Compared to natural lakes with relatively stable water levels, reservoirs are dynamic habitats where fluctuations in level and flow are typical conditions.
While many anglers can easily envision the large fluctuations in reservoir elevation that happen every year, such as occurs in Mississippi’s flood-control reservoirs. Equally obvious are the fluctuations in elevation over wet and dry years that occurs in water-supply reservoirs such as those in the Southwest. Less obvious are annual variations in water flow that occur in riverine reservoirs, such as those of the Tenn-Tom Waterway or Tennessee River reservoirs. A recent study by Dr. Michael Eggleton and graduate research assistant Clint Peacock at University of Arkansas-Pine Bluff document how variation in flow can affect largemouth and spotted bass growth.
Largemouth and spotted bass were collected from the lower 300 miles of the Arkansas River. This reach of the river became a series of 11 navigation pools with the completion of the McClellan-Kerr Arkansas River Navigation System in 1970. The pools of these “run-of-the-river” reservoirs are primarily main-channel habitat — among the 11 pools, 58% to 82% of the surface area of these pools is in the main channel.
Annual growth increments were calculated for each species and tested for association with April-to-September water flows. That time span corresponds roughly with the growing season for bass. Water flow, the volume of water moving through the reservoirs was classified as “low” when flows were in the lower 25% of the flows measured since 1970, “high” if the flows were in the upper 75% of flows measured since 1970, and “average” if the flows were greater than low and less than high — the middle 50% of flows measured since 1970.
The flows affected the growth of different size groups of bass differently. Growth of largemouth bass during their first and second years was greater during low- and average-flow years. Growth during the fourth and fifth years was greatest during the high-flow years. High flows enhanced the growth of larger largemouth but deterred the growth of smaller largemouth.
Like largemouths, spotted bass had greater growth during their first and second years when flows were low or average. Unlike largemouths, their growth during the next three seasons was not different among flow categories. Growth of young spotted bass was enhanced by low and average flows, but flow didn’t affect the growth of adult spots.
The consequences of the impaired growth of young bass during high-flow years can be consequential for the fishery. During periods of low and average flow, largemouth bass attained the 15-inch minimum length limit in approximately 4.6 years. In periods predominated by high flows, they grew to 15 inches in 5.1 years; it took six months longer for fish to recruit to the fishery. Similarly, it took spotted bass almost a full year longer to grow to 12 inches, the minimum legal length limit.
Mortality is always occurring, always removing fish from the population. In the years of faster growth, the largemouth bass larger than 15 inches were 17% of the population. In the years of slower growth, largemouth bass larger than 15 inches were 7% of the population, a 60% reduction in legal-length fish. The same reduction in proportion of fish larger than the minimum legal length limit applies to spotted bass. Higher flows, slower growth, fewer fish recruited to the fishery.
The researchers proposed that the slowed growth during high-flow years may create a “growth deficit,” whereby smaller fish, even if growing at a normal rate as larger adults, may never make up for the smaller size achieved during their first several years. The consequence is that it will take longer for the bass to reach 4, 5 or 6 pounds.
Various environmental factors could cause the reduction in growth during periods of high flow, but the lack of backwaters in this reservoir system is likely a significant factor, because bass are confined to the main channel where they are exposed to the high flows. Due to sedimentation, the backwater areas that remain are steadily being reduced in area and habitat quality. This is exactly the same condition that is occurring in the Tenn-Tom Waterway where, as of 2003 when the system was only 20 years old: 40% of the total aquatic habitat downstream of Bay Springs Lake was main-channel habitat, and some of the pools had lost 8% to 10% percent of backwater habitat. The losses of area of healthy backwater habitat since 2003 have not been assessed but undoubtedly are far greater.