January 03, 2024
This article originally appeared in the March-April 2022 issue of In-Fisherman.
Ecological Insights: Habitats of Co-Occurring Black Bass Species
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. Ecological theory predicts that these very similar fish should differ in how they use their shared environment. Fishery scientists at Mississippi State University and Tennessee Valley Authority (TVA) used 21 years of fall electrofishing catches by TVA biologists in 10 mainstem reservoirs of the Tennessee River to assess habitat segregation of adult largemouth, smallmouth, and spotted bass.* Numerous randomly selected shoreline stations were sampled in each of the reservoirs. At the completion of each sample, biologists categorized the prevalent habitat as sediment, gravel, natural rock, riprap, brush, or aquatic plants.
Largemouths selected aquatic plants and riprap habitat. Smallmouths and spotted bass selected rock and riprap but avoided aquatic plants. Smallmouths avoided habitats that were predominantly sediment, and spotted bass avoided gravel habitat. All three species neither selected nor avoided brush habitat.
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. In other words, they tended to congregate.
In natural lakes in the north, many of which are home to both largemouth and smallmouth bass (but not spotted bass), largemouths are in vegetation and smallmouths live in the rocks. Sure, you can catch a largemouths around rocks and a smallmouth in the weeds, but it’s rare. The species segregate by habitat.
But reservoirs, before impoundments, were streams. In natural, free-flowing streams, largemouths occupy slack-current backwaters that often have a lot of aquatic plants and wood. Smallmouths 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 largemouths or smallmouths. All three species 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 the adults of the three black basses in the Tennessee River reservoirs largely track the habitat preferences in their ancestral free-flowing streams but not, as measured by the co-occurrence indices, the habitat segregation observed in streams.
The co-occurrence appears to contradict ecological theory. Impoundment homogenizes habitat and, in the case of these Tennessee River reservoirs, stimulates production of the abundant shad that dominates the diets 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. Water depth is no longer an issue; all fish have access to ample deep and shallow water. Water current velocity—a common and important habitat-segregating variable among stream fish everywhere—is largely gone. Yes, the navigation and hydropower system that is now the Tennessee River has current, but it is intermittent.
Where black bass live is a function of habitats available and the available forage. But the habitat preferences of bass in rivers and streams, established over thousands of generations, still provide clues to where bass live after the rivers and streams are impounded, especially in riverine impoundments.
–Dr. Hal Schramm
*Miranda, L. E., K. M. Lakin, and N. M. Faucheux. 2021. Habitat associations of three black bass species in a reservoir system. Trans. Am. Fish. Soc. 150:538-547.
Cannibalistic Pike: Way of the Wild
It’s not easy being a young pike. It has to find enough food to grow, and avoid becoming a meal for one of its larger brethren. Small pike often constitute about 20 percent of the diet of medium-sized pike. But small is relative–sizable pike have been found in other pike’s gullets.
A pike may eat 75 percent of its weight per year in smaller pike, and another 2 to 3 times its weight in other fish. In a study conducted in Sweden, the risk of pike being eaten was reflected by their distribution in a lake.* By marking fish, researchers showed that pike distributed themselves on average 6 times farther from a pike large enough to eat them, compared to the distance they kept from pike of similar size. In high-density lakes, smaller pike were distributed about every 10 yards along suitable shoreline habitat. But there was considerable flux in their movements, leading to frequent redistribution. Nevertheless, they tended to maintain their distance from one another.
Another European study showed that larger pike occupied open waters in which preyfish were more abundant, whereas smaller pike tended to occupy vegetated ambush sites in shallow water. When larger pike are removed from a favorable hunting area, the smaller fish quickly fill the niche.
–Dr. Bruce Carlson
*Nilsson, P. A. 2006. Avoid your neighbours: size-determined spatial distribution patterns among northern pike individuals. Oikos 113:251-258.