Largemouth, smallmouth, and spotted bass live together in many middle-America waters, from Tennessee and West Virginia in the east to eastern Kansas and Oklahoma in the west. In Mississippi, the three black bass co-occur in Pickwick Lake and, potentially, Bay Springs Lake. Ecological theory predicts that these very similar fish should differ in how they use their shared environment.
Differences in food resources used, both for adult and juvenile stages, is a common way to achieve peaceful coexistence and sustained populations. But in Mississippi waters, and throughout much of their co-occurring range, adults of all three black bass species feed predominantly on the abundant shad; the juveniles share similar invertebrate diets until they transition to shad.
When diets overlap, segregation by habitat is a common ploy to allow coexistence. A recently published study by Dr. Steve Miranda and doctoral student Nicky Faucheux at Mississippi State and Kurt Lakin at the Tennessee Valley Authority (TVA) assessed habitat segregation of largemouth, smallmouth, and spotted bass.
The data were 21 years of fall electrofishing catches by TVA biologists in 10 main-stem reservoirs of the Tennessee River. Numerous samples at randomly selected shoreline stations were sampled in each of the reservoirs. At the completion of each sample, the biologists categorized the prevalent habitat as sediment, gravel, natural rock, riprap, brush, or aquatic plants.
Across all reservoirs, largemouth bass made up 67% of all black bass that were collected, smallmouth 14% and spots 19%.
Adult largemouth selected aquatic plants and riprap habitat. Adult smallmouth and spotted bass selected rock and riprap but avoided aquatic plants. Also, adult smallmouth avoided habitats that were predominantly sediment, and spotted bass avoided gravel habitat. All three species neither selected nor avoided brush habitat.
Juvenile habitat selection largely mirrored that of adults, but the young of all three species avoided areas with a lot of sediment.
Measures of co-occurrence for adult bass showed no evidence of one species avoiding the other in five of the habitats. In the brush habitat, all species were more likely to occur together than predicted by chance.
Juvenile bass, however, had a high likelihood of being collected together in all habitats except aquatic plants. In other words, they tended to congregate.
Is ecology wrong?
No. Indeed, fisheries is a child of ecology, and effective fisheries management relies heavily on fundamental ecological principles. In natural lakes in the north, many of which are home to both largemouth and smallmouth bass — but not spotted bass — largemouth are in the weeds and smallmouth live in the rocks. Sure, you can catch a largemouth around rocks and a smallmouth in the weeds, but it is rare. The species segregate by habitat.
Before they were impounded, reservoirs were streams. In natural, free-flowing streams where all three black bass species co-occur, largemouth occupy slack-current, backwaters that often have a lot of aquatic plants and wood. Smallmouth usually are found in clear, cool reaches with moderate current and rocky substrates. Spotted bass tend to be rather intermediate in their habitat conditions and live in faster current than largemouth and deeper water than either largemouth or smallmouth. All three species, both juveniles and adults, share similar diets. In their natural habitat, where diets are similar, the species segregate by habitat, as predicted by fundamental ecological principles.
But reservoirs are man-made habitats, not natural. The habitat associations of adult black bass in TVA reservoirs largely track their habitat preferences in free-flowing streams but not, as measured by the co-occurrence indices, the habitat segregation observed in streams.
Food, shelter rule
The Mississippi State and TVA team suggested some alternative reasons for the unexpected results. The congregation of the juveniles — the exact opposite of habitat segregation — is likely a consequence of shared primary needs: food and shelter from predation. Sites that provide refuge and abundant food are attractive to the young of all species.
Less clear is the co-occurrence of the adults. Impoundment homogenizes habitat and, in the case of these Tennessee River reservoirs, stimulates production of the abundant shad eaten by all three bass. The three primary environmental variables that segregate largemouth, smallmouth and spotted bass in streams where at least two of these species co-occur are bottom substrate, depth and current velocity. The substrate is unchanged after impoundment, although it is increasingly covered with sediment. Efforts to minimize sediment inputs will benefit all three species, which shared selection for rocks and riprap.
Water depth, obviously, is no longer an issue — all fish have deep and shallow water.
Water current velocity is a common and important habitat-segregating variable among stream fish everywhere. With current gone, missing is the last and probably most-important variable that black bass use to partition a habitat and coexist. Yes, the navigation and hydropower system that is now the Tennessee River has current, but it is intermittent. It is not the 2, 3 or 4 mph current that flowed 24/7 before impoundments and would determine where a bass called home.
Where black bass live is a function of available habitats and \available forage. But the habitat preferences of bass in rivers and streams, established over thousands of generations, provide clues to where bass live after the rivers and streams are impounded.
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