels of the food chain, starting with the nutrients which feed microscopic plankton, which feeds small insects, which feed bigger insects and small fish and so on. In reality, that's what happens in a well-managed fishery. Trophic levels cascade.

But, one of those little understood biological factors is how different organisms actually affect each trophic level. For example, bluegill are predator fish, limited by mouth size. They eat bugs and small fish. They impact the insect population, which has an effect at that trophic level. Threadfin shad are filter feeders, gleaning their food from the water column. Then, they reproduce most of the summer, providing baby fish to feed intermediate-sized predator fish. They affect three levels of the food chain. Gizzard Shad, especially big ones, feed on the bottom trough, rooting their food from the mud. Big bass eat big Gizzard Shad. Tilapia, on the other hand, are grazers. So are their babies. As soon as that daddy tilapia spits his newborns out of his mouth, they begin to graze, especially on algae. As a management tool, we use tilapia to help eliminate filamentous algae in ponds or lakes prone to that. A side benefit to tilapia is how fast they reproduce. They're more prolific than bluegill in southern lakes and ponds. Tilapia convert algae to flesh, then convert flesh up the chain when that fish is eaten.

Over the last decade, we've stocked tilapia in dozens, maybe hundreds, of lakes and ponds. There are several constants from that strategy. Stock at rates of twenty pounds per acre during spring or early summer months, and algae goes away within a month. The second common fact we are seeing is higher survival rates of young of year bluegill going into the winter months.

Yes, tilapia will die when the water temperature drops. My unproven theory is that lots of the baby tilapia are consumed by small predators, especially bigger bluegills and bass smaller than eight inches. Another unproven theory is that those intermediate-sized tilapia that do survive the year become sluggish as the temperatures drop, and intermediate-sized bass feed on them at that time. But, the biggest take-home point I have seen time and time again is that ponds with tilapia definitely see a marked increase in numbers of young bluegill going into the winter months. In ponds without tilapia and with overcrowded bass, just don't see significant numbers of young bluegill going into the winter. It just doesn't happen, unless there are other mitigating factors such as dense aquatic plant colonies.

That study was right on with its discovery of the significance of adding habitat. It was right on with what the scientists found in the stomachs of the bass they checked. No one can argue with that.

The conclusion that tilapia are inconsequential, just because very few were identified in bass stomachs, is shortsighted. Maybe that's another research question begging to be asked—and answered.

Reprinted with permission from Pond Boss Magazine